Topic 21. DIGESTIVE SYSTEM

The human digestive system is a digestive tube with glands located next to it, but outside it (salivary glands, liver and pancreas), the secret of which is involved in the process of digestion. Sometimes the digestive system is called the gastrointestinal tract.

The process of digestion is the process of chemical and mechanical processing of food, followed by the absorption of its breakdown products.

The role of the gastrointestinal tract in the human body is very large: from it comes the supply of substances that provide the body with the necessary energy and building materials to restore its constantly collapsing structures.

The entire digestive tract is very conditionally divided into three main sections - anterior, middle and posterior.

The anterior section includes the oral cavity with all its structural components, the pharynx and esophagus. In the anterior section, mainly the mechanical processing of food occurs.

The middle section includes the stomach, small and large intestines, liver and pancreas. In this department, the chemical processing of food takes place, the absorption of its breakdown products and the formation of feces.

The posterior section includes the caudal part of the rectum, which performs the function of evacuating undigested food residues from the alimentary canal.

Development of the digestive system

Tissue sources of development

Endoderm. In the early stages (4-week embryo), the rudiment of the digestive tract looks like an enterodermal tube (primary intestine), closed at both ends. In the middle part, the primary intestine communicates with the yolk sac by means of a yolk stalk. At the anterior end, a gill apparatus is formed.

Ectoderm. The invaginations of the ectoderm directed towards the blind ends of the primary intestine form the oral cavity and the anal bay.

The oral bay (stomodeum) is separated from the anterior end of the primary intestine by the oral (drain) plate.

The anal bay (proctodeum) is separated from the hindgut by a cloacal membrane.

Mesenchyme. The composition of the digestive wall includes derivatives of the mesenchyme - layers of connective tissue, smooth muscle cells and blood vessels.

The mesoderm forms the mesothelium of the serous integument, striated muscle fibers.

Neuroectoderm. Derivatives of the neuroectoderm (especially the neural crest) are an essential part of the gastrointestinal tract (enteric nervous system, part of the endocrine cells).

Development of the anterior gastrointestinal tract

Development of the face and mouth. Ectoderm, mesenchyme, neuroectoderm (neural crest and ectodermal placodes) are involved in the development of the face and oral cavity.

The ectoderm gives rise to stratified squamous epithelium of the skin, glands, and integumentary epithelium of the oral mucosa.

Mesenchyme. Derivatives of the mesenchyme of the head develop from several primordia.

The mesenchyme of the somites and the lateral plate of the head of the embryo forms the voluntary muscles of the craniofacial region, the skin itself, and the connective tissue of the dorsal region of the head.

The mesenchyme of the neural crest forms the structures of the face and pharynx - cartilage, bones, tendons, the skin itself, dentin, and the connective tissue stroma of the glands.

Ectodermal placodes. Some of the sensory neurons of the trigeminal ganglion (ganglion trigeminale) and the ganglion of the geniculi (ganglion geniculi) of the intermediate nerve originate from ectodermal placodes. From the same source, all neurons VIII (spiral ganglion, ganglion spirale cochleae), x (nodular ganglion, ganglion nodosum), IX (petrosal ganglion, ganglion petrosum) of the cranial nerve ganglia develop.

The face develops from seven rudiments: two early fused mandibular processes, two maxillary processes, two lateral nasal processes, and a medial nasal process. The maxillary and mandibular processes originate from the first gill arch.

In the facial region, by the 4th week, a frontal protrusion is formed, located along the midline and covering the forebrain. The frontal protrusion gives rise to the medial and lateral nasal processes. The emerging olfactory pits separate the medial nasal process from the lateral ones. Towards the midline, the maxillary processes grow, which together with the mandibular process form the corners of the mouth. Thus, the entrance to the oral cavity is limited by the medial nasal process, the paired maxillary processes, and the mandibular process.

By the 5th week, the maxillary processes are separated from the lateral nasal processes by the nasolacrimal groove, from which the nasolacrimal canal later develops. On the 6th week, during the formation of the upper jaw, the maxillary processes growing towards the midline bring together the nasal processes, which simultaneously increase and gradually cover the lower part of the frontal protrusion. At week 7, the maxillary and medial nasal processes fuse to form the philtrum. From the material of the fused maxillary processes, a maxillary segment is formed, from which the primary palate and the premaxillary part of the dental arch develop. The bone structures of the face are formed at the end of the 2nd - beginning of the 3rd month of development.

Development of the hard palate. The developing secondary palate separates the primary oral cavity into the nasal and secondary (final) oral cavity. On the inner surface of the maxillary processes, palatine processes are formed. On the 6th - 7th week, their edges are directed obliquely down and lie along the bottom of the oral cavity on the sides of the tongue. As the lower jaw develops and the volume of the oral cavity increases, the tongue descends, and the edges of the palatine processes rise up to the midline. After the fusion of the palatine processes and the formation of the secondary palate, the nasal chambers communicate with the nasopharynx through the final choanae.

With non-closure of the medial and lateral nasal processes, a gap of the upper lip is observed. The oblique facial fissure runs from the upper lip to the eye along the junction of the maxillary and lateral nasal processes. With incomplete connection of the maxillary and mandibular processes, an abnormally wide mouth develops - macrostomia. In addition to cosmetic defects, these malformations of the maxillofacial region cause serious respiratory and nutritional disorders in a child in the first days of life. With underdevelopment of the palatine processes, a cleft of the hard and soft palate is observed. Sometimes the cleft is only present in the soft palate.

Gill apparatus and its derivatives. In the initial section of the foregut, the branchial apparatus is formed, which is involved in the formation of the face, organs of the oral cavity and the cervical region. The gill apparatus consists of five pairs of pharyngeal pouches and the same number of gill arches and slits.

Development and role of pharyngeal pouches and gill slit. From the structures of the gill apparatus, the pharyngeal pockets are the first to appear. These are protrusions of the endoderm in the region of the lateral walls of the pharyngeal section of the primary intestine.

Towards the pharyngeal pockets of the endoderm, invaginations of the ectoderm of the cervical region grow, which are called gill slits.

Gill arches. The material between adjacent pharyngeal pouches and slits is called gill arches. There are four of them, the fifth gill arch is a rudimentary formation. Gill arches on the anterolateral surface of the neck form a ridge-like elevation. The mesenchymal base of each gill arch is penetrated by blood vessels (aortic arches) and nerves. Soon, muscles and a cartilaginous skeleton develop in each of them. The largest is the first gill arch, extramaxillary. The second gill arch is called the hyoid arch. The smaller third, fourth and fifth arches do not reach the median line and grow together with those located above. From the lower edge of the second gill arch, a gill fold (operculum) grows, covering the outside of the lower gill arches. This fold grows together with the skin of the neck, forming the anterior wall of the deep fossa (sinus cervicalis), at the bottom of which the lower gill arches are located. This sinus first communicates with the external environment, and then the hole above it overgrows. When the cervical sinus is not closed, a fistulous tract remains on the child's neck, communicating with the pharynx, if the second gill arch breaks through.

Development of the vestibule of the oral cavity. On the 7th week of development near the outer part of the jaw, in parallel with the formation of the epithelial dental plate, another growth of the epithelium occurs, called the labio-gingival plate (lamina labio-gingivalis). It forms a furrow that separates the rudiments of the upper and lower jaws from the lip.

Language development. The tongue develops from several rudiments that look like tubercles and are located at the bottom of the primary oral cavity in the region of the ventral gill arches. On the 8th - 9th week, the development of papillae on the upper surface of the anterior body of the tongue begins, while the lymphoid tissue develops in the back of the mucous membrane of the tongue. The muscles of the tongue originate from the myotomes of the upper (anterior) somites.

The material of all four gill arches is involved in the laying of the tongue. Two large lateral lingual tubercles and an unpaired lingual tubercle (tuberculum impar) originate from the first gill arch. The root of the tongue develops from a staple that originates from the second, third, and fourth gill arches. From the material between the unpaired lingual tubercle and the bracket, the thyroid gland is laid. The excretory duct (linguothyroid duct) of its rudiment opens on the surface of the rudiment of the tongue with a blind opening.

On the 4th week, an unpaired lingual tubercle (tuberculum impar) appears, located in the midline between the first and second gill arches. From this tubercle develops a small part of the back of the tongue, lying anterior to the blind basking (foramen coecum). In addition, on the inside of the first gill arch, two paired thickenings are formed, called lateral lingual tubercles. From these three protrusions, a significant part of the body of the tongue and its tip are formed.

The root of the tongue arises from a thickening of the mucous membrane lying behind the blind opening, at the level of the second, third and fourth gill arches. This is a bracket (copula).

The unpaired tubercle flattens rather quickly. All the rudiments of the tongue grow together, forming a single organ.

The boundary between the root and the body of a language. In the future, the boundary between the root and the body of the tongue is the line of location of the grooved papillae. At the top of this angle is a blind hole, the mouth of the lingual-thyroid duct. From the remnants of this duct, epithelial cysts can develop in the thickness of the tongue.

The digestive tube, despite the morphological and physiological features of its departments, has a general structural plan. Its wall consists of a mucous membrane lining the tube from the inside, a submucosa, a muscular membrane and an outer membrane, which is represented by a serous or adventitious membrane.

Mucous membrane. It got its name due to the fact that its surface is constantly moistened with mucus secreted by the glands. This membrane consists, as a rule, of three plates: the epithelium, the lamina propria of the mucosa and the muscular lamina of the mucosa. The epithelium in the anterior and posterior sections of the digestive tube (in the oral cavity, pharynx, esophagus, caudal part of the rectum) is stratified flat, and in the middle section, that is, in the stomach and intestines, it is single-layer cylindrical. Glands are located either endoepithelially (for example, goblet cells), or exoepithelially (in the lamina propria and in the submucosa), or outside the alimentary canal (in the liver, pancreas).

The composition of the mucous membrane includes its own plate, which lies under the epithelium, is separated from it by a basal membrane and is represented by loose fibrous unformed connective tissue. Blood and lymphatic vessels, nerve elements, accumulations of lymphoid tissue pass through it.

The location of the muscularis mucosa is the border with the submucosa. This plate consists of several layers formed by smooth muscle cells.

The relief of the mucous membrane throughout the entire alimentary canal is heterogeneous. It can be both smooth (lips, cheeks), and form indentations (pits in the stomach, crypts in the intestines), folds, villi (in the small intestine).

The submucosa is represented by a loose fibrous, unformed connective tissue, as it were, it connects the mucous membrane with the underlying formations (muscle membrane or bone base). Thanks to it, the mucous membrane has mobility and can form folds.

The muscular membrane is made up of smooth muscle tissue, in this case, the arrangement of muscle fibers can be circular (inner layer) and longitudinal (outer layer).

These layers are separated by connective tissue, which contains the blood and lymphatic vessels and the intermuscular nerve plexus. When the muscle membrane contracts, food is mixed and promoted during digestion.

Serous membrane. The bulk of the gastrointestinal tract is covered with a serous membrane - the visceral sheet of the peritoneum. The peritoneum consists of a connective tissue base, in which there are vessels and nerve elements, and of the mesothelium that surrounds it from the outside. At the same time, in relation to this shell, organs can be in several states: intraperitoneally (the organ is covered by it for the entire diameter), mesoperitoneally (the organ is covered by it only by 2/3) and extraperitoneally (the organ is covered by it from only one side).

Some sections (esophagus, part of the rectum) do not contain a serous membrane. In such places, the alimentary canal is covered on the outside with an adventitial membrane consisting of connective tissue.

The blood supply of the gastrointestinal tract is very abundant.

The most powerful plexuses are in the submucosal layer, they are closely related to the arterial plexuses that lie in the lamina propria of the mucous membrane. In the small intestine, arterial plexuses are also formed in the muscular membrane. Capillary networks are formed under the epithelium of the mucous membrane, around the glands, crypts, gastric pits, inside the villi, papillae of the tongue and in the muscle layers. Veins also form plexuses of the submucosa and mucosa.

Lymphatic capillaries take part in the formation of a network under the epithelium, around the glands in the lamina propria, as well as in the submucosa and muscularis.

The efferent innervation of all digestive organs comes from the ganglia of the autonomic nervous system, located either outside the digestive tube (extramural sympathetic ganglia) or in its thickness (intramural parasympathetic ganglia).

Afferent innervation is carried out by the endings of the dendrites of sensitive nerve cells, occurs due to the intramural ganglia, in which the endings are dendrites from the spinal ganglia. Sensitive nerve endings are located in the muscles, epithelium, fibrous connective tissue and nerve ganglia.

Oral cavity

The mucous membrane lining the oral cavity is distinguished by the following features: the presence of stratified squamous epithelium, the complete absence or weak development of the muscularis mucosa, and the absence of a submucosal layer in some areas. At the same time, there are places in the oral cavity where the mucous membrane is firmly fused with the underlying tissues and lies directly on the muscles (for example, in the back of the tongue) or on the bones (in the gums and hard palate). The mucous membrane can form folds in which accumulations of lymphoid tissue are located. Such areas are called tonsils.

In the mucous membrane there are many small blood vessels that shine through the epithelium and give it a characteristic pink color. A well-moistened epithelium is able to pass many substances into the underlying blood vessels, therefore, in medical practice, the introduction of drugs such as nitroglycerin, validol, and others through the oral mucosa is often used.

Lips. Three parts are distinguished in the lip - skin, transitional (or red) and mucous. In the thickness of the lip there is a striated muscle. The skin part of the lip has the structure of the skin. It is covered with stratified squamous keratinized epithelium and is supplied with sebaceous, sweat glands and hair. The epithelium of this part is located on the basal membrane, under which lies a loose fibrous connective tissue that forms high papillae that protrude into the epithelium.

The transitional (or red) part of the lip, in turn, consists of two zones: the outer (smooth) and the inner (villous). In the outer zone, the stratum corneum of the epithelium is preserved, but becomes thinner and more transparent. There is no hair in this area, the sweat glands gradually disappear, and only the sebaceous glands remain, opening with their ducts to the surface of the epithelium. There are more sebaceous glands in the upper lip, especially in the corner of the mouth. The lamina propria is a continuation of the connective tissue part of the skin, its papillae in this area are low. The inner zone in newborns is covered with epithelial papillae, which are sometimes called villi. These epithelial papillae, as the organism develops, gradually smooth out and become inconspicuous. The inner zone of the transitional part of the lip of an adult is characterized by a very high epithelium, devoid of the stratum corneum. In this zone, as a rule, sebaceous glands are absent. The lamina propria, protruding into the epithelium, forms very high papillae, in which there are numerous capillaries. The blood circulating in them shines through the epithelium and gives this area a reddish tint. The papillae contain a huge number of nerve endings, so the red edge of the lip is very sensitive.

The mucous part of the lip is covered with stratified squamous non-keratinized epithelium, but sometimes a small amount of keratin grains can still be detected in the cells of the surface layer of the epithelium.

The lamina propria also forms papillae here, but they are less high than in the adjacent villous zone of the lip. The muscular lamina of the mucous membrane is absent, therefore, its own lamina, without a sharp border, passes into the submucosa, adjacent directly to the striated muscles. In the submucosal base of the mucous part of the lip are the secretory sections of the salivary labial glands. Their excretory ducts open on the surface of the epithelium. The glands are quite large, sometimes reaching the size of a pea. By structure, these are complex alveolar-tubular glands. By the nature of the secret, they belong to the mixed mucous-protein glands. Their excretory ducts are lined with stratified squamous non-keratinized epithelium. In the submucosa of the mucous part of the lip, large arterial trunks pass, and there is also an extensive venous plexus, which also extends into the red part of the lip.

The cheeks are a muscular formation, which is covered on the outside with skin, and on the inside with a mucous membrane. Three zones are distinguished in the mucous membrane of the cheek - the upper (maxillary), middle (intermediate) and lower (mandibular). At the same time, a distinctive feature of the cheeks is that there is no muscular plate in the mucous membrane.

The maxillary part of the cheek has a structure similar to the structure of the mucous part of the lip. It is covered with stratified squamous non-keratinized epithelium, the papillae of the lamina propria are small in size. In these areas there are a large number of salivary glands of the cheek.

The middle (intermediate) zone of the cheek goes from the corner of the mouth to the branch of the lower jaw. The papillae of the lamina propria here, as in the transitional part of the lip, are large. There are no salivary glands. All these features indicate that the intermediate zone of the cheek, like the transitional part of the lip, is the zone of transition of the skin into the mucous membrane of the oral cavity.

The submucosa contains many blood vessels and nerves. The muscular membrane of the cheek is formed by the buccal muscle, in the thickness of which lie the buccal salivary glands. Their secretory sections are represented by mixed protein-mucous and purely mucous glands.

Gums are formations covered with a mucous membrane, tightly fused with the periosteum of the upper and lower jaws. The mucous membrane is lined with stratified squamous epithelium, which can become keratinized. The lamina propria forms long papillae, which consist of loose connective tissue. The papillae become lower in the part of the gum that is directly adjacent to the teeth. The lamina propria contains blood and lymph vessels. The gum is richly innervated. The epithelium contains free nerve endings, and the lamina propria contains encapsulated and non-encapsulated nerve endings.

Solid sky. It consists of a bone base covered with a mucous membrane.

The mucous membrane of the hard palate is lined with stratified squamous non-keratinizing epithelium, while the submucosa is absent.

The lamina propria of the mucous membrane of the hard palate is formed by fibrous unformed connective tissue.

The lamina propria has one peculiarity: bundles of collagen fibers are strongly intertwined and woven into the periosteum, this is especially pronounced in those places where the mucous membrane is tightly fused with the bone (for example, in the area of ​​​​the seam and the zone of transition to the gums).

The soft palate and uvula are represented by a tendon-muscle base covered with a mucous membrane. In the soft palate and uvula, oral (anterior) and nasal (posterior) surfaces are distinguished.

The mucous membrane of the oral part of the soft palate and uvula is covered with stratified squamous non-keratinized epithelium. The lamina propria, consisting of loose fibrous unformed connective tissue, forms high narrow papillae that protrude deeply into the epithelium. Deeper there is a pronounced submucosal base formed by loose fibrous unformed connective tissue with a large number of fatty elements and mucous salivary glands. The excretory ducts of these glands open on the oral surface of the soft palate and uvula.

The mucous membrane of the nasal surface of the soft palate is covered with a single-layer prismatic multi-row ciliated epithelium with a large number of goblet cells.

The human tongue, in addition to participating in taste perception, mechanical processing of food and the act of swallowing, performs an important function of the organ of speech. The basis of the tongue is striated muscle tissue, the contraction of which is arbitrary.

The relief of the mucous membrane covering it is different on the lower, lateral and upper surfaces of the tongue. The epithelium on the underside of the tongue is multi-layered, flat, non-keratinized, of small thickness. The mucous membrane of the upper and lateral surfaces of the tongue is fixedly fused with its muscular body. It contains special formations - papillae.

On the surface of the tongue there are four types of papillae: filiform, mushroom-shaped, surrounded by a shaft and leaf-shaped.

Most of the filiform papillae of the tongue. In size, they are the smallest among the papillae of the tongue. These papillae may be either filiform or conical in shape. In some forms of diseases, the process of rejection of superficial keratinizing epithelial cells can slow down, and epithelial cells, accumulating in large quantities on the tops of the papillae, thus form a film (plaque).

The second place in frequency of occurrence is occupied by fungiform papillae of the tongue, they are located on the back of the tongue among the filiform papillae (most of all on the tip of the tongue and along its edges). Most of them are mushroom-shaped.

Grooved papillae of the tongue (papillae of the tongue surrounded by a shaft) are located on the upper surface of the tongue in an amount of 6 to 12. They are located between the body and the root of the tongue along the boundary line. Unlike adults, the foliate papillae of the tongue are well developed only in children; they are located on the right and left edges of the tongue.

The mucous membrane of the root of the tongue does not have papillae. Elevations of the epithelium are formed due to the fact that in the own plate of the mucous membrane there are accumulations of lymphoid tissue, sometimes reaching 0,5 cm in diameter. Between these clusters, the epithelium forms depressions - crypts. The ducts of numerous mucous glands flow into the crypts. The collection of accumulations of lymphoid tissue in the root of the tongue is called the lingual tonsil.

The muscles of the tongue form the body of this organ, they are represented by a striated type of bundles, and are located in three mutually perpendicular directions.

The salivary glands of the tongue, according to the nature of the secret they secrete, can be divided into three types - proteinaceous, mucous and mixed.

The blood supply to the tongue is carried out by the lingual arteries.

The muscles of the tongue are innervated by branches of the hypoglossal nerve and the chorda tympani.

Sensitive innervation of the anterior 2/3 of the tongue is carried out by the branches of the trigeminal nerve, the posterior 1/3 by the branches of the glossopharyngeal nerve.

Salivary glands. In the oral cavity there are openings of the excretory ducts of three pairs of large salivary glands - parotid, submandibular and sublingual.

All salivary glands are complex alveolar or alveolar-tubular glands. They include the secretory ends of the departments and ducts that remove the secret.

Secretory sections according to the structure and nature of the secreted secret are of three types - lateral (serous), mucous and mixed (i.e., protein-mucous).

The excretory ducts of the salivary glands are divided into intercalary, striated, intralobular, interlobular excretory ducts and the common excretory duct.

The salivary glands perform exocrine and endocrine functions.

The exocrine function consists in the regular separation of saliva into the oral cavity. Saliva consists of water (about 99%), protein substances, including enzymes, non-protein substances (salts), inorganic substances, as well as cellular elements (epithelial cells, leukocytes).

The endocrine function of the salivary glands is ensured by the presence in saliva of biologically active substances such as hormones (kallikrein and bradykinin, an insulin-like substance, nerve growth factor, epithelial growth factor, thymocyte-transforming factor, lethality factor, etc.).

Teeth are the main part of the chewing apparatus. There are several types of teeth: first, falling (milk) teeth are formed, and then permanent ones. In the holes of the jaw bones, the teeth are strengthened by a dense connective tissue - periodontium, which forms a circular dental ligament in the region of the neck of the tooth. Collagen fibers of the dental ligament have a predominantly radial direction, while on the one hand they penetrate into the cement of the tooth root, and on the other - into the alveolar bone. The periodontium performs not only a mechanical, but also a trophic function, since blood vessels pass through it, feeding the root of the tooth.

Development of teeth. The laying of milk teeth begins at the end of the 2nd month of intrauterine development. The following structures are involved in the formation of the tooth germ: dental plate, enamel organ, dental papilla and dental sac.

The dental plate appears on the 7th week of intrauterine development as a thickening of the epithelium of the upper and lower jaws. At the 8th week, the dental lamina grows into the underlying mesenchyme.

Enamel organ - a local accumulation of cells of the dental plate, corresponding to the position of the tooth, determines the shape of the crown of the future tooth. The cells of the organ form the outer and inner enamel epithelium. Between them is localized loose mass of cells - enamel pulp. The cells of the inner enamel epithelium differentiate into cylindrical cells that form enamel - ameloblasts (enameloblasts). The enamel organ is connected to the dental plate, and then (on the 3rd - 5th month of intrauterine development) is completely separated from it.

Ameloblastoma is a benign but locally invasive tumor of the oral cavity originating from remnants of the epithelium of the enamel organ.

The dental papilla is a collection of mesenchymal cells originating from the neural crest and located within the goblet enamel organ. The cells form a dense mass that takes the shape of the crown of the tooth. Peripheral cells differentiate into odontoblasts.

dental pouch

The dental sac is the mesenchyme that surrounds the tooth germ. Cells that come into contact with root dentin differentiate into cementoblasts and deposit cementum. The outer cells of the dental sac form the periodontal connective tissue.

Milk tooth development. In a two-month-old fetus, the tooth rudiment is represented only by a formed dental plate in the form of an epithelial outgrowth into the underlying mesenchyme. The end of the dental plate is expanded. The enamel organ will develop from it in the future. In a three-month-old fetus, the formed enamel organ is connected to the dental plate with the help of a thin epithelial cord - the neck of the enamel organ. In the enamel organ, internal enamel cells of a cylindrical shape (ameloblasts) are visible. Along the edge of the enamel organ, the inner enamel cells pass into the outer ones, which lie on the surface of the enamel organ and have a flattened shape. The cells of the central part of the enamel organ (pulp) acquire a stellate shape. Part of the pulp cells, adjacent directly to the layer of enameloblasts, forms an intermediate layer of the enamel organ, consisting of 2-3 rows of cubic cells. The dental sac surrounds the enamel organ and then merges at the base of the tooth germ with the mesenchyme of the dental papilla. The dental papilla grows in size even deeper into the enamel organ. It is penetrated by blood vessels.

On the surface of the dental papilla, cells with dark basophilic cytoplasm differentiate from mesenchymal cells, arranged in several rows. This layer is separated from the ameloblasts by a thin basement membrane. In the circumference of the tooth germ, the crossbars of the bone tissue of the dental alveoli are formed. At the 6th month of development, the nuclei of ameloblasts move in the direction opposite to their original position. Now the nucleus is located in the former apical part of the cell, bordering the pulp of the enamel organ. In the dental papilla, a peripheral layer of regularly located pear-shaped odontoblasts is determined, the long process of which faces the enamel organ. These cells form a narrow strip of non-mineralized predentin, outside of which there is some mature mineralized dentin. On the side facing the dentin layer, a strip of organic matrix of enamel prisms is formed. The formation of dentin and enamel extends from the apex of the crown to the root, which is fully formed after the crown has erupted.

Laying of permanent teeth. Permanent teeth are laid at the end of the 4th month of intrauterine development. From the common dental plate behind each rudiment of a milk tooth, a rudiment of a permanent tooth is formed. First, the milk and permanent teeth are in a common alveolus. Later, a bony septum separates them. By the age of 6-7, osteoclasts destroy this septum and the root of the falling milk tooth.

Change of teeth. The first set of teeth (milk teeth) consists of 10 in the upper jaw and 10 in the lower jaw. The eruption of milk teeth in a child begins at the 6-7th month of life. The central (medial) and lateral incisors erupt first on both sides of the midline in the upper and lower jaws. In the future, canines appear lateral to the incisors, behind which two molars erupt. A full set of milk teeth is formed at about two years of age. Milk teeth serve for the next 4 years. The change of milk teeth occurs in the range from 6 to 12 years. Permanent front teeth (canines, small molars) replace the corresponding milk teeth and are called replacement permanent teeth. Premolars (permanent small molars) replace milk molars (large molars). The germ of the second large molar tooth is formed in the 1st year of life, and the third molar (wisdom tooth) - by the 5th year. The eruption of permanent teeth begins at the age of 6-7 years. The large molar (first molar) erupts first, then the central and lateral incisors. At 9-14 years old, premolars, canines and the second molar erupt. Wisdom teeth erupt later than all - at the age of 18 - 25 years.

The structure of the tooth. It includes two parts: hard and soft. In the hard part of the tooth, enamel, dentin and cement are isolated, the soft parts of the tooth are represented by the so-called pulp. Enamel is the outer shell and covers the crowns of the tooth. The thickness of the enamel is 2,5 mm along the cutting edge or in the region of the masticatory tubercles of the molars and decreases as it approaches the neck.

In the crown, under the enamel, there is a characteristically striated dentin, continuing in a continuous mass into the root of the tooth. Enamel formation (synthesis and secretion of the components of its organic matrix) involves cells that are absent in mature enamel and an erupted tooth - enameloblasts (ameloblasts), so enamel regeneration during caries is impossible.

Enamel has a high refractive index - 1,62, enamel density - 2,8 - 3,0 g per square centimeter of area.

Enamel is the hardest tissue in the body. However, enamel is fragile. Its permeability is limited, although there are pores in the enamel through which aqueous and alcoholic solutions of low molecular weight substances can penetrate. Relatively small water molecules, ions, vitamins, monosaccharides, amino acids can slowly diffuse in the enamel substance. Fluorides (drinking water, toothpaste) are included in the crystals of enamel prisms, increasing the resistance of enamel to caries. The permeability of enamel increases under the action of acids, alcohol, with a deficiency of calcium, phosphorus, fluorine.

Enamel is formed by organic substances, inorganic substances, water. Their relative content in weight percent: 1 : 96 : 3. By volume: organic matter 2%, water - 9%, inorganic matter - up to 90%. Calcium phosphate, which is part of hydroxyapatite crystals, makes up 3/4 of all inorganic substances. In addition to phosphate, calcium carbonate and fluoride are present in small amounts - 4%. Of the organic compounds, there is a small amount of protein - two fractions (soluble in water and insoluble in water and weak acids), a small amount of carbohydrates and lipids was found in the enamel.

The structural unit of enamel is a prism with a diameter of about 5 microns. The orientation of the enamel prisms is almost perpendicular to the boundary between enamel and dentine. Neighboring prisms form parallel beams. On sections parallel to the surface of the enamel, the prisms have the shape of a key nest: the elongated part of the prism of one row lies in the other row between the two bodies of adjacent prisms. Due to this shape, there are almost no spaces between the prisms in the enamel. There are prisms and a different (in cross section) shape: oval, irregular shape, etc. Perpendicular to the surface of the enamel and the enamel-dentin border, the course of the prisms has s-shaped bends. We can say that the prisms are helically curved.

There are no prisms on the border with dentin, as well as on the enamel surface (prismless enamel). The material surrounding the prism also has other characteristics and is called the "prism shell" (the so-called gluing (or soldering) substance), the thickness of such a shell is about 0,5 microns, in some places the shell is absent.

Enamel is an exceptionally hard tissue, which is explained not only by the high content of calcium salts in it, but also by the fact that calcium phosphate is found in enamel in the form of hydroxyapatite crystals. The ratio of calcium and phosphorus in crystals normally varies from 1,3 to 2,0. With an increase in this coefficient, the stability of the enamel increases. In addition to hydroxyapatite, other crystals are also present. The ratio of different types of crystals: hydroxyapatite - 75%, carbonate apatite - 12%, chlorine apatite - 4,4%, fluorapatite - 0,7%.

Between the crystals there are microscopic spaces - micropores, the totality of which is the medium in which diffusion of substances is possible. In addition to micropores, there are spaces between prisms in enamel - pores. Micropores and pores are the material substrate of enamel permeability.

There are three types of lines in the enamel, reflecting the uneven nature of enamel formation in time: transverse striation of enamel prisms, Retzius lines and the so-called neonatal line.

The transverse striation of enamel prisms has a period of about 5 µm and corresponds to the daily periodicity of prism growth.

Due to differences in optical density due to lower mineralization, Retzius lines are formed at the boundary between the elementary units of enamel. They look like arches arranged in parallel at a distance of 20 - 80 microns. The lines of Retzius may be interrupted, there are especially many of them in the neck area. These lines do not reach the surface of the enamel in the region of the masticatory tubercles and along the cutting edge of the tooth. The elementary units of enamel are rectangular spaces delimited from each other by vertical lines - the boundaries between prisms and horizontal lines (transverse striation of prisms). In connection with the unequal rate of enamel formation at the beginning and at the end of amelogenesis, the value of elementary units, which differs between the surface and deep layers of enamel, is also important. Where the Retzius lines reach the surface of the enamel, there are furrows - perichyma, running in parallel rows along the surface of the tooth enamel.

The neonatal line delimits the enamel formed before and after birth, it is visible as an oblique strip, clearly visible against the background of prisms and passing at an acute angle to the tooth surface. This line consists predominantly of prismless enamel. The neonatal line is formed as a result of changes in the mode of enamel formation at birth. These enamels are found in the enamel of all temporary teeth and, as a rule, in the enamel of the first premolar.

The surface areas of the enamel are denser than its underlying parts, the concentration of fluorine is higher here, there are grooves, pits, elevations, prismatic areas, pores, micro-holes. Various layers may appear on the surface of the enamel, including colonies of microorganisms in combination with amorphous organic matter (dental plaques). When inorganic substances are deposited in the plaque area, tartar is formed.

Huntero-Schreger bands in enamel are clearly visible in polarized light in the form of alternating bands of different optical density, directed from the border between the dentin almost perpendicular to the surface of the enamel. The stripes reflect the fact that the prisms deviate from the perpendicular position with respect to the enamel surface or to the enamel-dentin border. In some areas, enamel prisms are cut longitudinally (light stripes), in others - transversely (dark stripes).

Dentin is a type of mineralized tissue that makes up the bulk of the tooth. Dentin in the area of ​​the crown is covered with enamel, in the area of ​​the root - with cement. Dentin surrounds the cavity of the tooth in the area of ​​the crown, and in the area of ​​the root - the root canal.

Dentin is denser than bone tissue and cementum, but much softer than enamel. Density - 2,1 g/cm³. The permeability of dentin is much greater than the permeability of enamel, which is associated not so much with the permeability of the dentin substance itself, but with the presence of tubules in the mineralized dentin substance.

Composition of dentin: organic matter - 18%, inorganic matter - 70%, water - 12%. By volume - organic matter is 30%, inorganic matter - 45%, water - 25%. Of the organic substances, the main component is collagen, much less chondroitin sulfate and lipids. Dentin is highly mineralized, the main inorganic component being hydroxyapatite crystals. In addition to calcium phosphate, calcium carbonate is present in dentin.

Dentin is permeated with tubules. The direction of the tubules is from the border between the pulp and dentin to the dentin-enamel and dentin-cement junctions. Dentinal tubules are parallel to each other, but have a tortuous course (S-shaped on vertical sections of the tooth). The diameter of the tubules is from 4 µm closer to the pulpal edge of the dentin to 1 µm along the periphery of the dentin. Closer to the pulp, the tubules account for up to 80% of the volume of dentin, closer to the dentin-enamel junction - about 4%. In the root of the tooth, closer to the dentin-cement border, the tubules not only branch, but also form loops - the region of the granular layer of Toms.

On a section running parallel to the enamel-dentin junction, heterogeneities of dentin mineralization are visible. The lumen of the tubules is covered by a double concentric cuff with a dense periphery - peritubular dentin, dental (or Neumann) sheaths. The dentin of the Neumann sheaths is more mineralized than the intertubular dentin. The outermost and innermost parts of the peritubular dentin are less mineralized than the median part of the cuff. There are no collagen fibrils in peritubular dentin, and hydroxyapatite crystals are organized differently in peritubular and intertubular dentin. Closer to the predentin, the peritubular dentin is practically absent. Peritubular dentin is constantly formed, therefore, in adults, peritubular dentin is significantly larger than in children, respectively, the permeability of dentin in children is higher.

In different parts of the tooth, dentin is heterogeneous.

Primary dentin is formed during mass dentinogenesis. In the mantle (superficial) and near-pulp dentin, the orientation of collagen fibers is different. The mantle dentin is less mineralized than the peripulpal dentin. Raincoat dentin is located on the border with enamel. The peripulpal dentin is the bulk of the dentin.

Granular and hyaline layers of dentin. In the root of the tooth, between the main mass of dentin and acellular cement, there are granular and hyaline layers of dentin. In the hyaline layer, the orientation of the fibers is felt-like. The granular layer consists of alternating areas of hypo- or completely non-mineralized dentin (interglobular spaces) and fully mineralized dentin in the form of spherical formations (dentinal balls or calcospherites).

Secondary dentin (or irritant dentin) is deposited between the bulk of dentin (primary dentin) and predentin. Irritation dentin is constantly formed throughout life by abrasion of chewing surfaces or destruction of dentin.

Regular dentin (organized dentin) is located in the region of the root of the tooth.

Irregular irritation dentin (disorganized dentin) is located at the apex of the tooth cavity.

Predentin (or non-mineralized dentin) is located between the layer of odontoblasts and dentin. Predentin is newly formed and non-mineralized dentin. Between the predentin and the peripulpal dentin there is a plate of mineralizing predentin - an intermediate dentin of calcification.

There are several types of breaklines in dentin. The lines are perpendicular to the dentinal tubules. The following main types of lines are distinguished: the Schreger and Owen lines associated with the bends of the dentinal tubules, the Ebner lines and the mineralization lines associated with uneven mineralization, violations of mineralization and its rhythm. In addition, there is a neonatal line.

Owen's lines are visible in polarized light and are formed when the secondary bends of the dentinal tubules are superimposed on each other. Owen's contour lines are quite rare in primary dentin, they are more often located on the border between primary and secondary dentin.

These lines are located perpendicular to the tubules at a distance of about 5 µm from each other.

Lines of mineralization are formed due to the uneven rate of calcification during dentinogenesis. Since the mineralization front is not necessarily strictly parallel to the predentin, the course of the lines can be tortuous.

The neonatal lines, as in enamel, reflect the fact of a change in the mode of dentinogenesis at birth. These lines are expressed in milk teeth and in the first permanent molar.

The cement covers the root dentin with a thin layer, thickening towards the root apex. The cement located closer to the neck of the tooth does not contain cells and is called acellular. The top of the root is covered with cement containing cells - cementocytes (cellular cement). Acellular cement consists of collagen fibers and an amorphous substance. Cell cement resembles coarse fibrous bone tissue, but does not contain blood vessels.

The pulp is the soft part of the tooth, represented by loose connective tissue and consists of peripheral, intermediate and central layers. The peripheral layer contains odontoblasts - analogues of bone osteoblasts - high cylindrical cells with a process extending from the apical pole of the cell to the border between dentin and enamel. Odontoblasts secrete collagen, glycosaminoglycans (chondroitin sulfate) and lipids that are part of the organic matrix of dentin. With the mineralization of predentin (non-calcified matrix), the processes of odontoblasts become immured in the dentinal tubules. The intermediate layer contains odontoblast precursors and emerging collagen fibers. The central layer of the pulp is a loose fibrous connective tissue with many anastomosing capillaries and nerve fibers, the terminals of which branch out in the intermediate and peripheral layers. In the elderly, in the pulp, irregularly shaped calcified formations - denticles are often found. True denticles consist of dentin surrounded on the outside by odontoblasts. False denticles are concentric deposits of calcified material around necrotic cells.

Pharynx

This is the intersection of the respiratory and digestive tracts. According to the functional conditions in the pharynx, three sections are distinguished, which have a different structure - nasal, oral and laryngeal. All of them differ in the structure of the mucous membrane, which is represented by various types of epithelium.

The mucous membrane of the nasal part of the pharynx is covered with multi-row ciliated epithelium, contains mixed glands (respiratory type of mucous membrane).

The mucous membrane of the oral and laryngeal sections is lined with stratified squamous epithelium, located on the lamina propria of the mucous membrane, in which there is a well-defined layer of elastic fibers.

Esophagus

The esophagus is a hollow tube that consists of the mucosa, submucosa, muscularis and adventitia.

The mucous membrane, together with the submucosa, forms 7–10 longitudinally located folds in the esophagus, protruding into its lumen.

The mucous membrane of the esophagus consists of the epithelium, its own and muscular plates. The epithelium of the mucous membrane is multilayered, flat, non-keratinizing.

The lamina propria of the esophageal mucosa is a layer of loose, fibrous, unformed connective tissue that protrudes into the epithelium in the form of papillae.

The muscular plate of the mucous membrane of the esophagus consists of bundles of smooth muscle cells located along it, surrounded by a network of elastic fibers.

The submucosa of the esophagus, formed by loose fibrous unformed connective tissue, provides greater mobility of the mucosa in relation to the muscular membrane. Together with the mucosa, it forms numerous longitudinal folds, which straighten out during the swallowing of food. In the submucosa are the own glands of the esophagus.

The muscular membrane of the esophagus consists of an inner circular and outer longitudinal layers, separated by a layer of loose fibrous unformed connective tissue. At the same time, in the upper part of the esophagus muscles belong to striated tissue, on average - to striated tissue and smooth muscles, and in the lower part - only to smooth.

The adventitial membrane of the esophagus consists of loose fibrous unformed connective tissue, which, on the one hand, is associated with layers of connective tissue in the muscular membrane, and on the other hand, with the connective tissue of the mediastinum surrounding the esophagus.

The abdominal esophagus is covered with a serous membrane.

The blood supply of the esophagus is produced from the artery entering the esophagus, and plexuses are formed in the submucosa (large-loop and small-loop), from which blood enters the large-loop plexus of the lamina propria.

Innervation. The intramural nervous apparatus is formed by three interconnected plexuses: adventitious (most developed in the middle and lower thirds of the esophagus), subadventitial (lying on the surface of the muscular membrane and well expressed only in the upper parts of the esophagus), intermuscular (located between the circular and longitudinal muscle layers).

Stomach

The main function of the stomach is secretory. It consists in the production of gastric juice by the glands. It consists of the enzymes pepsin (promoting the breakdown of proteins), chymosin (contributing to the curdling of milk), lipase (promoting the breakdown of lipids), as well as hydrochloric acid and mucus.

The mechanical function of the stomach is to mix food with gastric juice and push the processed food into the duodenum.

Also, the wall of the stomach produces an anti-anemic factor, which promotes the absorption of vitamin B¹².

The endocrine function of the stomach consists in the production of a number of biologically active substances - gastrin, histamine, serotonin, motilin, enteroglucagon, etc. Together, these substances have a stimulating or inhibitory effect on the motility and secretory activity of the glandular cells of the stomach and other parts of the digestive tract.

Structure. The wall of the stomach consists of the mucous membrane, submucosa, muscular and serous membranes.

The mucous membrane of the stomach has an uneven surface due to the presence of three types of formations in it - folds, fields and pits.

The epithelium lining the surface of the gastric mucosa and pits is single-layer cylindrical. The peculiarity of this epithelium is its glandular character: all epithelial cells constantly secrete a mucoid (mucus-like) secret. Each glandular cell is clearly divided into two parts: basal and apical.

The lamina propria of the gastric mucosa is represented by loose, fibrous, unformed connective tissue. In it, in greater or lesser quantities, there are always accumulations of lymphoid elements in the form of either diffuse infiltrates or solitary (single) lymphatic follicles.

The muscular plate of the gastric mucosa is located on the border with the submucosa. It consists of three layers formed by smooth muscle tissue: inner and outer circular and middle longitudinal. Each of these layers is made up of bundles of smooth muscle cells.

The glands of the stomach in its various departments have an unequal structure. There are three types of gastric glands: own gastric, pyloric and cardiac.

Own glands of the stomach contain several types of glandular cells - the main, parietal (cooking), mucous, cervical and endocrine (argyrophilic).

The main cells of their own glands are located mainly in the region of their bottom and bodies. They distinguish between the basal and apical parts. The basal part of the cell is located at the base on the basal membrane, bordering on the lamina propria, and has a well-defined basophilia. Granules of protein secretion are found in the apical part of the cell. Chief cells secrete pepsinogen, a proenzyme that, in the presence of hydrochloric acid, is converted to its active form, pepsin. It is believed that chymosin, which breaks down milk proteins, is also produced by chief cells.

The parietal cells of the own glands are located outside the main and mucous cells, tightly adhering to their basal ends. In size they are larger than the main cells, their shape is irregularly rounded.

The main role of the parietal cells of the own glands of the stomach is the production of chlorides, from which hydrochloric acid is formed.

The mucous cells of the own glands of the stomach are represented by two types. Some are located in the body of their own glands and have a compacted nucleus in the basal part of the cells.

In the apical part of these cells, many round or oval granules, a small amount of mitochondria, and a lamellar complex were found. Other mucous cells (cervical) are located only in the neck of their own glands.

The pyloric glands of the stomach are located in a small area near its exit into the duodenum. The secret produced by the pyloric glands is alkaline. In the neck of the glands there are also intermediate (cervical) cells, which have already been described in the own glands of the stomach.

Cardiac glands of the stomach are simple tubular glands with highly branched terminal sections. Apparently, the secretory cells of these glands are identical to the cells lining the pyloric glands of the stomach and the cardiac glands of the esophagus.

Endocrine argyrophilic cells. Several types of endocrine cells have been identified in the stomach according to morphological, biochemical and functional characteristics.

EC cells - the largest group of cells, located in the area of ​​the bottom of the glands between the main cells. These cells secrete serotonin and melatonin.

G-cells (gastrin-producing) are located mainly in the pyloric glands, as well as in the cardiac glands, located in the area of ​​\uXNUMXb\uXNUMXbtheir body and bottom, sometimes the neck. The gastrin secreted by them stimulates the secretion of pepsinogen by the chief cells and hydrochloric acid by parietal cells, as well as gastric motility.

P-cells secrete bombesin, which stimulates the release of hydrochloric acid and enzyme-rich pancreatic juice, and also increases the contraction of the smooth muscles of the gallbladder.

ECX cells (enterochromaffin-like) are characterized by a variety of shapes and are located mainly in the body and bottom of the fundic glands. These cells produce histamine, which regulates the secretory activity of parietal cells that produce hydrochloric acid.

The submucosa of the stomach consists of loose fibrous irregular connective tissue containing a large number of elastic fibers. This layer contains arterial and venous plexuses, a network of lymphatic vessels and a submucosal nerve plexus.

The muscular coat of the stomach is characterized by weak development in the region of its bottom, good expression in the body and the achievement of the greatest development in the pylorus. In the muscular membrane of the stomach, there are three layers formed by smooth muscle tissue.

The serous membrane of the stomach forms the outer part of its wall. It is based on loose fibrous unformed connective tissue adjacent to the muscular membrane of the stomach. From the surface, this connective tissue layer is covered with a single-layer squamous epithelium - mesothelium.

The arteries that feed the wall of the stomach pass through the serous and muscular membranes, giving them the corresponding branches, and then pass into a powerful plexus in the submucosa. The main sources of nutrition include the right and left ventricular arteries. From the stomach, blood flows into the portal vein.

Innervation. The stomach has two sources of efferent innervation - parasympathetic (from the vagus nerve) and sympathetic (from the borderline sympathetic trunk).

In the wall of the stomach there are three nerve plexuses - intermuscular, submucosal and subserous.

Small intestine

In the small intestine, all kinds of nutrients - proteins, fats and carbohydrates - undergo chemical processing. Protein digestion involves the enzymes enterokinase, kinasogen and trypsin, which break down simple proteins, erepsin (a mixture of peptidases), which breaks down peptides into amino acids, and nuclease, which digests complex proteins (nucleoproteins). Digestion of carbohydrates occurs due to amylase, maltose, sucrose, lactose and phosphatase, and fats - the enzyme lipase.

In the small intestine, the process of absorption of the breakdown products of proteins, fats and carbohydrates into the blood and lymphatic vessels also takes place.

Also, the small intestine performs a mechanical function: it pushes the chyme in the caudal direction.

The endocrine function, performed by special secretory cells, consists in the production of biologically active substances - serotonin, histamine, motilin, secretin, enteroglucagon, cholecystokinin, pancreozymin, gastrin and gastrin inhibitor.

Structure. The wall of the small intestine consists of a mucous membrane, submucosa, muscular and serous membranes.

The relief due to the presence of a number of formations (folds, villi and crypts) is very specific for the mucous membrane of the small intestine.

These structures increase the overall surface of the small intestine mucosa, which contributes to the performance of its main functions.

From the surface, each intestinal villus is lined with a single-layer cylindrical epithelium. In the epithelium, three types of cells are distinguished - border, goblet and endocrine (argyrophilic).

Enterocytes with a striated border make up the bulk of the epithelial layer covering the villus. They are characterized by a pronounced polarity of the structure, which reflects their functional specialization - ensuring the resorption and transport of substances from food.

On the apical surface of the cells, a border formed by many microvilli is visible. Due to such a large number of villi, the absorption surface of the intestine increases by 30-40 times.

It was revealed that the breakdown of nutrients and their absorption most intensively occur in the region of the striated border. This process is called parietal digestion, in contrast to the cavity, which takes place in the lumen of the intestinal tube, and intracellular.

Goblet intestinal. By structure, these are typical mucous cells. They show cyclical changes associated with the accumulation and subsequent secretion of mucus.

Beneath the epithelium of the villi is a weakly expressed basement membrane, followed by a loose, fibrous, unformed connective tissue of the lamina propria.

In the stroma of the villi, there are always separate smooth muscle cells: derivatives of the muscular layer of the mucous membrane. Bundles of smooth muscle cells are wrapped in a network of reticular fibers that connect them to the stroma of the villus and the basement membrane.

The contraction of myocytes promotes the absorption of food hydrolysis products into the blood and lymph of the intestinal villi.

Intestinal crypts of the small intestine are tubular depressions of the epithelium, lying in its own plate of its mucous membrane, and the mouth opens into the lumen between the villi.

The epithelial lining of intestinal crypts contains the following types of cells: bordered, borderless intestinal cells, goblet, endocrine (argyrophilic) and intestinal cells with acidophilic granularity (Paneth cells). Intestinal enterocytes with a striated border make up the bulk of the epithelial lining of the crypts.

The lamina propria of the small intestine mucosa mainly consists of a large number of reticular fibers. They form a dense network throughout the lamina propria and, approaching the epithelium, participate in the formation of the basement membrane. Process cells with a pale oval nucleus are closely associated with reticular fibers. In appearance, they resemble the reticular cells of the hematopoietic organs.

The mucosa contains many single lymphatic follicles and aggregates of follicles. Single (solitary) lymphatic follicles are found throughout the small intestine. Large follicles lying in the distal small intestine penetrate into the muscularis mucosa and are partially located in the submucosa. Larger accumulations of lymphoid tissue - aggregates (or group lymphatic follicles (Peyer's patches)), as a rule, are located in the ileum, but sometimes occur in the jejunum and duodenum.

The submucosa contains blood vessels and nerve plexuses.

The muscular coat is represented by two layers of smooth muscle tissue - internal (circular) and external (longitudinal).

The serous membrane covers the intestine from all sides, with the exception of the duodenum, which is covered by the peritoneum only in front.

The blood supply to the small intestine is carried out at the expense of the arteries entering the wall of the small intestine with the formation of a plexus in it in all layers of the intestinal membrane.

The lymphatic vessels of the small intestine are represented by a very widely branched network. In each intestinal villus there is a centrally located, blindly ending at its top, a lymphatic capillary.

Innervation. The small intestine is innervated by sympathetic and parasympathetic nerves.

Afferent innervation is carried out by a sensitive musculo-intestinal plexus formed by sensitive nerve fibers of the spinal ganglia and their receptor endings.

Efferent parasympathetic innervation is carried out due to the musculo-intestinal and submucosal nerve plexuses. The muscular-intestinal plexus is most developed in the duodenum, where numerous, densely located large ganglia are observed.

Colon

In the large intestine, water is absorbed from the chyme and feces are formed. A significant amount of mucus is secreted in the large intestine, which facilitates the movement of contents through the intestines and helps to glue undigested food particles. Excretion processes also take place in the large intestine. A number of substances are released through the mucous membrane of this intestine, for example, calcium, magnesium, phosphates, salts of heavy metals, etc. There is also evidence that vitamin K is produced in the large intestine, and the bacterial flora that is constantly present in the intestine takes part in this. Bacteria in the large intestine help digest fiber.

The large intestine is divided into the colon and the rectum.

Colon. The wall of the colon, as well as the entire gastrointestinal tract, consists of a mucous membrane, submucosa, muscular and serous membranes.

The mucous membrane has a large number of folds and crypts, which significantly increase its surface, but there are no villi.

Folds are formed on the inner surface of the intestine from the mucous membrane and submucosa. They are located across and have a crescent shape (hence the name - crescent folds). Crypts in the colon are better developed than in the small intestine. At the same time, the epithelium is single-layer prismatic, it consists of cells of the intestinal epithelium with a striated border, goblet and intestinal cells without a border.

The lamina propria consists of loose, fibrous, unformed connective tissue. Its thin layers are visible between the intestinal crypts.

The muscular plate of the mucous membrane is more pronounced than in the small intestine, and consists of two strips. Its inner strip is denser, formed mainly by circularly located bundles of smooth muscle cells. The outer strip is represented by bundles of smooth muscle cells, oriented partly longitudinally, partly obliquely with respect to the axis of the intestine.

The submucosa consists of loose fibrous irregular connective tissue, in which there are many fat cells. Here are the vascular and nerve submucosal plexuses. There are always a lot of lymphatic follicles in the submucosa of the colon, they spread here from the lamina propria.

The muscular coat is represented by two layers of smooth muscle tissue: internal (or circular) and external (or longitudinal), which forms three ribbons stretching along the entire length of the intestine.

In the parts of the intestine lying between the ribbons, only a thin layer is found, consisting of a small amount of longitudinally arranged bundles of smooth muscle cells. These areas form swellings - gaustra.

The serous membrane covers the colon, however, there are sections covered with a serous membrane on all sides, and there are sections covered only on three sides - mesoperitoneally (ascending and descending sections of the colon).

The appendix is ​​a rudimentary formation of the large intestine, it contains large accumulations of lymphoid tissue. The mucous membrane of the appendix has crypts that are located radially with respect to its lumen.

The epithelium of the mucous membrane is cylindrical, bordered, with a small number of goblet cells.

The lamina propria of the mucosa consists of loose, fibrous, unformed connective tissue, which, without a sharp border (due to the weak development of the muscular mucosal lamina), passes into the submucosa.

In the submucosal base of the appendix, formed by loose fibrous unformed connective tissue, blood vessels and the nerve submucosal plexus lie.

The muscular coat is also formed by two layers.

The appendix performs a protective function. It has been established that differentiation of B-lymphocytes occurs in the follicles.

Rectum. The rectum is a continuation of the colon.

In the anal part of the intestine, three zones are distinguished - columnar, intermediate and skin. In the columnar zone, the longitudinal folds form the anal columns.

The mucous membrane of the rectum consists of the epithelium, its own and muscular plates. The epithelium in the upper section of the rectum is single-layered, cylindrical, in the columnar zone of the lower section - multi-layered, cubic, in the intermediate - multi-layered, flat, non-keratinizing, in the skin - multi-layered, flat, keratinizing. The transition from stratified, cuboidal epithelium to stratified, squamous epithelium stands out as a zigzag line.

The lamina propria is made up of loose, fibrous, unformed connective tissue. She takes part in the formation of the folds of the rectum. Here are single lymphatic follicles and vessels. In the region of the columnar zone in this plate lies a network of thin-walled blood lacunae, the blood from which flows into the hemorrhoidal veins.

In the intermediate zone of the rectum, the lamina propria contains a large number of elastic fibers, elements of lymphoid tissue.

In the skin area surrounding the anus, hair joins the sebaceous glands. Sweat glands in the lamina propria of the mucous membrane appear at a distance of 1 - 1,5 cm from the anus, they are tubular glands.

The muscular plate of the mucous membrane, as in other parts of the large intestine, consists of two strips.

The submucosa is represented by loose fibrous unformed connective tissue. It contains the vascular and nerve plexuses. In the submucosa lies the plexus of hemorrhoidal veins. In case of violation of the tone of the walls of these vessels, varicose expansions appear.

The muscular coat is formed by smooth muscle tissue and consists of two layers - inner (circular) and outer (longitudinal). The circular layer at different levels of the rectum forms two thickenings, which stand out as separate anatomical formations - sphincters.

The serous membrane covers the rectum in its upper part, in the lower sections the rectum has a connective tissue membrane.

Liver

The liver is one of the major glands of the digestive tract, performing numerous functions.

The following processes take place in it:

1) neutralization of various metabolic products;

2) destruction of various biologically active substances;

3) destruction of sex hormones;

4) various protective reactions of the organism;

5) it takes part in the formation of glycogen (the main source of glucose);

6) the formation of various proteins;

7) hematopoiesis;

8) it accumulates vitamins;

9) formation of bile.

Structure. The liver is an unpaired organ located in the abdominal cavity, covered with peritoneum on all sides. It has several lobes, 8 segments.

The main structural and functional unit of the liver is the hepatic lobule. It is a hexagonal prism of liver cells (hepatocytes collected in the form of beams). Each lobule is covered with a connective tissue membrane, in which the bile ducts and blood vessels pass. From the periphery of the lobule (through the system of capillaries of the portal vein and the hepatic artery) to its center, the blood passes through the blood vessels, being cleansed, and through the central vein of the hepatic lobule enters the collecting veins, then into the hepatic veins and into the inferior vena cava.

Bile capillaries pass between the rows of hepatocytes that form the beam of the hepatic lobule. These capillaries do not have their own wall. Their wall is formed by contiguous surfaces of hepatocytes, on which there are small depressions that coincide with each other and together form the lumen of the bile capillary.

Summarizing the above, we can conclude that the hepatocyte has two surfaces: one is capillary (facing the blood vessel), the other is biliary (facing the lumen of the bile capillary).

At the same time, you need to know that the lumen of the bile capillary does not communicate with the intercellular gap due to the fact that the membranes of neighboring hepatocytes in this place fit tightly to each other, forming end plates, which, in turn, prevents the penetration of bile into the blood vessels. In these cases, bile spreads throughout the body and stains its tissues yellow.

Basic cell types

Hepatocytes form hepatic plates (strands), contain in abundance almost all organelles. The nucleus has 1 - 2 nucleoli and is most often located in the center of the cell. 25% of hepatocytes have two nuclei. The cells are characterized by polyploidy: 55-80% of hepatocytes are tetraploid, 5-6% are octaploid and only 10% are diploid. The granular and smooth endoplasmic reticulum is well developed. Elements of the Golgi complex are present in various parts of the cell. The number of mitochondria in a cell can reach 2000. Cells contain lysosomes and peroxisomes. The latter have the form of a bubble surrounded by a membrane with a diameter of up to 0,5 μm. Peroxisomes contain oxidative enzymes - amino oxidase, urate oxidase, catalase. As in mitochondria, oxygen is utilized in peroxisomes. Direct relation to the formation of these organelles has a smooth endoplasmic reticulum. Numerous inclusions, mainly of glycogen, are present in the cytoplasm. Each hepatocyte has two poles - sinusoidal and bile (or biliary).

The sinusoidal pole faces the space of Disse. It is covered with microvilli, which are involved in the transport of substances from the blood to hepatocytes and vice versa. Microvilli of hepatocytes are in contact with the surface of endothelial cells. The biliary pole also has microvilli, which facilitates the excretion of bile components. Bile capillaries are formed at the point of contact of the biliary poles of two hepatocytes.

Cholangiocytes (or epithelial cells of the intrahepatic bile ducts) make up 2-3% of the total liver cell population. The total length of the intrahepatic bile ducts is approximately 2,2 km, which plays an important role in the formation of bile. Cholangiocytes are involved in the transport of proteins and actively secrete water and electrolytes.

stem cells. Hepatocytes and cholangiocytes are among the growing cell populations of the endodermal epithelium. The stem cells for both are oval cells located in the bile ducts.

Sinusoid cells of the liver. Four cell types are known and intensively studied that are constantly present in the sinusoids of the liver: endothelial cells, Kupffer stellate cells, Ito cells and pit cells. According to the data of morphometric analysis, sinusoid cells occupy about 7% of the liver volume.

Endothelial cells contact with the help of numerous processes, separating the lumen of the sinusoid from the space of Disse. The nucleus is located along the cell membrane from the space of Disse. The cells contain elements of a granular and smooth endoplasmic reticulum. The Golgi complex is located between the nucleus and the lumen of the sinusoid. The cytoplasm of endothelial cells contains numerous pinocytic vesicles and lysosomes. Fenestra, not tightened by diaphragms, occupy up to 10% of the endothelium and regulate the entry of particles larger than 0,2 in diameter into the space of Disse, for example, chylomicrons. Endothelial cells of sinusoids are characterized by endocytosis of all types of molecules and particles with a diameter of not more than 0,1 μm. The absence of a typical basement membrane, the capacity for endocytosis, and the presence of fenestrations distinguish the endothelium of the sinusoids from the endothelium of other vessels.

Kupffer cells belong to the system of mononuclear phagocytes and are located between endothelial cells as part of the wall of the sinusoid. The main site of localization of Kupffer cells are the periportal areas of the liver. Their cytoplasm contains lysosomes with high peroxidase activity, phagosomes, iron inclusions, and pigments. Kupffer cells remove foreign material from the blood, fibrin, an excess of activated blood coagulation factors, participate in the phagocytosis of aging and damaged red blood cells, hemoglobin and iron metabolism. Iron from destroyed erythrocytes or from the blood accumulates in the form of hemosiderin for subsequent use in the synthesis of Hb. Metabolites of arachidonic acid, platelet activating factor cause the activation of Kupffer cells. Activated cells, in turn, begin to produce a complex of biologically active substances, such as oxygen radicals, plasminogen activator, tumor necrosis factor TNF, IL-1, IL-6, transforming growth factor, which can cause toxic damage to hepatocytes.

Pit cells (Pit-cells) - lymphocytes located on endothelial cells or between them. It is suggested that pit cells may be NK cells and act against tumor and virus-infected cells. Unlike Kupffer cells, which require activation, the cytolytic effect of pit cells appears spontaneously, without prior activation from other cells or biologically active substances.

Fat-accumulating cells (lipocytes, Ito cells) have a process shape, are localized in the space of Disse or between hepatocytes. Ito cells play an important role in the metabolism and accumulation of retinoids. About 50 - 80% of vitamin A in the body accumulates in the liver, and up to 90% of all liver retinoids are deposited in fat drops of Ito cells. Retinol esters enter hepatocytes as part of chylomicrons. In hepatocytes, retinol esters are converted to retinol and a complex of vitamin A with retin-binding protein is formed. The complex is secreted into the space of Disse, from where it is deposited by Ito cells. In vitro, Ito cells have been shown to be able to synthesize collagen, which suggests their involvement in the development of cirrhosis and fibrosis of the liver.

The main functions of the liver

Secretion of bile. Hepatocytes produce and secrete bile through the biliary pole into the bile capillaries. Bile is an aqueous solution of electrolytes, bile pigments, bile acids. Bile pigments are the end products of the metabolism of Hb and other porphyrins. Hepatocytes take up free bilirubin from the blood, conjugate it with glucuronic acid, and secrete non-toxic, conjugated bilirubin into the bile capillaries. Bile acids are the end product of cholesterol metabolism and are essential for the digestion and absorption of lipids. Physiologically active substances, such as conjugated forms of glucocorticoids, are also excreted from the body with bile. As part of the bile, class A immunoglobulins from the spaces of Disse enter the intestinal lumen.

Synthesis of proteins. Hepatocytes secrete albumins (fibrinogen, prothrombin, factor III, angiotensinogen, somatomedins, thrombopoietin, etc.) into the space of Disse. Most plasma proteins are produced by hepatocytes.

Metabolism of carbohydrates. Excess glucose in the blood that occurs after a meal is absorbed by hepatocytes with the help of insulin and stored in the form of glycogen. With glucose deficiency, glucocorticoids stimulate gluconeogenesis in hepatocytes (the conversion of amino acids and lipids into glucose).

lipid metabolism. Chylomicrons from the spaces of Disse enter hepatocytes, where they are stored as triglycerides (lipogenesis) or secreted into the blood as lipoproteins.

Storage. Triglycerides, carbohydrates, iron, copper are stored in hepatocytes. Ito cells accumulate lipids and up to 90% of retinoids deposited in the liver.

Detoxification. Inactivation of Hb metabolic products, proteins, xenobiotics (eg, drugs, drugs, industrial chemicals, toxic substances, metabolic products of bacteria in the intestine) occurs with the help of enzymes during oxidation, methylation and binding reactions. A non-toxic form of bilirubin is formed in hepatocytes, urea is synthesized from ammonia (the end product of protein metabolism), which is to be excreted through the kidneys, and sex hormones are degraded.

Body protection. Kupffer cells remove microorganisms and their waste products from the blood. Pit cells are active against tumor and virus-infected cells. Hepatocytes transport IgA from the space of Disse to the bile and then to the intestinal lumen.

Hematopoietic. The liver is involved in prenatal hematopoiesis. In the postnatal period, thrombopoietin is synthesized in hepatocytes.

The bile ducts are a system of bile vessels that transport bile from the liver to the lumen of the duodenum. Allocate intrahepatic and extrahepatic bile ducts. The intrahepatic ones include the interlobular bile ducts, and the extrahepatic ones include the right and left hepatic ducts, the common hepatic, cystic and common bile ducts (choledochus).

The gallbladder is a hollow organ with a thin wall (about 1,5 - 2 mm). It holds 40 - 60 ml of bile. The wall of the gallbladder consists of three membranes: mucous, muscular and adventitial. The latter from the side of the abdominal cavity is covered with a serous membrane.

The mucous membrane of the gallbladder forms folds that anastomose with each other, as well as crypts or sinuses in the form of pockets.

In the region of the neck of the bladder, there are alveolar-tubular glands in it that secrete mucus. The epithelium of the mucous membrane has the ability to absorb water and some other substances from the bile that fills the bladder cavity. In this regard, cystic bile is always thicker in consistency and darker in color than bile that comes directly from the liver.

The muscular coat of the gallbladder consists of smooth muscle cells (arranged in a network in which their circular direction predominates), which are especially well developed in the region of the neck of the gallbladder. Here are the sphincters of the gallbladder, contributing to the retention of bile in the lumen of the gallbladder.

The adventitia of the gallbladder is composed of dense fibrous connective tissue.

Innervation. In the capsule of the liver there is a vegetative nerve plexus, the branches of which, accompanying the blood vessels, continue into the interlobular connective tissue.

Pancreas

The pancreas is an organ of the digestive system, which includes exocrine and endocrine parts. The exocrine part is responsible for the production of pancreatic juice containing digestive enzymes (trypsin, lipase, amylase, etc.), which enters the duodenum through the excretory ducts, where its enzymes are involved in the breakdown of proteins, fats and carbohydrates to final products. In the endocrine part, a number of hormones are synthesized (insulin, glucagon, somatostatin, pancreatic polypeptide), which are involved in the regulation of carbohydrate, protein and fat metabolism in tissues.

Structure. The pancreas is an unpaired organ of the abdominal cavity, on the surface covered with a connective tissue capsule, fused with the visceral sheet of the peritoneum. Its parenchyma is divided into lobules, between which connective tissue strands pass. They contain blood vessels, nerves, intramural nerve ganglia, lamellar bodies (Vater-Pacini bodies) and excretory ducts.

The acinus is a structural and functional unit. It consists of cells of the pancreas, includes a secretory section and an insertion section, from which the ductal system of the gland begins.

Acinar cells perform a secretory function, synthesizing the digestive enzymes of pancreatic juice. They have the shape of a cone with a narrowed apex and a wide base lying on the basement membrane of the acinus.

The secretion of hormones occurs cyclically. The secretion phases are the same as those of other glands. However, secretion according to the merocrine type occurs depending on the physiological needs of the body for digestive enzymes, this cycle can be reduced or, conversely, increased.

The released secret passes through the ducts (intercalary, interacinar, intralobular), which, uniting, flow into the Wirsung duct.

The walls of these ducts are lined with a single layer of cuboidal epithelium. Their cytolemma forms internal folds and microvilli.

The endocrine part of the pancreas is in the form of islets (round or oval) lying between the acini, while their volume does not exceed 3% of the volume of the entire gland.

Islets consist of endocrine insular cells - insulocytes. Between them are fenestrated blood capillaries. The capillaries are surrounded by a pericapillary space. Hormones secreted by the insular cells first enter this space and then through the capillary wall into the blood.

There are five main types of insular cells: B cells (basophilic), A cells (acidophilic), D cells (dendritic), D1 cells (argyrophilic), and PP cells.

B cells make up the bulk of islet cells (about 70-75%). Granules of B-cells consist of the hormone insulin, A-cells make up approximately 20 - 25% of the total mass of insular cells. In the islets, they occupy a predominantly peripheral position.

The hormone glucagon was found in the A-cell granules. It acts as an insulin antagonist.

The number of D-cells in the islets is small - 5 - 10%.

D cells secrete the hormone somatostatin. This hormone delays the release of insulin and glucagon by A- and B-cells, and also inhibits the synthesis of enzymes by pancreatic acinar cells.

PP cells (2 - 5%) produce a pancreatic polypeptide that stimulates the secretion of gastric and pancreatic juice.

These are polygonal cells with very small grains in the cytoplasm (the size of the granules is not more than 140 nm). PP cells are usually localized along the periphery of the islets in the head of the gland, and also occur outside the islets among the exocrine compartments and ducts.

The blood supply to the pancreas comes from the branches of the celiac trunk. Venous blood flows from the pancreas into the portal vein.

Innervation. The efferent innervation of the pancreas is carried out by the vagus and sympathetic nerves.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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