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Lecture notes, cheat sheets
Histology. Female reproductive system
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 26. FEMALE REGENERAL SYSTEM The female reproductive system consists of paired ovaries, uterus, fallopian tubes, vagina, vulva, and paired mammary glands. The main functions of the female reproductive system and its individual organs: 1) the main function is reproductive; 2) the ovaries perform a germinal function, participating in the processes of oogenesis and ovulation, as well as an endocrine function; estrogen is produced in the ovaries; during pregnancy, the corpus luteum is formed in the ovaries, which synthesizes progesterone; 3) the uterus is intended for bearing the fetus; 4) the fallopian tubes communicate between the ovaries and the uterine cavity to advance the fertilized egg into the uterine cavity, followed by implantation; 5) the cervical canal and vagina form the birth canal; 6) mammary glands synthesize milk for feeding a newborn baby. The body of a non-pregnant woman is constantly undergoing cyclic changes, which is associated with cyclic changes in the hormonal background. Such a complex of changes in a woman's body is called the "ovarian-menstrual cycle". The ovarian cycle is the cycle of ovogenesis, i.e., the phases of growth and maturation, ovulation and the formation of the corpus luteum. The ovarian cycle is under the influence of follicle-stimulating and luteinizing hormones. The menstrual cycle is a change in the mucous membrane of the uterus, the purpose of which is to prepare the most favorable conditions for the implantation of the embryo, and in its absence, they end with the rejection of the epithelium, manifested by menstruation. The average duration of the ovarian-menstrual cycle is about 28 days, but the duration can be purely individual. Female sex hormones All female sex hormones can be divided into two groups - estrogens and progestins. Estrogens are produced by follicular cells, corpus luteum and placenta. There are the following hormones estrogen: 1) estradiol - a hormone formed from testosterone, with the help of aromatization of the latter under the influence of the enzymes aromatase and estrogen synthetase. The formation of these enzymes is induced by follitropin. It has significant estrogenic activity; 2) estrol is formed by aromatization of androstenedione, has little estrogenic activity, is excreted in the urine of pregnant women. It is also found in the follicular fluid of growing ovarian follicles and in the placenta; 3) estriol - a hormone formed from estrol, excreted in the urine of pregnant women, found in a significant amount in the placenta. Progestins include the hormone progesterone. It is synthesized by the cells of the corpus luteum during the luteal phase of the ovarian-menstrual cycle. Synthesis of progesterone is also carried out by chorion cells during pregnancy. The formation of this hormone is stimulated by lutropin and human chorionic gonadotropin. Progesterone is the pregnancy hormone. The structure of the ovary Outside, the ovary is covered with a single layer of cuboidal epithelium. Under it is a thick connective tissue plate (or albuginea) of the ovary. The transverse section shows that the ovary consists of a cortex and a medulla. The medulla of the ovary is formed by loose connective tissue, it contains many elastic fibers, blood vessels and nerve plexuses. The ovarian cortex contains primordial follicles, growing primary and secondary follicles, corpus luteum and white, and atretic follicles. ovarian cycle. Features of the structure of the primary, secondary and tertiary follicles The ovarian cycle has two halves: 1) follicular phase. In this phase, under the influence of follicle-stimulating hormone, the development of primordial follicles occurs; 2) luteal phase. Under the influence of luteal hormone, the corpus luteum of the ovary is formed from the cells of the Graafian body, which produces progesterone. Between these two phases of the cycle, ovulation occurs. The development of the follicle is carried out as follows: 1) primordial follicle; 2) primary follicle; 3) secondary follicle; 4) tertiary follicle (or Graafian vesicle). During the ovarian cycle, there are changes in the level of hormones in the blood. Structure and development of primordial follicles. Primordial follicles are located under the ovarian albuginea in the form of compact groups. The primordial follicle consists of one first-order oocyte, which is covered by a single layer of flat follicular cells (granulomatous tissue cells) and surrounded by a basement membrane. After birth, a girl’s ovaries contain about 2 million primordial follicles. During the reproductive period, about 98% of them die, the remaining 2% reach the stage of primary and secondary follicles, but only no more than 400 follicles develop into the Graafian vesicle, after which ovulation occurs. During one ovarian-menstrual cycle, 1, extremely rarely 2 or 3 first-order oocytes ovulate. With a long lifespan of the first-order oocyte (up to 40-50 years in the mother's body), the risk of various gene defects increases significantly, which is associated with the effect of environmental factors on the follicle. During one ovarian-menstrual cycle, from 3 to 30 primordial follicles, under the influence of follicle-stimulating hormone, enter the growth phase, resulting in the formation of primary follicles. All follicles that have begun their growth but have not reached the stage of ovulation undergo atresia. Atrezated follicles consist of a dead oocyte, a wrinkled transparent membrane that is surrounded by degenerated follicular cells. Between them are fibrous structures. In the absence of folliculotropic hormone, primordial follicles develop only to the stage of the primary follicle. This is possible during pregnancy, before puberty, as well as when using hormonal contraceptives. Thus, the cycle will be anovulatory (no ovulation). Structure of primary follicles. After the growth stage and its formation, the flat-shaped follicular cell turns into a cylindrical one and begins to actively divide. During division, several layers of follicular cells are formed that surround the first-order oocyte. Between the oocyte of the first order and the resulting environment (follicular cells) there is a fairly thick transparent membrane. The outer shell of the growing follicle is formed from the elements of the ovarian stroma. In the outer shell, one can distinguish the inner layer containing interstitial cells that synthesize androgens, a rich capillary network and the outer layer, which is formed by connective tissue. The inner cell layer is called the theca. The resulting follicular cells have receptors for follicle-stimulating hormone, estrogen and testosterone. Follicle-stimulating hormone promotes the synthesis of aromatase in granulose cells. It also stimulates the formation of estrogens from testosterone and other steroids. Estrogens stimulate the proliferation of follicular cells, while the number of granulose cells increases significantly, and the follicle increases in size, they also stimulate the formation of new receptors for follicle-stimulating hormone and steroids. Estrogens enhance the effect of follitropin on follicular cells, thereby preventing follicular atresia. Interstitial cells are cells of the parenchyma of the ovary, they have the same origin as the cells of the theca. The functions of interstitial cells are the synthesis and secretion of androgens. Norepinephrine acts on granulose cells through α2-adrenergic receptors, stimulates the formation of steroids in them, facilitates the action of gonadotropic hormones on steroid production, and thereby accelerates the development of the follicle. The structure of the secondary follicle. With the growth of the primary follicle between the follicular cells, rounded cavities filled with fluid are formed. Secondary follicles are characterized by further growth, while a dominant follicle appears, which is ahead of the rest in its development, the theca is most pronounced in its composition. Follicular cells increase estrogen production. Estrogens by an autocrine mechanism increase the density of follitropin recipes in the membranes of follicular cells. Follitropin stimulates the appearance of lutropin receptors in the membrane of follicular cells. The high content of estrogen in the blood blocks the synthesis of follitropin, which inhibits the development of other primary follicles and stimulates the secretion of LH. At the end of the follicular stage of the cycle, the level of lutropin rises, luteinizing hormone is formed, which stimulates the formation of androgens in the theca cells. Androgens from the theca through the basement membrane (vitreous membrane at later stages of follicle development penetrate deep into the follicle, into granulose cells, where they are converted into estrogens with the help of aromatase. The structure of the tertiary follicle. The tertiary follicle (or Graafian vesicle) is a mature follicle. It reaches 1 - 2,5 cm in diameter, primarily due to the accumulation of fluid in its cavity. A mound of follicular cells protrudes into the cavity of the Graaffian vesicle, inside which the egg is located. The egg at the stage of the oocyte of the first order is surrounded by a transparent membrane, outside of which follicular cells are located. Thus, the wall of the Graafian vesicle consists of a transparent and granular membrane, as well as theca. 24 - 36 hours before ovulation, the increasing level of estrogen in the body reaches its maximum values. The content of LH increases until the middle of the cycle. 12-14 hours after the onset of the peak of estrogen, its content also increases significantly. Lutropin stimulates luteinization of granulosa and theca cells (in this case, accumulation of lipids, yellow pigment occurs) and induces preovulatory synthesis of progesterone. This increase facilitates the reverse positive effect of estrogens, and also induces a preovulatory follitropin peak by enhancing the pituitary response to GnRH. Ovulation occurs 24 to 36 hours after the estrogen peak or 10 to 12 hours after the LH peak. Most often on the 11th - 13th day of a 28-day cycle. However, theoretically, ovulation is possible from 8 to 20 days. Under the influence of prostaglandins and the proteolytic action of granulose enzymes, thinning and rupture of the follicle wall occur. A first order oocyte undergoes the first meiotic division, resulting in a second order oocyte and a polar body. The first meiosis is completed already in the mature follicle before ovulation against the background of the LH peak. The second meiosis is completed only after fertilization. The structure and functions of the corpus luteum. Under the influence of LH in the luteal stage of the ovarian-menstrual cycle, the menstrual corpus luteum forms at the site of the burst follicle. It develops from the Graafian vesicle and consists of luteinized follicles and theca cells, between which sinusoidal capillaries are located. In the luteal stage of the cycle, the menstrual corpus luteum functions, which maintains a high level of estrogen and progesterone in the blood and prepares the endometrium for implantation. Subsequently, the development of the corpus luteum is stimulated by chorionic gonadotropin (only under the condition of fertilization). If fertilization does not occur, then the corpus luteum undergoes involution, after which the levels of progesterone and estrogen in the blood decrease significantly. The menstrual corpus luteum functions until the completion of the cycle before implantation. The maximum level of progesterone is observed 8 - 10 days after ovulation, which approximately corresponds to the time of implantation. Under the condition of fertilization and implantation, the further development of the corpus luteum occurs under the stimulating effect of chorionic gonadotropin, which is produced in the trophoblast, resulting in the formation of the corpus luteum of pregnancy. During pregnancy, trophoblast cells secrete chorionic gonadotropin, which through LH receptors stimulates the growth of the corpus luteum. It reaches a size of 5 cm and stimulates the synthesis of estrogens. A high level of progesterone, which is formed in the corpus luteum, and estrogen allows you to keep the pregnancy. In addition to progesterone, cells of the corpus luteum synthesize relaxin, a hormone of the insulin family, which reduces the tone of the myometrium and reduces the density of the pubic articulation, which are also very important factors for maintaining pregnancy. The corpus luteum of pregnancy functions most actively in the first and early second trimesters, then its function gradually fades away, and the synthesis of progesterone begins to be carried out by the formed placenta. After degeneration of the corpus luteum, a connective tissue scar, called the white body, is formed in its original place. Hormonal regulation of the ovarian-menstrual cycle The ovarian-menstrual cycle is regulated by the pituitary hormones - follicle-stimulating hormone and luteinizing hormone. The regulation of the synthesis of these hormones is under the influence of releasing factors of the hypothalamus. Ovarian hormones - estrogens, progesterone, inhibin - affect the synthesis of hormones of the hypothalamus and pituitary gland according to the feedback principle. Gonadoliberin. The secretion of this hormone is carried out in a pulsating manner: within a few minutes there is an increased secretion of the hormone, which is replaced by several hours of interruptions with low secretory activity (usually the interval between secretion peaks is 1-4 hours). The regulation of GnRH secretion is under the control of estrogen and progesterone levels. At the end of each ovarian-menstrual cycle, there is an involution of the corpus luteum of the ovary. Accordingly, the concentration of estrogen and progesterone decreases significantly. According to the feedback principle, a decrease in the concentration of these hormones stimulates the activity of neurosecretory cells of the hypothalamus, which leads to the release of GnRH with peaks lasting several minutes and with intervals between them of about 1 hour. Initially, the hormone is secreted from the pool stored in neurosecretory cell granules, and then immediately after secretion. The active mode of GnRH secretion activates the gonadotropic cells of the adenohypophysis. In the luteal stage of the ovarian-menstrual cycle, the corpus luteum is actively functioning. There is a constant synthesis of progesterone and estrogens, the concentration of which in the blood is significant. In this case, the interval between the peak of secretory activity of the hypothalamus increases to 2-4 hours. Such secretion is insufficient for the activation of gonadotropic hormones of the adenohypophysis. Follitropin. The secretion of this hormone is carried out in the follicular stage, at the very beginning of the ovarian-menstrual cycle, against the background of a reduced concentration of estrogens and progesterone in the blood. Stimulation of secretion is carried out under the influence of gonadoliberin. Estrogens, the peak of which is observed a day before ovulation, and inhibin suppress the secretion of follicle-stimulating hormone. Follitropin has an effect on follicular cells. Estradiol and follicle-stimulating hormone increase the number of receptors on the membranes of granulose cells, which enhances the effect of follitropin on follicular cells. Follitropin has a stimulating effect on the follicles, causing their growth. The hormone also activates aromatase and estrogen secretion. Lutropin. The secretion of lutropin occurs at the end of the follicular stage of the cycle. Against the background of a high concentration of estrogens, the release of follitropin is blocked and the secretion of lutropin is stimulated. The highest concentration of lutropin is observed 12 hours before ovulation. A decrease in the concentration of lutropin is observed during the secretion of progesterone by granulose cells. Lutropin interacts with specific receptors located on the membranes of theca and granulose cells, while luteinization of follicular cells and theca cells occurs. The main action of lutropin is the stimulation of androgen synthesis in theca cells and the induction of progesterone by granulose cells, as well as the activation of proteolytic enzymes of granulose cells. At the peak of lutropin, the first meiotic division is completed. Estrogen and progesterone. Estrogens are secreted by granulosa cells. Secretion gradually increases in the follicular stage of the cycle and reaches a peak one day before ovulation. The production of progesterone begins in the granulosa cells before ovulation, and the main source of progesterone is the corpus luteum of the ovary. The synthesis of estrogen and progesterone is greatly enhanced during the luteal stage of the cycle. Sex hormones (estrogens) interact with specific receptors located on the membranes of neurosecretory cells of the hypothalamus, gonadotrophic cells of the adenohypophysis, ovarian follicular cells, alveolar cells of the mammary glands, mucous membranes of the uterus, fallopian tubes and vagina. Estrogens and progesterone have a regulatory effect on the synthesis of GnRH. With a simultaneously high concentration of estrogen and progesterone in the blood, the peaks of secretion of gonadotropic hormones increase to 3-4 hours, and at their low concentration they decrease to 1 hour. Estrogens control the proliferative phase of the menstrual cycle - they contribute to the restoration of the functionally active epithelium of the uterus (endometrium). Progesterone controls the secretory phase - it prepares the endometrium for implantation of a fertilized egg. A simultaneous decrease in the concentration of progesterone and estrogens in the blood leads to rejection of the functional layer of the endometrium, the development of uterine bleeding - the menstrual phase of the cycle. Under the influence of estrogens, progesterone, prolactin, as well as chorionic somatomammotropin, the differentiation of secretory cells of the mammary gland is stimulated. The structure and function of the fallopian tubes In the wall of the fallopian tube (oviduct), three membranes can be distinguished - internal mucosa, middle muscular and external serous. There is no mucous membrane in the intrauterine section of the tube. The mucous membrane of the fallopian tube surrounds its lumen. It forms a huge number of branching folds. The epithelium of the mucous membrane is represented by a single layer of cylindrical cells, among which ciliated and secretory cells are distinguished. The lamina propria of the mucosa consists of loose fibrous unformed connective tissue, rich in blood vessels. Secretory cells of the mucous membrane have a pronounced granular endoplasmic reticulum and the Golgi complex. In the apical part of such cells there is a significant amount of secretory granules. Cells are more active during the secretory stage of the ovarian-menstrual cycle and carry out mucus production. The direction of mucus movement is from the fallopian tube to the uterine cavity, which contributes to the movement of a fertilized egg. Ciliated cells have cilia on their apical surface that move towards the uterus. These cilia help move the fertilized egg from the distal fallopian tube, where fertilization occurs, to the uterine cavity. The muscular membrane of the fallopian tube is represented by two layers of smooth muscles - the outer circular and the inner longitudinal. Between the layers is a layer of connective tissue, which has a large number of blood vessels. The contraction of smooth muscle cells also promotes the movement of the fertilized egg. The serous membrane covers the surface of the fallopian tube facing the abdominal cavity. Uterus The wall of the uterus consists of three layers - mucous, muscular and serous. The mucous membrane of the uterus (endometrium) is formed by a single-layer cylindrical epithelium, which lies on its own plate of the mucosa, represented by loose fibrous unformed connective tissue. Epithelial cells can be divided into secretory and ciliated. In the lamina propria of the mucous membrane there are uterine glands (crypts) - long curved simple tubular glands that open into the lumen of the uterus. The muscular layer (myometrium) consists of three layers of smooth muscle tissue. The outer layer is represented by longitudinal fibers, the middle layer is circular, and the inner layer is also longitudinal. The middle layer contains a large number of blood vessels. During pregnancy, the thickness of the muscle membrane increases significantly, as well as the size of smooth muscle fibers. Outside, the uterus is covered with a serous membrane, represented by connective tissue. The structure of the cervix. The cervix is the lower segment of the organ, partly protruding into the vagina. Allocate the supravaginal and vaginal parts of the cervix. The supravaginal part of the cervix is located above the place of attachment of the walls of the vagina and opens into the lumen of the uterus with the internal uterine os. The vaginal part of the cervix opens with the external uterine os. Outside, the vaginal part of the cervix is covered with stratified squamous epithelium. This epithelium is completely renewed every 4 to 5 days by desquamation of the superficial and proliferation of basal cells. The cervix is a narrow canal, slightly expanding in the middle part. The wall of the cervix consists of dense connective tissue, among the collagen and elastic fibers of which there are separate smooth muscle elements. The mucous membrane of the cervical canal is represented by a single-layer cylindrical epithelium, which in the area of the external pharynx passes into a stratified squamous epithelium, and its own layer. In the epithelium, glandular cells that produce mucus and cells that have cilia are distinguished. In the lamina propria there are numerous branched tubular glands that open into the lumen of the cervical canal. There are no spiral arteries in the own layer of the mucous membrane of the cervix, therefore, during the menstrual stage of the cycle, the mucous membrane of the cervix is not rejected like the endometrium of the body of the uterus. Vagina This is a fibromuscular tube, consisting of three layers - mucous, muscular and adventitious. The mucosa is represented by stratified squamous epithelium and lamina propria. The stratified squamous epithelium consists of basal, intermediate, and superficial cells. Basal cells are germ cells. Due to them, there is a constant renewal of the epithelium and its regeneration. The epithelium undergoes partial keratinization - keratohyalin granules can be found in the surface layers. The growth and maturation of the epithelium is under hormonal control. During menstruation, the epithelium becomes thinner, and during the reproductive period, it increases due to division. In its own layer of the mucous membrane there are lymphocytes, granular leukocytes, sometimes lymphatic follicles can be found. During menstruation, leukocytes can easily enter the lumen of the vagina. The muscular coat consists of two layers - the inner circular and the outer longitudinal. The adventitia is composed of fibrous connective tissue and connects the vagina to the surrounding structures. The structure of the external genitalia Large labia The labia majora are two skin folds located on the sides of the genital slit. From the outside, the labia majora are covered with skin that has sebaceous and sweat glands. There are no hair follicles on the inner surface. In the thickness of the labia majora there are venous plexuses, fatty tissue and Bartholin's glands of the vestibule. Bartholin's glands are paired formations, have a size no larger than a pea and are located on the border of the anterior and middle thirds of the labia. The glands are tubular-alveolar structures that open into the vestibule of the vagina. Their secret moisturizes the mucous membrane of the vestibule and the entrance to the vagina during sexual arousal. Small labia The labia minora are located medially from the large ones and are normally hidden by the large ones. The labia minora do not have adipose tissue. They are composed of numerous elastic fibers, as well as blood vessels in the form of plexuses. Pigmented skin contains sebaceous and small mucous glands that open into the vestibule of the vagina. Clitoris The clitoris is analogous to the dorsal surface of the male penis. It consists of two cavernous bodies that form the head at the distal end of the clitoris. The clitoris has a mucous membrane outside, consisting of a stratified squamous epithelium with weak keratinization (no hair, sebaceous and sweat glands). The skin contains numerous free and encapsulated nerve endings. Menstrual cycle Cyclic changes in the lining of the uterus are called the menstrual cycle. During each cycle, the endometrium goes through menstrual, proliferative, and secretory phases. The endometrium is divided into functional and basal layers. The basal layer of the endometrium is supplied with blood from the rectus arteries and is preserved in the menstrual phase of the cycle. The functional layer of the endometrium, which is shed during menstruation, is supplied with blood from the spiral arteries that sclerosis during the menstrual phase, resulting in ischemia of the functional layer. After menstruation and rejection of the functional layer of the endometrium, a proliferative phase develops, which lasts until ovulation. At this time, there is an active growth of the follicle and at the same time, under the influence of estrogens, the proliferation of cells of the basal layer of the endometrium. The epithelial cells of the glands of the basal layer migrate to the surface, proliferate and form a new epithelial lining of the mucosa. New uterine glands are formed in the endometrium, new spiral arteries grow from the basal layer. After ovulation and until the onset of menstruation, the secretory phase lasts, depending on the total length of the cycle, it can vary from 12 to 16 days. In this phase, the corpus luteum functions in the ovary, which produces progesterone and estrogens. Due to the high level of progesterone, favorable conditions are created for implantation. In this stage, the uterine glands expand, they become tortuous. Glandular cells stop dividing, hypertrophy and begin to secrete glycogen, glycoproteins, lipids and mucin. This secret rises to the mouth of the uterine glands and is released into the lumen of the uterus. In the secretory phase, the spiral arteries become more tortuous and approach the mucosal surface. The number of connective tissue cells increases in the surface of the compact layer, and glycogen and lipids accumulate in the cytoplasm. Collagen and reticular fibers are formed around the cells, which are formed by collagen types I and III. The stromal cells acquire the features of placental decidual cells. Thus, two zones are created in the endometrium - compact, facing the lumen of the uterine cavity, and spongy - deeper. The menstrual phase of the ovarian-menstrual cycle is the rejection of the functional layer of the endometrium, which is accompanied by uterine bleeding. If fertilization and implantation occur, then the menstrual corpus luteum undergoes involution, and the level of ovarian hormones - progesterone and estrogens - increases significantly in the blood. This leads to twisting, sclerosis and a decrease in the lumen of the spiral arteries that supply blood to two-thirds of the functional layer of the endometrium. As a result of these changes, a change occurs - a deterioration in the blood supply to the functional layer of the endometrium. During menstruation, the functional layer is completely rejected, and the basal layer is preserved. The duration of the ovarian-menstrual cycle is about 28 days, but it is subject to significant variations. The duration of menstruation is from 3 to 7 days. Changes in the vagina during the ovarian-menstrual cycle. During the onset of the follicular stage, the vaginal epithelium is thin and pale. Under the influence of estrogens, the proliferation of the epithelium occurs, which reaches its maximum thickness. At the same time, a significant amount of glycogen used by the vaginal microflora accumulates in the cells. The resulting lactic acid prevents the development of pathogenic microorganisms. The epithelium shows signs of keratinization. In the luteal stage, the growth and maturation of epithelial cells is blocked. Leukocytes and horny scales appear on the surface of the epithelium. The structure of the mammary gland The mammary gland is a derivative of the epidermis and belongs to the skin glands. The development of the gland depends on sex - on the type of sex hormones. In prenatal development, milk lines are laid - epidermal ridges that lie on both sides of the body from the armpit to the groin. In the midthoracic region, the epithelial cords of the ridges grow into the skin itself and subsequently differentiate into complex tubular alveolar glands. The histological structure of the mammary gland depends on the degree of its maturity. There are cardinal differences between the juvenile mammary gland, mature inactive and active glands. The juvenile mammary gland is represented by interlobular and intralobular ducts separated by connective tissue septa. There are no secretory sections in the juvenile gland. A mature inactive gland is formed during puberty. Under the influence of estrogens, its volume increases significantly. The excretory ducts become more branched, and adipose tissue accumulates among the connective tissue bridges. Secretory departments are absent. The lactating gland is formed under the influence of progesterone in combination with estrogens, prolactin and chorionic somatomammotropin. Under the action of these hormones, the differentiation of the secretory sections of the mammary gland is induced. At the 3rd month of pregnancy, kidneys are formed from the growing terminal sections of the intralobular ducts, which differentiate into secretory sections - alveoli. They are lined with cuboidal, secretory epithelium. Outside, the wall of the alveoli and excretory ducts is surrounded by numerous myoepithelial cells. The intralobular ducts are lined with single-layered cuboidal epithelium, which in the milk ducts becomes stratified squamous. In the lactating gland, the connective tissue septa that separate the lobules of the mammary gland are less pronounced compared to the juvenile and functionally inactive glands. Secretion and excretion of milk is carried out in the glands under the influence of prolactin. The greatest secretion is carried out in the morning hours (from 2 to 5 am). Under the influence of prolactin in the membranes of alveolar cells, the density of receptors for both prolactin and estrogens increases. During pregnancy, the concentration of estrogen is high, which blocks the action of prolactin. After the birth of a child, the level of estrogen in the blood decreases significantly, and then prolactin increases, which allows it to induce milk secretion. In the first 2-3 days after birth, the mammary gland secretes colostrum. The composition of colostrum differs from milk. It has more protein, but less carbohydrates and fats. In colostrum, cell fragments can be found, and sometimes whole cells containing nuclei - colostrum bodies. During active lactation, alveolar cells secrete fats, casein, lactoferrin, serum albumin, lysozyme, and lactose. Milk also contains fat and water, salts and class A immunoglobulins. The secretion of milk is carried out according to the apocrine type. The main components of milk are isolated by exocytosis. The only exceptions are fats, which are released by a section of the cell membrane. The hormones that regulate lactation include prolactin and oxytocin. Prolactin maintains lactation during breastfeeding. The maximum secretion of prolactin is carried out at night - from 2 to 5 in the morning. The secretion of prolactin is also stimulated by the sucking of the breast by the child, while within half an hour the concentration of the hormone in the blood increases sharply, after which the active secretion of milk by the alveolar cells for the next feeding begins. Against the background of lactation, the secretion of gonadotropic hormones is suppressed. This is due to an increase in the level of endorphins, which block the release of GnRH by neurosecretory cells of the hypothalamus. Oxytocin is a hormone from the posterior pituitary gland that stimulates the contraction of myoepithelial cells, which promotes the movement of milk in the ducts of the gland. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Sexual system >> Forward: Organ of vision
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