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Lecture notes, cheat sheets
Histology. Endocrine system
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 20. ENDOCRINE SYSTEM The endocrine system together with the nervous system have a regulatory effect on all other organs and systems of the body, forcing it to function as a single system. The endocrine system includes glands that do not have excretory ducts, but release highly active biological substances into the internal environment of the body, acting on cells, tissues and organs of substances (hormones), stimulating or weakening their functions. Cells in which the production of hormones becomes the main or predominant function are called endocrine. In the human body, the endocrine system is represented by the secretory nuclei of the hypothalamus, pituitary, epiphysis, thyroid, parathyroid glands, adrenal glands, endocrine parts of the sex and pancreas, as well as individual glandular cells scattered in other (non-endocrine) organs or tissues. With the help of hormones secreted by the endocrine system, the body functions are regulated and coordinated and brought into line with its needs, as well as with irritations received from the external and internal environment. By chemical nature, most hormones belong to proteins - proteins or glycoproteins. Other hormones are derivatives of amino acids (tyrosine) or steroids. Many hormones, entering the bloodstream, bind to serum proteins and are transported throughout the body in the form of such complexes. The connection of the hormone with the carrier protein, although it protects the hormone from premature degradation, but weakens its activity. The release of the hormone from the carrier occurs in the cells of the organ that perceives this hormone. Since hormones are released into the blood stream, an abundant blood supply to the endocrine glands is a prerequisite for their functioning. Each hormone acts only on those target cells that have specific chemical receptors in their plasma membranes. The target organs, usually classified as non-endocrine, include the kidney, in the juxtaglomerular complex of which renin is produced; salivary and prostate glands, in which special cells are found that produce a factor that stimulates the growth of nerves; as well as special cells (enterinocytes) localized in the mucous membrane of the gastrointestinal tract and producing a number of enteric (intestinal) hormones. Many hormones (including endorphins and enkephalins), which have a wide spectrum of action, are produced in the brain. Relationship between the nervous and endocrine systems The nervous system, sending its efferent impulses along the nerve fibers directly to the innervated organ, causes directed local reactions that come on quickly and stop just as quickly. Distant hormonal influences play a predominant role in the regulation of such general body functions as metabolism, somatic growth, and reproductive functions. The joint participation of the nervous and endocrine systems in ensuring the regulation and coordination of body functions is determined by the fact that the regulatory influences exerted by both the nervous and endocrine systems are implemented by fundamentally the same mechanisms. At the same time, all nerve cells exhibit the ability to synthesize protein substances, as evidenced by the strong development of the granular endoplasmic reticulum and the abundance of ribonucleoproteins in their perikarya. The axons of such neurons, as a rule, end in capillaries, and the synthesized products accumulated in the terminals are released into the blood, with the current of which they are carried throughout the body and, unlike mediators, have not a local, but a distant regulatory effect, similar to the hormones of the endocrine glands. Such nerve cells are called neurosecretory, and the products produced and secreted by them are called neurohormones. Neurosecretory cells, perceiving, like any neurocyte, afferent signals from other parts of the nervous system, send their efferent impulses through the blood, that is, humorally (like endocrine cells). Therefore, neurosecretory cells, physiologically occupying an intermediate position between nervous and endocrine cells, unite the nervous and endocrine systems into a single neuroendocrine system and thus act as neuroendocrine transmitters (switches). In recent years, it has been established that the nervous system contains peptidergic neurons, which, in addition to mediators, secrete a number of hormones that can modulate the secretory activity of the endocrine glands. Therefore, as noted above, the nervous and endocrine systems act as a single regulatory neuroendocrine system. Classification of the endocrine glands At the beginning of the development of endocrinology as a science, endocrine glands were tried to be grouped according to their origin from one or another embryonic rudiment of the germ layers. However, further expansion of knowledge about the role of endocrine functions in the body showed that the commonality or proximity of embryonic anlages does not at all prejudge the joint participation of the glands developing from such rudiments in the regulation of body functions. According to modern concepts, the following groups of endocrine glands are distinguished in the endocrine system: neuroendocrine transmitters (secretory nuclei of the hypothalamus, pineal gland), which, with the help of their hormones, switch information entering the central nervous system to the central link in the regulation of adenohypophysis-dependent glands (adenohypophysis) and the neurohemal organ (posterior pituitary, or neurohypophysis). The adenohypophysis, thanks to the hormones of the hypothalamus (liberins and statins), secretes an adequate amount of tropic hormones that stimulate the function of the adenohypophysis-dependent glands (adrenal cortex, thyroid and gonads). The relationship between the adenohypophysis and the endocrine glands dependent on it is carried out according to the feedback principle (or plus or minus). The neurohemal organ does not produce its own hormones, but accumulates the hormones of the large cell nuclei of the hypothalamus (oxytocin, ADH-vasopressin), then releases them into the bloodstream and thus regulates the activity of the so-called target organs (uterus, kidneys). In functional terms, the neurosecretory nuclei, the pineal gland, the adenohypophysis, and the neurohemal organ constitute the central link of the endocrine system, while the endocrine cells of non-endocrine organs (digestive system, airways and lungs, kidneys and urinary tract, thymus), adenohypophysis-dependent glands (thyroid gland, adrenal cortex , gonads) and adenohypophysis-independent glands (parathyroid glands, adrenal medulla) are peripheral endocrine glands (or target glands). Summarizing all of the above, we can say that the endocrine system is represented by the following main structural components. 1. Central regulatory formations of the endocrine system: 1) hypothalamus (neurosecretory nuclei); 2) pituitary gland; 3) epiphysis. 2. Peripheral endocrine glands: 1) thyroid gland; 2) parathyroid glands; 3) adrenal glands: a) cortical substance; b) the adrenal medulla. 3. Organs that combine endocrine and non-endocrine functions: 1) gonads: a) testis; b) ovary; 2) placenta; 3) pancreas. 4. Single hormone-producing cells: 1) neuroendocrine cells of the POPA group (APUD) (nervous origin); 2) single hormone-producing cells (not of nervous origin). Hypothalamus The hypothalamus occupies the basal region of the diencephalon and borders the lower part of the third ventricle of the brain. The cavity of the third ventricle continues into the funnel, the wall of which becomes the pituitary stalk and at its distal end gives rise to the posterior lobe of the pituitary gland (or neurohypophysis). In the gray matter of the hypothalamus, its nuclei (over 30 pairs) are isolated, which are grouped in the anterior, middle (mediobasal or tuberal) and posterior sections of the hypothalamus. Some of the hypothalamic nuclei are clusters of neurosecretory cells, while others are formed by a combination of neurosecretory cells and neurons of the usual type (mainly adrenergic). In the nuclei of the middle hypothalamus, hypothalamic adenohypophysotropic hormones are produced, which regulate the secretion (and probably also production) of hormones in the anterior and middle lobes of the pituitary gland. Adenohypophysotropic hormones are low molecular weight proteins (oligopeptides) that either stimulate (liberins) or inhibit (statins) the corresponding hormone-forming functions of the adenohypophysis. The most important nuclei of this part of the hypothalamus are localized in the gray tubercle: the arcuate, or infundibular, nucleus and the ventromedial nucleus. The ventromedial nucleus is large and turns out to be the main site for the production of adenohypophysotropic hormones, but along with it, this function is also inherent in the arcuate nucleus. These nuclei are formed by small neurosecretory cells in combination with adrenergic neurons of the usual type. The axons of both small neurosecretory cells of the mediobasal hypothalamus and adjacent adrenergic neurons are directed to the medial emission, where they end at the loops of the primary capillary network. Thus, the neurosecretory formations of the hypothalamus are divided into two groups: cholinergic (large cell nuclei of the anterior hypothalamus) and adrenergic (small neurosecretory cells of the mediobasal hypothalamus). The division of neurosecretory formations of the hypothalamus into peptidocholinergic and peptidoadrenergic reflects their belonging, respectively, to the parasympathetic or sympathetic part of the hypothalamus. The connection of the anterior hypothalamus with the posterior pituitary gland, and the mediobasal hypothalamus with the adenohypophysis allows us to divide the hypothalamic-pituitary complex into the hypothalamic-neurohypophyseal and hypothalamic-adenohypophyseal systems. The significance of the posterior lobe of the pituitary lies in the fact that it accumulates and releases into the blood the neurohormones produced by the large-celled peptidocholinergic nuclei of the anterior hypothalamus. Consequently, the posterior lobe of the pituitary gland is not a gland, but is an auxiliary neurohemal organ of the hypothalamic-neurohypophyseal system. A similar neurohemal organ of the hypothalamic-adenohypophyseal system is medial emission, in which adenohypophysotropic hormones (liberins and statins) are accumulated and enter the blood, produced by peptidoadrenergic neurosecretory cells of the mediobasal hypothalamus. Pituitary There are several lobes in the pituitary gland: adenohypophysis, neurohypophysis. In the adenohypophysis, the anterior, middle (or intermediate) and tuberal parts are distinguished. The anterior part has a trabecular structure. Trabeculae, strongly branching, are woven into a narrow-loop network. The gaps between them are filled with loose connective tissue, through which numerous sinusoidal capillaries pass. In each trabecula, several types of glandular cells (adenocytes) can be distinguished. Some of them, located along the periphery of the trabeculae, are larger in size, contain secretory granules and are intensely stained on histological preparations, therefore these cells are called chromophilic. Other cells are chromophobic, occupying the middle of the trabeculae, differ from the chromophilic cells by a weakly staining cytoplasm. Due to the quantitative predominance of chromophobic cells in the composition of trabeculae, they are sometimes called the main ones. Chromophilic cells are divided into basophilic and acidophilic. Basophilic cells, or basophils, produce glycoprotein hormones, and their secretory granules on histological preparations are stained with basic colors. Among them, two main varieties are distinguished - gonadotropic and thyrotropic. Some of the gonadotropic cells produce follicle-stimulating hormone (follitropin), while others are attributed to the production of luteinizing hormone (lutropin). If the body is deficient in sex hormones, the production of gonadotropins, especially follitropin, is so enhanced that some gonadotropic cells hypertrophy and are strongly stretched by a large vacuole, as a result of which the cytoplasm takes the form of a thin rim, and the nucleus is pushed to the edge of the cell ("castration cells"). The second variety - a thyrotropic cell that produces thyrotropic hormone (thyrotropin) - is distinguished by an irregular or angular shape. In case of insufficiency of the thyroid hormone in the body, the production of thyrotropin increases, and thyrotropocytes are partially transformed into thyroidectomy cells, which are characterized by larger sizes and a significant expansion of the cisterns of the endoplasmic reticulum, as a result of which the cytoplasm takes the form of coarse foam. In these vacuoles, aldehyde fuchsinophilic granules are found, larger than the secretory granules of the original thyrotropocytes. For acidophilic cells, or acidophiles, large dense granules are characteristic, stained on preparations with acidic dyes. Acidophilic cells are also divided into two varieties: somatotropic, or somatotropocytes that produce somatotropic hormone (somatotropin), and mammotropic, or mammotropocytes that produce lactotropic hormone (prolactin). The function of these cells is similar to basophilic ones. A corticotropic cell in the anterior pituitary gland produces adrenocorticotropic hormone (ACTH or corticotropin), which activates the adrenal cortex. The middle part of the adenohypophysis is a narrow strip of stratified epithelium, homogeneous in structure. Adenocytes of the middle lobe are able to produce a protein secret, which, accumulating between neighboring cells, leads to the formation of follicle-like cavities (cysts) in the middle lobe. In the middle part of the adenohypophysis, melanocyte-stimulating hormone (melanotropin) is produced, which affects pigment metabolism and pigment cells, as well as lipotropin, a hormone that enhances the metabolism of fat-lipoid substances. The tuberal part is a section of the adenohypophyseal parenchyma adjacent to the pituitary stalk and in contact with the lower surface of the medial hypothalamic emission. The functional properties of the tuberal part are not sufficiently elucidated. The posterior lobe of the pituitary gland - neurohypophysis - is formed by neuroglia. Glial cells of this lobe are represented mainly by small process or spindle-shaped cells - pituicites. The axons of the neurosecretory cells of the supraoptic and paraventricular nuclei of the anterior hypothalamus enter the posterior lobe. In the posterior lobe, these axons terminate in expanded terminals (storage bodies, or Herring bodies) that are in contact with the capillaries. The posterior pituitary gland accumulates antidiuretic hormone (vasopressin) and oxytocin produced by neurosecretory cells of the supraoptic and paraventricular nuclei of the anterior hypothalamus. It is possible that pituicytes are involved in the transfer of these hormones from storage bodies into the blood. Innervation. The pituitary gland, as well as the hypothalamus and pineal gland, receive nerve fibers from the cervical ganglia (mainly from the upper ones) of the sympathetic trunk. Extirpation of the upper cervical sympathetic ganglia or transection of the cervical sympathetic trunk leads to an increase in the thyrotropic function of the pituitary gland, while irritation of the same ganglia causes its weakening. Blood supply. The superior pituitary arteries enter the medial emission, where they break up into the primary capillary network. Its capillaries form loops and glomeruli that penetrate into the medial emission ependyma. The axons of peptidoadrenergic cells of the mediobasal hypothalamus approach these loops, forming axovasal synapses (contacts) on the capillaries, in which the hypothalamic liberins and statins are transferred into the blood stream. Then the capillaries of the primary network are collected in the portal veins, which run along the pituitary stalk to the parenchyma of the adenohypophysis, where they again break up into a secondary capillary network, the sinusoidal capillaries of which, branching, braid the trabeculae. Finally, the sinusoids of the secondary network merge into the efferent veins, which divert blood enriched with adenohypophyseal hormones into the general circulation. Thyroid gland The thyroid gland has two lobes (right and left, respectively) and an isthmus. Outside, it is surrounded by a dense connective tissue capsule, from which partitions extend into the gland. Composing the stroma of the gland, they branch and divide the thyroid parenchyma into lobules. The functional and structural unit of the thyroid gland are follicles - closed spherical or rounded formations of varying sizes with a cavity inside. Sometimes the walls of the follicles form folds, and the follicles become irregular in shape. In the lumen of the follicles, a secretory product accumulates - a colloid, which during life has the consistency of a viscous liquid and consists mainly of thyroglobulin. In addition, in the connective tissue layers there are always lymphocytes and plasma cells, the number of which in a number of diseases (thyrotoxicosis, autoimmune thyroiditis) increases dramatically up to the appearance of lymphoid accumulations and even lymphoid follicles with reproduction centers. In the same interfollicular layers, parafollicular cells are found, as well as mast cells (tissue basophils). Thyrocytes - glandular cells of the thyroid gland, which make up the wall (lining) of the follicles and are located in one layer on the basement membrane, limit the follicle from the outside. The shape, volume and height of thyrocytes change in accordance with shifts in the functional activity of the thyroid gland. When the body's needs for thyroid hormone increase and the functional activity of the thyroid gland increases (hyperfunctional state), the thyrocytes of the follicular lining increase in volume and height and take on a prismatic shape. The intrafollicular colloid becomes more liquid, numerous vacuoles appear in it, and on histological preparations it takes the form of foam. The apical surface of the thyrocyte forms microvilli protruding into the lumen of the follicle. As the functional activity of the thyroid gland increases, the number and size of microvilli increase. At the same time, the basal surface of thyrocytes, which is almost flat during the period of functional rest of the thyroid gland, becomes folded when it is activated, which leads to an increase in the contact of thyrocytes with the pericapillary spaces. The secretory cycle of any glandular cell consists of the following phases: the absorption of the starting materials, the synthesis of the hormone and its release. production phase. The production of thyroglobulin (and, consequently, thyroid hormone) begins in the cytoplasm of the basal part of the thyrocyte and ends in the cavity of the follicle on its apical surface (on the border with the intrafollicular colloid). The initial products (amino acids, salts), brought to the thyroid gland by the blood and absorbed by thyrocytes through their base, are concentrated in the endoplasmic reticulum, and the synthesis of the polypeptide chain, the basis of the future thyroglobulin molecule, takes place on the ribosomes. The resulting product accumulates in the cisterns of the endoplasmic reticulum and then moves to the zone of the lamellar complex, where thyroglobulin condenses (but not yet iodinated) and small secretory vesicles are formed, which then move to the upper part of the thyrocyte. Iodine is taken up by thyrocytes from the blood in the form of iodide, and thyroxine is synthesized. Elimination phase. It is carried out by reabsorption of intrafollicular colloid. Depending on the degree of activation of the thyroid gland, endocytosis occurs in different forms. Excretion of the hormone from the gland, which is in a state of functional rest or weak excitation, proceeds without the formation of apical pseudopodia and without the appearance of drops of intracellular colloid inside thyrocytes. It is carried out by proteolysis of thyroglobulin, which takes place in the peripheral layer of the intrafollicular colloid at the border with microvilli, and subsequent micropinocytosis of the products of this cleavage. Parafollicular cells (calcitoninocytes), found in the thyroid parenchyma, differ sharply from thyrocytes in their lack of ability to absorb iodine. As mentioned above, they produce a protein hormone - calcitonin (thyrocalcitonin), which lowers the level of calcium in the blood and is an antagonist of parathyrin (parathyroid hormone). Parathyroid glands (parathyroid glands) It is believed that at each of the poles of the thyroid gland there are parathyroid glands (there are 4-6 of them in total). Each parathyroid gland is surrounded by a thin connective tissue capsule. Their parenchyma is formed by epithelial strands (trabeculae) or accumulations of glandular cells (parathyrocytes) separated by thin layers of loose connective tissue with numerous capillaries. Among parathyrocytes, there are main, intermediate and acidophilic (oxyphilic) cells, which, however, should not be considered as separate types of glandular cells of the parathyroid glands, but as functional or age-related states of parathyrocytes. During the increase in the secretory activity of the parathyroid glands, the chief cells swell and increase in volume, the endoplasmic reticulum and the lamellar complex hypertrophy in them. The release of parathyrin from the glandular cells into the intercellular gaps is carried out by exocytosis. The released hormone enters the capillaries and is carried out into the general circulation. The blood supply to the thyroid and parathyroid glands comes from the superior and inferior thyroid arteries. Adrenal Paired organs formed by a combination of two independent glands of different origin and different physiological significance: cortical and cerebral (medullary). Adrenal hormones are involved in the protective and adaptive reactions of the body, the regulation of metabolism and the activity of the cardiovascular system. In the adrenal glands, there are: a cortical layer and a medulla. The adrenal cortex is divided into three zones: glomerular, fascicular and reticular. The glomerular (external) zone is formed by elongated glandular cells (adrenocorticocytes), which are layered on top of each other, forming rounded clusters, which determines the name of this zone. In the cells of the glomerular zone, there is a high content of ribonucleoproteins and a high activity of enzymes involved in steroidogenesis. The zona glomeruli produces aldosterone, a hormone that regulates the level of sodium in the body and prevents the body from losing this element in the urine. Therefore, aldosterone can be called the mineralocorticoid hormone. Mineralocorticoid function is indispensable for life, and therefore the removal or destruction of both adrenal glands, which captures their zona glomeruli, is fatal. At the same time, mineralocorticoids accelerate the course of inflammatory processes and promote the formation of collagen. The middle part of the cortical substance is occupied by the largest beam zone in width. Adrenocorticocytes of this zone are large and cubic or prismatic in shape, their axis is oriented along the epithelial cord. The fascicular zone of the adrenal cortex produces glucocorticoid hormones - corticosterone, cortisol (hydrocortisone) and cortisone. These hormones affect the metabolism of carbohydrates, proteins and lipids, enhance the processes of phosphorylation and promote the formation of substances that store and release energy in the cells and tissues of the body. Glucocorticoids promote gluconeogenesis (i.e., the formation of glucose at the expense of proteins), the deposition of glycogen in the liver and myocardium, and the mobilization of tissue proteins. Glucocorticoid hormones increase the body's resistance to the action of various damaging agents of the environment, such as severe injuries, poisoning with poisonous substances and intoxication with bacterial toxins, as well as in other extreme conditions, mobilizing and enhancing the protective and compensatory reactions of the body. At the same time, glucocorticoids increase the death of lymphocytes and eosinophils, leading to lymphocytopenia and blood eosinopenia, and weaken both inflammatory processes and immunogenesis (antibody formation). In the inner reticular zone, the epithelial strands lose their correct location and, branching out, form a loose network, in connection with which this zone of the cortex got its name. Adrenocorticocytes in this zone decrease in volume and become diverse in shape (cubic, round or polygonal). In the reticular zone, androgenic hormone is produced (male sex hormone, similar in chemical nature and physiological properties to testosterone testis). Therefore, tumors of the adrenal cortex in women are often the cause of the development of male secondary sexual characteristics, such as mustaches and beards. In addition, female sex hormones (estrogen and progesterone) are also formed in the reticular zone, but in small quantities. The medulla of the adrenal glands is separated from the cortical part by a thin, in some places interrupted, internal connective tissue capsule. The adrenal medulla is formed by an accumulation of relatively large cells, mostly round in shape, located between the blood vessels. These cells are modified sympathetic neurons, they contain catecholamines (norepinephrine and adrenaline). Both catecholamines are similar in physiological action, but norepinephrine is a mediator that mediates the transmission of a nerve impulse from a postganglionic sympathetic neuron to an innervated effector, while adrenaline is a hormone and does not have a mediator property. Norepinephrine and epinephrine exhibit a vasoconstrictive effect and increase blood pressure, but the vessels of the brain and striated muscles expand under the influence of adrenaline. Adrenaline increases the level of glucose and lactic acid, increasing the breakdown of glycogen in the liver, and this is less common for norepinephrine. The blood supply to the adrenal gland comes from the adrenal arteries. The innervation of the adrenal glands is represented mainly by the fibers of the celiac and vagus nerves. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: The cardiovascular system >> Forward: Digestive system
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