The adenohypophysis has a highly dense and complex vascular network. Hypertrophy and hyperplasia of TSH cells under these conditions are due to a lack of negative feedback by T3 and, to a lesser extent, T4 on the hypothalamus and thyrotrophs. Cells of the anterior lobe, viewed microscopically using hematoxylin and eosin staining, include acidophils, basophils, and chromophobes, constituting approximately 40%, 10%, and 50% of the total cell population, respectively. However, the sparsely-granulated cells with smaller granules often fail to stain with these dyes, and appear chromophobic. HPD effectively disconnected the pituitary gland from hypothalamic control whilst the pars distalis was not deprived of its blood supply. The pituitary fossa is surrounded by a layer of dura, continuous with the diaphragma sellae above (the dural anatomy is discussed in more detail in the pituitary fossa article). Dopamine also suppresses ACTH production by corticotrophs in the pars intermedia of some species. FIGURE 17.3. In the adult, vestiges of Rathke’s pouch, if present, are seen either as a cleft or cyst like spaces located between the anterior and posterior lobes. Neuroradiology. isolated mesenchyme-free hypophyseal pouches from 4- to 5-day chick embryos and cultured them as chorioallantoic grafts. pars intermedia Process of secretion Neurohypophysis Porcine Stress Syndrome MATERIALS AND MET!iODS ' RESULTS DISCUSSION SUMMARY AND CONCLUSIONS BIBLIOGRAPHY ACKNOWLEDGMENTS APPENDIX 1111408 Page 1 4 4 4 5 7 7 13 25 28 36 41 45 52 56 64 72 73 85 86 Schuenke M, Schulte E. Head and Neuroanatomy (THIEME Atlas of Anatomy). The anterior pituitary secretes six hormones, each with distinct functions and target organs: Most of the aforementioned hormones' release is controlled by releasing hormones secreted by the hypothalamus into the portal circulation. The pars intermedia is a common site of hyperplasia and neoplasia in older horses and somewhat less so in dogs, in which pars intermedia cells can produce substantial bioactive ACTH. When we stain the adenohypophysis with laboratory dyes, some cells acquire a pink color. Handbook of Toxicologic Pathology (Second Edition), Pathologic Basis of Veterinary Disease (Sixth Edition), Neurohypophysis [Posterior Pituitary Gland, The Hypothalamus–Pituitary System in Non-Mammalian Vertebrates, Best Practice & Research Clinical Endocrinology & Metabolism. Temporary recurring or persistent occlusive vasospasm has been linked to necrosis of the anterior pituitary. The pars intermedia surrounds the residual Rathke's pouch and separates the pars nervosa (see Neurohypophysis [Posterior Pituitary Gland]) from the pars distalis. A second hypothalamic release-inhibiting hormone is somatostatin (somatotropin release-inhibiting hormone, SRIH). Conversely, prolactin secreting adenomas (prolactinomas) can be treated medically with dopamine agonists (e.g. This invagination of cells then extends toward the neurohypophysis growing from the embryonic brain. However, immunohistochemistry, using antibodies against the trophic hormones of interest, is more reliable than histochemical tinctorial characteristics in identifying the trophic hormone produced by a particular cell type in the adenohypophysis. Nippon Rinsho. TPE. The adenohypophysis is nonneural tissue that develops from an ectodermal evagination of the roof of the buccal cavity, Rathke's pouch. They tend to be concentrated in the dorsocephalic region of the adenohypophysis. The anterior pituitary is by far the largest part of the gland and is responsible for synthesis and release of most pituitary hormones (with the exception of oxytocin and antidiuretic hormone (ADH) which are released by the posterior pituitary). The Developing Human, Clinically Oriented Embryology. However, in teleosts, the rostral proximal pars distalis receives blood supply from the extensive looping’s of arte­rioles which are found near the interface with the pars distalis (Fig. These cells are widely distributed within the pars distalis, and form the bulk of the pars intermedia. The posterior portion of the adenohypophysis is the pars intermedia, which is responsible for synthesis of a-MSH and endorphins. Pregnancy and lactation are associated with a marked degree of hyperplasia of prolactin-secreting cells. ACTH was not demonstrable by immunocytochemistry in the explants on retrieval. The posterior pituitary (aka neurohypophysis) is a direct extension from the hypothalamus, connected to it via the infundibular stalk, which is also considered part of the neurohypophysis. The pituitary gland (or hypophysis cerebri), together with its connections to the hypothalamus, acts as the main endocrine interface between the central nervous system and the rest of the body. The cell bodies reside in two hypothalamic nuclei: The pituitary gland volume changes depending on hormonal status, most dramatically during pregnancy when it can be markedly enlarged, sometimes mistaken for an adenoma. The adenohypophysis may enlarge as a result of stimulation from the hypothalamus and mimic a neoplasm. Importantly, the median eminence and the pituitary gland are not encumbered with a blood-brain barrier. Thus, dopamine antagonists (e.g. Cortisol in turn raises blood sugar levels, reduces inflammation, and combats the effects of stress. “Thyroidectomy cells” are enlarged and vacuolated TSH cells that contain prominent, often dilated granular endoplasmic reticulum, conspicuous Golgi complexes, and few secretory granules. Margaret A. Miller, in Pathologic Basis of Veterinary Disease (Sixth Edition), 2017. Although these categories are not as clear cut as was originally thought, corticotropes are basophilic and show strong periodic acid Schiff (PAS) staining. Thyrotropin-producing (TSH) basophils or thyrotrophs account for about 5% of the cells of the adenohypophysis. 3. Here the cells are responsible for the synthesis of growth hormone (GH), prolactin (PRL) or luteotropic hormone (LTH), and adrenocorticotropin (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) or thyrotropin. 7. These cells are distributed throughout the adenohypophysis and are generally round to ovoid in shape. Fig 1.3 – The blood supply to the anterior and posterior lobes of the pituitary gland. Melanotrophs do not express glucocorticoid receptors, so generally they do not respond to cortisol concentrations in the peripheral blood. thyroxine), which in turn control the rate at which our tissues use oxygen. One type contains secretory granules, which measure 200–300 nm in diameter, whereas the second type has secretory granules, which measure 600–900 nm. The exception is prolactin which, except during pregnancy when it too is stimulated by a releasing hormone, is tonically inhibited by prolactin inhibitory hormone (PIH) which is actually dopamine. Generally speaking, young adults have larger glands than older individuals, and hormonally active individuals (puberty/pregnancy) have the largest glands. 2. These axons release various releasing and inhibiting factors which are then taken down the infundibulum in the hypophyseal portal venous plexus, and thus delivered to the anterior pituitary where they control the release of its numerous hormones 2,4,5. The caudal part of the pars distalis may be spared, and the necrotic areas may be sharply demarcated. Hisham M. Dahmoush, ... Arastoo Vossough, in Handbook of Clinical Neurology, 2016. 8. A portal system is a group of blood vessels that connects sets of capillaries of different organs.2 A portal venous systemis one in which the joining blood vessels are veins. Recent News. Any condition that leads to hypofunction of the thyroid, either as a result of thyroidectomy or the administration of goitrogenic drugs or chemicals that inhibit the biosynthesis of triiodothyronine (T3) and thyroxine (T4) or increase the hepatic degradation of thyroid hormones, results in the development of thyroidectomy cells in the pituitary gland. The pituitary gland receives its blood supply from the superior and inferior hypophysial arteries. (2005) ISBN:0443071683. 11. Gomitoli are 1- to 2-mm-long arteries surrounded by a thick layer of muscle and capillaries. These plump glands completely fill the pituitary fossa, and have a convex upper border, whereas older individuals will have a largely empty pituitary fossa, with a deflated and thinned gland lying on the floor of the sella. Learn vocabulary, terms, and more with flashcards, games, and other study tools. In some situations, SRIH also inhibits the secretion of PRL and ACTH. Pituitary gland 1. The pars distalis is vascularized by hypophyseal portal vessels that arise from the capillary beds within the median eminence of the hypothalamus. When the groove closed, tissue from the ventral neural ridge was reported to become incorporated into the floor of the diencephalon. Wanda M. Haschek, ... Matthew A. Wallig, in Fundamentals of Toxicologic Pathology (Second Edition), 2010. edit Edit this Article Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally to neurohypophysis. The releasing hormones are transported via axonal processes to the median eminence where they are released into capillaries and are carried by the hypophyseal portal system to trophic hormone-producing cells in the adenohypophysis. Function : Somatotropin (growth hormone, GH) stimulates kidney and liver to synthesize and secrete somatomedin, which stimulates the growth of long bones . It is important to understand the blood supply of the adrenal gland. George Kontogeorgos, ... Eva Horvath, in The Human Nervous System (Third Edition), 2012. superior hypophyseal arteries. Ultrastructurally, two types of prolactin cells have been recognized. Because FSH and LH play vital roles in the function of the gonads, they are grouped under the term gonadotropic hormones or “gonadotropins.” The pituitary cells that release gonadotropins are termed gonadotropes. The pars tuberalis forms a cuff around the infundibulum and the pars distalis wraps partially around the pars nervosa. One of these hormones is the glycoprotein thyrotropin. AJR Am J Roentgenol. The hypophyseal trophic hormones act on targeted endocrine cells and cells in other organs and tissues. FIGURE 1.13. The trophic hormones produced by pars distalis cells include adrenocorticotrophic hormone (ACTH) produced by corticotrophs, growth hormone (also known as somatotrophin) produced by somatotrophs, prolactin produced by lactotrophs, and thyroid-stimulating hormone produced by thyrotrophs. Occasional larger granules may also be present within corticotrophs. These may give rise to Rathke cleft cysts (also known as pars intermedia cysts 6,7). The infundibulum grows ventrally towards the stomodeum while simultaneously Rathke pouch grows dorsally. There are three categories of tropic hormones based on chemical structure. Aminergic and peptidergic neurons terminate in the median eminence where they regulate release of RHs and RIHs into the portal circulation. The portal system connects the median eminence to the adenohypophysis and brings RHs and RIHs to the tropic hormone-producing cells. bromocriptine and cabergoline). Hyperplasia resolves with alteration of the physiologic status responsible for the hyperplasia or treatment of the underlying cause. These cells are widely distributed within the pars distalis and form the bulk of the pars intermedia. Charles C. Capen, ... John T. Yarrington, in Handbook of Toxicologic Pathology (Second Edition), 2002. The pars intermedia in a thin layer of epithelial cells located between pars distalis and neurohypophysis. Start studying EN2 - Endocrine glands and the action of hormones + melanotrophs. Classically, cells of the pars distalis have been characterized into three groups based on their stainability: acidophils, basophils, and chromophobes. The pituitary, connected to the hypothalamus at the base of the brain, has two lobes. The blood supply of the pituitary is unique because it is connected to the hypothalamus. Figure 21. These cells tend to occur in small clusters within the pars distalis and have a stellate to polygonal shape. We use cookies to help provide and enhance our service and tailor content and ads. inferior hypophyseal arteries. Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through the aorta. Pars distalis: This is the portion in which the majority of the hormone production occurs. The arterial supply is scarce, whereas the majority of blood reaches the parenchyma through the portal veins (Figure 17.3). The adenohypophysis consists of the pars intermedia and the pars distalis. During embryonic development, the adenohypophysis forms from an invagination (inpocketing) of the cell layer of the embryo that later becomes the roof of the mouth. The hypothalamus serves as the major regulator of the adenohypophysis. Gonadotrophs undergo hypertrophy and the cytoplasm becomes vacuolated due to distention profiles of endoplasmic reticulum with finely granular material in response to long-term stimulation by gonadotropin-releasing hormone (GnRH) from the hypothalamus. Prolactin cells, particularly those with larger granules, stain positively with erythrosine and carmoisine. By continuing you agree to the use of cookies. This tetradecapeptide inhibits the secretion of both growth hormone and TSH. These cells are distributed throughout the adenohypophysis, and are generally round to ovoid in shape. Of particular interest to our discussion of human reproductive biology are the final two hormones secreted by basophils of the pars distalis. Dopamine serves as the major prolactin inhibitory factor by suppressing virtually all aspects of prolactin secretion, and also inhibits cell division and DNA synthesis of this cell type. Immunoperoxidase stains have revealed that some cells contain both FSH and LH, whereas others contain only LH or FSH. Both sets of authors consider that there is no good reason to reject the classical view of the formation of the adenohypophysis from the ectoderm of the stomodeum. At the ultrastructural level, actively synthesizing cells have few secretory granules, but abundant granular endoplasmic reticulum and Golgi complexes. The chromophils can be further divided into acidophils (alpha cells) and basophils (beta cells). These vessels are invaded into the pars distalis together with the interdegitations of the anterior neurohypophysis. Given that capillaries of the pituitary are fenestrated, they lie outside the blood–brain barrier. These cells are called acidophils (phil, meaning “love”) because they have an affinity for acid dyes. In some situations, SRIH also inhibits the secretion of PRL and ACTH. drain primary capillary plexus of median eminence which deliver blood into secondary plexus located in pars distalis. The embryonic origin of the secretory cells in the adenohypophysis appears to be neural in origin similar to the neurohypophysis and hypothalamus. Hypertrophy and hyperplasia of TSH cells under these conditions are due to a lack of negative feedback by T3 and, to a lesser extent, T4 on the hypothalamus and thyrotrophs. ADVERTISEMENT: Radiopaedia is free thanks to our supporters and advertisers. Pars distalis. A Tsunoda, O Okuda, K Sato. The long portal vessels supply the adenohypophysis with 70–90% of blood and carry the hypothalamic releasing and inhibiting hormones that regulate secretion and multiplication of hormone-producing cells. The growth hormone-producing cells stain positively with orange C and eosin, and these cells typically have numerous secretory granules, which have an average diameter of 300 nm. If the first hypothesis were correct, one wonders where the neural crest cells would have come from in the latter situation. A reason for considering this second possibility less likely was that the various adenohypophyseal cell types are able to differentiate in hypophyseal pouches grown with heterologous mesenchyme. The internal plexus gives rise to long veins that contribute to the formation of the portal system. The distal part of the infundibulum differentiates to form the posterior pituitary (neurohypophysis) and retains the connection with the hypothalamus as the stalk. Dopamine also suppresses ACTH production by corticotrophs in the pars intermedia of some species. Introduction Measurements Situation Relations Presenting parts Anterior Lobe- Gross Anatomy , Histology , Function Posterior Lobe- Gross Anatomy , Histology , Function Development Blood Supply Pharyngeal Hypophysis Applied Anatomy ACTH-producing cells (corticotrophs) are round to ovoid, may be chromophobic or lightly basophilic (in humans), and can stain weakly positive with PAS. The pars tuberalis is a group of cells surrounding the pituitary stalk. Category 1 includes the glycoproteins, LH, FSH and TSH. Unable to process the form. Stellate cells typically have elongate processes and prominent cytoplasmic filaments (Figure 17.16). The blood supply derives from two groups of vessels coming off the internal carotid artery and drained by the hypophyseal vein. In the adult human, the pars intermedia is sparse or absent. Gonadotroph cells undergo a series of changes following castration, resulting in the formation of “gonadectomy cells.” As a result of the lack of negative feedback by gonadal steroids, gonadotrophs are actively stimulated to synthesize and secrete FSH and LH. Thyrotropin-producing (TSH) basophils or thyrotrophs account for about 5% of the cells of the adenohypophysis. 5, 7). Complete severance of hypothalamo-pituitary connections also caused atrophy of the pars nervosa and enlargement of the cells of the pars intermedia. At the ultrastructural level, actively synthesizing cells have few secretory granules but abundant granular endoplasmic reticulum and Golgi complexes. Follicular (stellate) cell, F; colloid, C. Bar: 1 μm. Prolactin-secreting acidophils (luteotrophs) compose 15–25% of the total cell population. Not uncommonly the diaphragma sellae is incomplete (considered a normal variant) and CSF may enter into the pituitary fossa and may give rise to the "empty sella sign" on neuroimaging. It lies between the more rostral optic chiasma, and the more caudal mammillary bodies. ACTH-producing cells (corticotrophs) are round to ovoid, may be chromophobic or lightly basophilic (in humans), and can stain weakly positive with PAS. These neurosecretory neurons are innervated by regular neurons (e.g., aminergic, cholinergic, serotonergic, and/or peptidergic) which influence their release. -. The gland is divided into anterior and posteriorparts, with different embryological origins reflecting a difference i… Molecular techniques can be used to detect mRNA in cases in which a cell has the genetic machinery to make a particular trophic hormone but does not (1) produce it in sufficient quantity for immunohistochemical detection or (2) release sufficient bioactive hormone into the circulation for detection by plasma assay. Rathke's cleft cyst. 12-4). The superior hypophyseal arteries give superior branches which supply the superior most infundibulum which receives axons from a number of hypothalamic nuclei. Large PAS-positive lipofuscin granules often are found within these cells. The growth hormone-producing cells stain positively with orange C and eosin, and these cells typically have numerous secretory granules, which have an average diameter of 300 nm. Some acidophils synthesize and secrete growth hormone (GH). The diaphragma sellae has a variably sized opening for the infundibular stalk. It has three parts: pars distalis, tuberalis, and intermedia. hypophyseal portal veins. The pituitary gland sits atop the base of the skull in a concavity within the sphenoid bone called the sella turcica (pituitary fossa), immediately below the hypothalamus and optic chiasm. They maintained that the neural ridges that flank the neural plate extended beyond the tip of the anterior neuropore onto the ventral surface of the head and reached the stomodeum as a “ventral neural ridge.” This was held to form a groove, the floor of which perforated through to the underlying prosocoele. 21). The adenohypophysis (adeno, meaning “glandular”) consists of three regions: the pars distalis, the pars intermedia, and the pars tuberalis (Figure 1.10). It is especially well developed in certain chelonians, crocodilians, most snakes, and in the anoline lizards. Immunoperoxidase stains have revealed that some cells contain both FSH and LH, whereas others contain only LH or FSH. Crenshaw WB, Chew FS. It is the distal part of the pituitary and forms the majority adenohypophysis. The pars distalis is the largest part of the pituitary gland. A.G. Watts, in Encyclopedia of Stress (Second Edition), 2007. Instead, they are controlled (inhibited) by dopamine released from hypothalamic neurons. The infundibulum extends from the tuber cinereum and pierces the diaphragma sella before being surrounded by the pars tuberalis. These granules however, are difficult to study in human glands since it is not easy to obtain fresh specimens. The pars tuberalis is the part of the adenohypophysis which surrounds the anterior aspect of the infundibular stalk. 18 (3): 551. The adenohypophysis represents the largest portion of the pituitary gland. The pituitary is usually divided (in practice) into anterior and posterior parts, which actually refers to groupings of four subparts 4,5: The anterior lobe of the pituitary is surrounded by a thin fibrous capsule, similar to that of salivary glands, continuous with thin septae which extend into the gland 11. Historically, trophic hormone–producing cells of the adenohypophysis were classified by their tinctorial characteristics as acidophils, basophils, or chromophobes. The pars tuberalis was derived from A(1) + A(2). and the . Ultrastructurally, gonadotrophs have granules that vary from 200 to 300 nm in diameter. Schematic diagram showing structure of the avian hypothalamus and pituitary gland. Category 3 includes the very similar smaller peptides, ACTH, a-MSH, and LPH. The other type of acidophil in the adenohypophysis synthesizes and secretes prolactin (PRL), which is also a large protein. Here neurosecretory fibres arise from a localized region of the hypothalamus, called the nucleus lateralis tuberis, and end in contact either with the various types of secretory cells or with blood capillaries related to them. Cells of pars distalis: three histochemocal types of cells: basophils (10%), acidophils (40%), and chromophobes (50%). The anterior pituitary is also known as the adenohypophysis or pars distalis and is derived from Rathke's pouch, a growth of the primitive oral cavity. Mosby. The neurohypophysis stores and releases two hormones made in the hypothalamus: oxytocin and vasopressin. Vasopressin acts on the kidneys to cause water retention. However, some endocrine glands or cells—for example, thyroid medullary C cells, parathyroid chief cells, and the adrenal medulla—are not under the influence of pituitary trophic hormones and are not regulated by a hypothalamic-hypophyseal-target organ axis. The main variations are in size/number 8: The pituitary gland has a dual origin (ectoderm of the primitive mouth cavity and neuroectoderm of the diencephalon) which reflects the two distinct parts in the adult gland. (2011) ISBN:1437720021. At approximately 24 days gestation the beginnings of the adenohypophysis and neurohypophysis begin to form as Rathke pouch, which forms as an ectodermal outpouching of stomodeum (primitive oral cavity lined by ectoderm) and the infundibulum which forms in the floor of the diencephalon (part of the neural tube). It consists of the pars distalis, a structure composed of strings of specialized cells that secrete hormones associated with growth and development (trophic hormones). The growth hormone-secreting acidophils (somatotrophs) constitute approximately 50% of the total cell population in the pars distalis (Figure 17.16). They tend to be concentrated in the dorsocephalic region of the pars distalis. Sparsely granulated acidophils (SG) have few mature secretory granules but many dilated cisternae of granular endoplasmic reticulum and well-developed Golgi regions (G). Pairs of the superior and inferior hypophysial arteries, both branches of the internal carotid arteries, enter the median eminence and form the superficial external plexus that gives rise to a network of capillaries and the gomitoli (also named internal plexus). There is no explanation for the absence of the pars tuberalis in squamate reptiles. The venous drainage of the pars distalis is exerted mainly by two large veins, which bilaterally leave the dorso-lateral region (venous pole) of the pars distalis and by a few small veins, which drain into the wide, sinus-like vessel, which curves around the dorso-caudal region of the pars distalis and joins bilaterally the vena hypophysea transversa. The pars tuberalis contains some stainable cell types and secretes tuberalin that stimulates PRL release. The granules have an average diameter of 250 nm and are frequently aligned along the plasma membranes. Each type of trophic hormone–producing cell in the pars distalis is under the control of a specific releasing hormone or factor from the hypothalamus (see Fig. The hypophyseal pouch was considered to arise from the ventral neural ridge between the optic chiasma and the stomodeum. 12-3). A second hypothalamic release-inhibiting hormone is somatostatin (somatotropin release-inhibiting hormone, or SRIH), which inhibits the secretion of both growth hormone and TSH. In mammals, the most important pathway is the sympathetic innervation of pinealocytes that is controlled by two retinal pathways both passing through the superior cervical ganglion. Gonadotropin-producing (GTH) basophils or gonadotrophs are relatively large round to oval cells, and account for approximately 10% of the cells concentrated in the dorsocephalic region in the pars distalis. (1980) could find no evidence for a “ventral neural ridge” or for incorporation of ventral ectoderm into the diencephalic floor. The Rhynchocephalia (Sphenodon punctatus), Chelonia (turtles), and Crocodilia (crocodiles, alligators, etc.) Corticotrophs are typically chromophobes, and somatotrophs are typically acidophils (Fig. This means the blood flows from the capillaries to … The adenohypophysis represents the largest portion of the pituitary gland. They synthesize proopiomelanocortin (POMC), which is cleaved first into adrenocorticotrophic hormone (ACTH) and then into α-melanocyte-stimulating hormone (MSH), β-endorphin, and corticotrophin-like intermediate peptide (CLIP), with little ACTH remaining. forming the anterior lobe of the gland, or pars distalis, secrete theirhormonesdirectlyinto the blood flowing through the sinusoids that run between the cells (Figs. The pituitary gland sits atop the base of the skull in a concavity within the sphenoid bone called the sella turcica (pituitary fossa), immediately below the hypothalamus and optic chiasm. Dopamine serves as the major prolactin inhibitory factor. For a more in-depth discussion refer to elevated prolactin (differential) article. The long portal vessels originate in the proximal part of the hypophysial stalk and terminate in fenestrated capillaries within the pars distalis. …is also found in the pars distalis of bony fishes. 10. Occasional growth hormone-containing secretory granules may measure up to 500 nm in diameter. The adenohypophysis has long been regarded as arising from the hypophyseal (Rathke's) pouch and the latter, as an outgrowth of the ectoderm of the stomodeum. Still other basophils in the adenohypophysis secrete the polypeptide hormones lipotropin (LPH) and melanophore-stimulating hormone (MSH). Other cells in the adenohypophysis, the basophils, stain darkly with basic dyes. The topographical anatomy of these vessels is described below, and more detailed descriptions of their distribution to the vertebral bodies, the spinal nerve roots and intervertebral discs are provided under separate headings. The growth hormone-secreting acidophils (somatotrophs) constitute approximately 50% of the total cell population in the pars distalis (Fig. It is composed of cords of epithelial cells individually specialized to secrete trophic hormones acting on various target organs (see below).
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