Pituitary

The pituitary gland (hypophysis, s.glandula pituitaria) is located in the hypophyseal fossa of the Turkish saddle of the sphenoid bone and is separated from the cranial cavity by the appendage of the hard shell of the brain, which forms the diaphragm of the saddle. Through the hole in this diaphragm, the pituitary is connected to the funnel of the hypothalamus of the midbrain. The transverse size of the pituitary gland is 10-17 mm, anteroposterior - 5-15 mm, vertical - 5-10 mm. The weight of the pituitary gland in men is approximately 0.5 g, in women it is 0.6 g. Outside, the pituitary gland is covered with a capsule.

In accordance with the development of the pituitary gland from two different primordia, two parts are distinguished in the organ - anterior and posterior. Adenohypophysis, or anterior lobe (adenohypophysis, s.lobus anterior), larger, is 70-80% of the total pituitary mass. It is more dense than the posterior lobe. In the anterior part, the distal part (pars distalis) is located, which occupies the anterior part of the pituitary fossa, the intermediate part (pars intermedia) located on the border with the posterior lobe, and the tuberous part (pars tuberalis), which extends upwards and joins with the funnel of the hypothalamus. Due to the abundance of blood vessels, the anterior lobe is pale yellow, with a reddish hue. The parenchyma of the anterior lobe of the pituitary gland is represented by several types of glandular cells, between the strands of which are sinusoidal blood capillaries. Half (50%) of adenohypophysis cells are chromophilic adenocytes, which have fine granules in their cytoplasm, well colored with chromium salts. These are acidophilic adenocytes (40% of all adenohypophysis cells) and basophilic adenocytes (10%). Among the basophilic adenocytes are gonadotropic, corticotropic and thyrotropic endocrinocytes. Chromophobic adenocytes are small, they have a large nucleus and a small amount of cytoplasm. These cells are considered precursors of chromophilic adenocytes. Other 50% of adenohypophysis cells are chromophobic adenocytes.

The neurohypophysis, or posterior lobe (neurohypophysis, s.lobus posterior), consists of the lobus nervosus, which is located in the posterior part of the pituitary fossa, and the funnel (infundibulum) located behind the tubercular part of the adenohypophysis. The posterior part of the pituitary gland is formed by neuroglytic cells (pituitary cells), nerve fibers from the neurosecretory nuclei of the hypothalamus into the neurohypophysis, and neurosecretory bodies.

The pituitary gland with the help of nerve fibers (pathways) and blood vessels is functionally connected with the hypothalamus of the intermediate brain, which regulates the activity of the pituitary gland. The pituitary and hypothalamus, together with their neuroendocrine, vascular and nerve connections, are usually considered as a hypothalamic-pituitary system.

Hormones of the anterior and posterior lobes of the pituitary gland affect many functions of the body, primarily through other endocrine glands. In the anterior part of the pituitary acidophilic adenocytes (alpha cells) produce somotropic hormone (growth hormone), which takes part in the regulation of the growth and development of the young organism. Corticotropic endocrinocytes secrete adrenocorticotropic hormone (ACTH), which stimulates the secretion of steroid hormones by the adrenal glands. Thyrotropic endocrinocytes secrete thyrotropic hormone (TSH), which affects the development of the thyroid gland and activates the production of its hormones. Gonadotropic hormones: follicle stimulating (FSH), luteinizing (LH) and prolactin - affect the sexual maturation of the body, regulate and stimulate the development of follicles in the ovary, ovulation, growth of the mammary glands and milk production in women, the process of spermatogenesis in men. These hormones are produced by the basophilic adenocytes of the beta cell ). Here, lipotropic factors of the pituitary gland are secreted, which affect the mobilization and utilization of fats in the body. In the intermediate part of the anterior lobe, a melanocyte-stimulating hormone is formed that controls the formation of melanin pigments in the body.

Neurosecretory cells of the supraoptic and paraventricular nuclei in the hypothalamus produce vasopressin and oxytocin. These hormones are transported to the cells of the posterior lobe of the pituitary gland along the axons that make up the hypothalamic-pituitary tract. From the posterior lobe of the pituitary gland these substances enter the blood. The hormone vasopressin has a vasoconstrictor and antidiuretic effect, for which it was also called the antidiuretic hormone (ADH). Oxytocin exerts a stimulating effect on the contractility of the uterine musculature, increases the secretion of milk by the lactating mammary gland, inhibits the development and function of the yellow body, affects the change in the tone of the smooth (undistorted) muscles of the gastrointestinal tract.

Development of the pituitary gland

The anterior part of the pituitary gland develops from the epithelium of the dorsal wall of the oral bay in the form of a ring-shaped outgrowth (Rathke's pocket). This ectodermal protrusion grows toward the bottom of the future III ventricle. Toward him from the lower surface of the second cerebral bladder (the future bottom of the third ventricle) grows an outgrowth from which the gray crater of the funnel and the posterior lobe of the pituitary gland develop.

[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]

Vessels and nerves of the pituitary gland

The upper and lower pituitary arteries are directed from the internal carotid arteries and blood vessels of the cerebral arterial circle to the pituitary gland. The upper hypophyseal arteries go to the gray nucleus and funnel of the hypothalamus, anastomose here with each other and form capillaries, the primary hemocapillary network that enter the brain tissue. From the long and short loops of this network, portal veins are formed, which are directed to the anterior lobe of the pituitary gland. In the parenchyma of the anterior lobe of the pituitary gland these veins dissociate into wide sinusoidal capillaries, which form a secondary hemocapillary network. The posterior lobe of the pituitary gland is primarily blood flowing through the lower pituitary artery. Between the upper and lower pituitary arteries there are long arterial anastomoses. The outflow of venous blood from the secondary hemocapillary network is carried out by a system of veins that flow into the cavernous and intercellular sinuses of the hard shell of the brain.

The innervation of the pituitary gland involves sympathetic fibers that penetrate the organ together with the arteries. Postganglionic sympathetic nerve fibers move away from the interweaving of the internal carotid artery. In addition, in the posterior lobe of the pituitary gland numerous outgrowths of the processes of neurosecretory cells located in the nuclei of the hypothalamus are found.

Age features of the pituitary gland

The average weight of the pituitary gland in newborns reaches 0.12 g. The body weight doubles to 10 and triples by 15 years. By the age of 20 the weight of the pituitary gland reaches a maximum (530-560 mg) and in the subsequent age periods it almost does not change. After 60 years, there is a slight decrease in the mass of this endocrine gland.

[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]

Pituitary Hormones

The unity of the nervous and hormonal regulation in the body is ensured by the close anatomical and functional connection of the pituitary and hypothalamus. This complex determines the state and functioning of the entire endocrine system.

The main gland of internal secretion, which produces a number of peptide hormones that directly regulate the function of the peripheral glands, is the pituitary gland. This is a reddish-gray bean-shaped formation, covered with a fibrous capsule weighing 0.5-0.6 g. It varies slightly depending on the sex and age of the person. It is common to divide the pituitary gland into two parts, different in development, structure and functions: anterior distal - adenohypophysis and posterior - neurohypophysis. The first is about 70% of the total mass of the gland and is conditionally divided into the distal, funnel and intermediate parts, the second - to the back, or the share, and the pituitary foot. The gland is located in the hypophyseal fossa of the Turkish saddle of the sphenoid bone and is connected to the brain through the foot. The upper part of the anterior lobe is covered by the visual crossover and visual pathways. The blood supply of the pituitary gland is very abundant and is carried out by the branches of the internal carotid artery (upper and lower pituitary arteries), as well as branches of the arterial circle of the large brain. The upper pituitary arteries participate in the blood supply of the adenohypophysis, and the lower ones - the neurohypophysis, while contacting with the neurosecretory endings of the axons of large-cell hypothalamic nuclei. The first enter the middle elevation of the hypothalamus, where they scatter into the capillary network (the primary capillary plexus). These capillaries (with which the axon axons of small neurosecretory cells contact the mediobasal hypothalamus) gather in portal veins descending along the pituitary pedicle to the parenchyma of the adenohypophysis, where they are again divided into a network of sinusoidal capillaries (secondary capillary plexus). So, the blood, after passing through the middle elevation of the hypothalamus, where it is enriched with hypothalamic adenohypophysotropic hormones (releasing hormones), gets to the adenohypophysis.

The outflow of blood saturated with adenohypophyseal hormones from the numerous capillaries of the secondary plexus is carried out through the veins, which in turn flow into the venous sinuses of the dura mater and further into the total blood flow. Thus, the portal system of the pituitary gland with a descending direction of blood flow from the hypothalamus is a morphofunctional component of the complex mechanism of neurohumoral control of the trophic functions of the adenohypophysis.

The innervation of the pituitary gland is carried out by sympathetic fibers that follow the pituitary arteries. Beginning they are given postganglionic fibers, going through the internal carotid plexus, connected with the upper cervical nodes. There is no direct innervation of the adenohypophysis from the hypothalamus. The nerve fibers of the neurosecretory nuclei of the hypothalamus enter the posterior lobe.

Adenohypophysis in histological architectonics is a very complex formation. It distinguishes two types of glandular cells - chromophobic and chromophilic. The latter in turn are divided into acidophilic and basophilic (a detailed histological description of the pituitary gland is given in the corresponding section of the manual). However, it should be noted that the hormones produced by the glandular cells that make up the parenchyma of the adenohypophysis due to the variety of the latter are somewhat different in their chemical nature, and the fine structure of the secreting cells must correspond to the peculiarities of the biosynthesis of each of them. But sometimes in adenohypophysis one can observe transitional forms of glandular cells, which are capable of producing several hormones. There is evidence that a variety of glandular cells of the adenohypophysis is not always determined genetically.

Under the diaphragm of the Turkish saddle is the funnel part of the anterior lobe. It covers the pituitary foot, contacting the gray hillock. This part of the adenohypophysis is characterized by the presence in it of epithelial cells and abundant blood supply. It is also hormone-active.

The intermediate (middle) part of the pituitary gland consists of several layers of large secretion-active basophilic cells.

The pituitary through its hormones carries a variety of functions. In its anterior lobe, adrenocorticotropic (ACTH), thyrotropic (TTG), follicle-stimulating (FSH), luteinizing (LH), lipotropic hormones are produced, as well as growth hormone - somatotropic (STO and prolactin, melanocyte-stimulating hormone (MSH) is synthesized in the intermediate lobe, and In the back, vasopressin and oxytocin accumulate.

ACTH

Hypophyseal hormones represent a group of protein and peptide hormones and glycoproteins. Of the hormones of the anterior lobe of the pituitary gland ACTH is the most studied. It is produced by basophil cells. Its main physiological function is the stimulation of biosynthesis and the secretion of steroid hormones by the adrenal cortex. ACTH also exhibits melanocyte-stimulating and lipotropic activity. In 1953 it was isolated in its pure form. Later, its chemical structure was established, consisting of 39 amino acid residues in a human and a number of mammals. ACTH does not have specific specificity. At present, chemical synthesis of both the hormone itself and various, more active than natural hormones, fragments of its molecule is carried out. In the structure of the hormone, two sections of the peptide chain, one of which provides detection and binding of ACTH to the receptor, and the other - gives a biological effect. With the ACTH receptor, it seems that it binds due to the interaction of the electrical charges of the hormone and the receptor. The role of the biological effector ACTH performs a fragment of the molecule 4-10 (Met-Glu-Gis-Fen-Arg-Tri-Tri).

Melanocyte-stimulating activity of ACTH is due to the presence in the molecule of the N-terminal region, consisting of 13 amino acid residues and repeating the structure of alpha-melanocyte-stimulating hormone. The same site contains heptapeptide, present in other pituitary hormones and possesses some adrenocorticotropic, melanocyte-stimulating and lipotropic activities.

The key point in the action of ACTH is the activation of the protein kinase enzyme in the cytoplasm with the participation of cAMP. Phosphorylated protein kinase activates the enzyme esterase, which converts cholesterol esters into a free substance in fatty drops. The protein synthesized in the cytoplasm as a result of ribosome phosphorylation stimulates the binding of free cholesterol to cytochrome P-450 and its transfer from lipid droplets to the mitochondria, where all the enzymes that ensure the conversion of cholesterol into corticosteroids are present.

[27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]

Thyroid-stimulating hormone

TSH - thyrotropin - the main regulator of the development and functioning of the thyroid gland, the processes of synthesis and secretion of thyroid hormones. This complex protein - glycoprotein - consists of alpha and beta subunits. The structure of the first subunit coincides with the alpha subunit of luteinizing hormone. Moreover, it largely coincides in different animal species. The sequence of amino acid residues in the human beta-subunit of human TSH is deciphered and consists of 119 amino acid residues. It can be noted that beta subunits of human TSH and cattle are similar in many respects. The biological properties and character of the biological activity of glycoprotein hormones are determined by the beta subunit. It also ensures the interaction of the hormone with the receptors in various target organs. However, the beta subunit in most animals shows a specific activity only after its connection with the alpha-subunit, acting as a kind of activator of the hormone. The latter, with the same probability, induces luteinizing, follicle-stimulating and thyrotropic activities, determined by the properties of the beta subunit. The similarity found allows us to conclude that these hormones originate in the course of evolution from one common precursor, the beta subunit determines the immunological properties of hormones. There is an assumption that the alpha subunit protects the beta subunit from the action of proteolytic enzymes, and also facilitates its transportation from the pituitary to peripheral target organs.

Gonadotropic hormones

Gonadotropins are present in the body in the form of LH and FSH. The functional purpose of these hormones generally reduces to providing reproductive processes in individuals of both sexes. They, like TTG, are complex proteins - glycoproteins. FSH induces the maturation of follicles in the ovaries in females and stimulates spermatogenesis in males. LH causes in females rupture of the follicle with the formation of a yellow body and stimulates the secretion of estrogens and progesterone. In males, this same hormone accelerates the development of interstitial tissue and the secretion of androgens. Effects of gonadotropins are dependent on each other and proceed synchronously.

The dynamics of gonadotropin secretion in women varies during the menstrual cycle and is studied in sufficient detail. In the preovulatory (follicular) phase of the cycle, the content of LH is at a rather low level, and FSH is increased. As the follicle ripens, the secretion of estradiol rises, which increases the production of gonadotropins by the pituitary gland and the emergence of cycles of both LH and FSH, i.e., sex steroids stimulate the secretion of gonadotropins.

At present, the structure of LH is determined. Like TTG, it consists of 2 subunits: a and p. The structure of the alpha subunit of LH in different animal species largely coincides, it corresponds to the structure of the alpha-subunit of TSH.

The structure of the beta subunit of LH differs markedly from the structure of the beta-subunit of TSH, although it has four identical segments of the peptide chain consisting of 4-5 amino acid residues. In TTG, they are localized in positions 27-31, 51-54, 65-68 and 78-83. Since the beta-subunit of LH and TTG determines the specific biological activity of hormones, it can be assumed that homologous sites in the structure of LH and TSH should ensure the connection of beta subunits with the alpha subunit, and the different regions in the structure - responsible for the specificity of the biological activity of hormones.

Native LH is very stable to the action of proteolytic enzymes, however, the beta subunit is rapidly cleaved by chymotrypsin, and the a-subunit is difficult to hydrolyse by the enzyme, i.e., it plays a protective role preventing the access of chymotrypsin to peptide bonds.

As for the chemical structure of FSH, at present the researchers have not received final results. Just like LH, FSH consists of two subunits, however, the beta-subunit of FSH differs from the beta-subunit of LH.

Prolactin

In the processes of reproduction, another hormone, prolactin (lactogenic hormone), actively participates. The main physiological properties of prolactin in mammals are manifested in the form of stimulation of development of mammary glands and lactation, growth of sebaceous glands and internal organs. It promotes the effect of steroids on secondary sexual characteristics in males, stimulates the secretory activity of the yellow body in mice and rats, and participates in the regulation of fat metabolism. Much attention is paid to prolactin in recent years as a regulator of maternal behavior, this polyfunctionality is explained by its evolutionary development. It is one of the ancient pituitary hormones and is found even in amphibians. At present, the structure of prolactin of some mammalian species has been completely deciphered. However, until recently, scientists have expressed doubts about the existence of such a hormone in humans. Many believed that its function is performed by growth hormone. Now we have convincing evidence of the presence of prolactin in humans and partially deciphered its structure. Prolactin receptors actively bind growth hormone and placental lactogen, which indicates a single mechanism of action of the three hormones.

Somatotropin

An even wider spectrum of action than prolactin has growth hormone - somatotropin. Like prolactin, it is produced by acidophilic cells of the adenohypophysis. STG stimulates the growth of the skeleton, activates the biosynthesis of the protein, gives a fat-mobilizing effect, promotes an increase in body size. In addition, he coordinates the exchange processes.

The involvement of the hormone in the latter is confirmed by the fact of a sharp increase in its secretion by the pituitary gland, for example, with a decrease in the sugar content in the blood.

The chemical structure of this human hormone is now fully established - 191 amino acid residues. Its primary structure is similar to the structure of chorionic somatomamotropin or placental lactogen. These data indicate a significant evolutionary proximity of the two hormones, although they exhibit differences in biological activity.

It is necessary to emphasize the high specific specificity of the hormone in question - for example, the STH of animal origin is inactive in humans. This is due to both the reaction between the human and animal STH receptors, and the structure of the hormone itself. Currently, studies are underway to identify active sites in a complex structure of STH that exhibit biological activity. We study individual fragments of a molecule that exhibit other properties. For example, after hydrolysis of human STH with pepsin, a peptide consisting of 14 amino acid residues and corresponding to the region of molecule 31-44 was isolated. He did not have the effect of growth, but by lipotropic activity was significantly superior to the native hormone. Human growth hormone, in contrast to a similar hormone in animals, has a significant lactogenic activity.

In the adenohypophysis a lot of both peptide and protein substances with a fat-mobilizing effect are synthesized, and the trophic hormones of the pituitary - ACTH, STH, TTG and others - have a lipotropic effect. In recent years, especially beta-and y-lipotropic hormones (LPG) have been singled out. The biological properties of beta-LPG, which, in addition to lipotropic activity, also have melanocyte-stimulating, corticotropin-stimulating and hypocalcemic action, as well as an insulin-like effect, have been studied in more detail.

At present, the primary structure of sheep LPG (90 amino acid residues), lipotropic hormones of pigs and cattle is deciphered. This hormone has specific specificity, although the structure of the central portion of beta-LPG in different species is the same. It determines the biological properties of the hormone. One of the fragments of this site is found in the structure of alpha-MSH, beta-MSH, ACTH and beta-LPG. It is suggested that these hormones originated from the same precursor in the course of evolution. Y-LPG has a weaker lipotropic activity than beta-LPG.

Melanocyte-stimulating hormone

This hormone, synthesized in the intermediate lobe of the pituitary gland, stimulates the biosynthesis of skin pigment melanin by its biological function, contributes to the increase in the size and quantity of melanocyte pigment cells in the skin of amphibians. These qualities of MSH are used in biological testing of the hormone. There are two types of hormone: alpha and beta-MSH. It is shown that alpha-MSH does not have specific specificity and has the same chemical structure in all mammals. Its molecule is a peptide chain consisting of 13 amino acid residues. Beta-MSH, in contrast, has specific specificity, and its structure differs in different animals. In most mammals, the β-MSH molecule consists of 18 amino acid residues, and only in humans it is elongated from the amino terminus to four amino acid residues. It should be noted that alpha-MSH has some adrenocorticotropic activity, and its effect on the behavior of animals and humans has now been proven.

Oxytocin and vasopressin

In the posterior lobe of the pituitary gland, vasopressin and oxytocin accumulate, which are synthesized in the hypothalamus: vasopressin - in the neurons of the supraoptic nucleus, and oxytocin - paraventriculatory. Then they are transferred to the pituitary gland. It should be emphasized that in the hypothalamus, the precursor of the hormone vasopressin is first synthesized. At the same time, a neurofizin protein of the 1 st and 2 nd types is produced there. The first binds oxytocin, and the second - vasopressin. These complexes migrate in the form of neurosecretory granules in the cytoplasm along the axon and reach the posterior lobe of the pituitary where the nerve fibers terminate in the vessel wall and the contents of the granules enter the blood. Vasopressin and oxytocin are the first pituitary hormones with a fully established amino acid sequence. In their chemical structure, they are nonapeptides with one disulfide bridge.

The hormones under consideration give a variety of biological effects: they stimulate the transport of water and salts through the membranes, have a vasopressor effect, enhance the contractions of the smooth muscles of the uterus during labor, increase the secretion of mammary glands. It should be noted that vasopressin has an antidiuretic activity higher than oxytocin, whereas the latter acts more strongly on the uterus and the mammary gland. The main regulator of vasopressin secretion is water intake, in the kidney canals it binds to receptors in cytoplasmic membranes with subsequent activation of the enzyme adenylate cyclase in them. For binding the hormone to the receptor and for the biological effect, different parts of the molecule are responsible.

The pituitary body, connected through the hypothalamus with the entire nervous system, unites the endocrine system participating in the constancy of the internal environment of the organism (homeostasis) into a functional whole. Inside the endocrine system, homeostatic regulation is performed on the basis of the principle of feedback between the anterior pituitary gland and target glands (thyroid gland, adrenal cortex, gonads). The excess of the hormone produced by the target gland inhibits, and its deficiency stimulates the secretion and release of the corresponding tropic hormone. The feedback system includes the hypothalamus. It is in it are sensitive to the hormones of iron targets, the receptor zones. Specifically binding to circulating hormones and changing the response depending on the concentration of hormones, the hypothalamic receptors transmit their effect to the corresponding hypothalamic centers that coordinate the work of the adenohypophysis, releasing the hypothalamic adenohypophysotropic hormones. Thus, the hypothalamus should be considered as a neuro-endocrine brain.