Hypophysis anterior

Pituitary (hypophysis) Anterior (adenophypophysis) Base of the brain Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Gonadotropins (follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Growth hormone (GH), prolactin, melanocyte-stimulating hormone (MSH)... 

In a hyperfunctioning of the thyroid gland, secretion of an excess quantity of thyroid hormones leads to a hyperthyroid condition (Basedow s disease, goiter). In this condition, drags are used that suppress production of thyrotropic hormones in the anterior lobe of the hypophysis (diiodotyrosine), in the thyroid gland (propylthiouracil, methylthiouracil,... 

Drugs used for hyperthyroidism can be classified as drugs that suppress thyroid hormone synthesis in the anterior lobe of the hypophysis, and they consist of diiodotyrosine and iodine, as well as drugs that suppress thyroid hormone synthesis in thyroid glands (propylthiouracil, methylthiouracil, methimazole, and carbimazole). 

Diiodotyrosine does not possess pronounced hormonal activity. However, it stops production of thyrotropic hormone by the anterior lobe of the hypophysis, which activates thyroid gland activity. 

Formation of these drugs is under the direct control of a polypeptide adrenocorticotropic hormone (ACTH, corticotropin), which is processed by the anterior lobe of the hypophysis. Human ACTH consists of 39 amino acids and has a molecular weight of about 4500. It differs from animal ACTH in the amino acid compositions at positions 29-33. 

Pituitary Hormones. The hormones of the hypophysis (pituitary gland) are quite numerous, being secreted variously in three parts of the gland — the ncurohypophysis (posterior lobel. the adenohypophysis (anterior lithe), and the pars intermedia, which connects the other two. 

The TIDA neurosecretory neurons terminating in the median eminence lack true synapses and the DA released from these neurons into the extracellular fluid diffuses through fenestrated capillaries of the hypophysial portal system where it is transported to the anterior pituitary (Fig. 3 Bottom Panel). Considering this unique cytoarchitecture, it is not surprising that the TIDA neurons are reported to lack inhibitory autoreceptors (Demarest and Moore, 1979b Timmerman et al., 1995a), have lower levels of DA transporter mRNA (Meister and Elde, 1993) and protein (Ciliax et al., 1995 Revay et al.,... 

Regulation of growth harmone (GH) secretion in mammals involves a complex interaction between inhibitory (somatostatin) and stimulatory (GH releasing hormone GHRH) neuropeptides synthesized and released by neurosecretory neurons terminating in the median eminence (Tuomisto and Mannisto, 1985 McMahon et al., 2001). Somatostatin and GHRH are transported in the hypophysial portal blood to the anterior pituitary... 

The master coordinator of hormonal activity in mammals is the hypothalamus, which acts on input that it receives from the central nervous system. Hormone secretion from the anterior pituitary gland is regulated by releasing hormones secreted by the hypothalamus. Neuroendocrine neurons in the hypothalamus project axons to the median eminence, at the base of the brain these neurons release substances into small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophysial portal vessels). While the anterior pituitary... 

GRF has been isolated and characterized from a human tumor of the pancreas (61) as well as from rat (62), porcine (63), bovine (64), caprine (65), ovine (65), and human (65) hypothalamus stalk-median eminence. It is a 40-44-amino-acid peptide hormone produced in the arcuate nucleus of the hypothalamus and released from neurosecretory nerve terminals of these arcuate neurons, and it is carried by the hypothalamo-hypophysial portal circulation to the anterior pituitary gland where it stimulates growth hormone (GH) secretion (66). 

Two neuropeptides, corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) are released from parvoceUular neurons in the hypothalamic PVN to initiate a stress response. The terminal endings of these neurons, located in the median eminence of the hypothalamus, release CRH and AVP into the hypothalamic-hypophysial portal vessel system, where they travel to the anterior pituitary. The two neuropeptides act syn-ergistically on pituitary corticotrophs to activate the synthesis of pro-opiomelanocortin (POMC). This peptide, discussed in detail below, is processed to produce several peptides including adrenocorticotrophic hormone (ACTH), or corticotropin. ACTH released from corticotrophs travels via the bloodstream to act on cells in the zona fasciculata layer of the adrenal cortex, stimulating the synthesis and release of the glucocorticoids, cortisol (in humans) or corticosterone (in rodents). 

The pituitary gland (hypophysis) is located at the base of the skull (Figure 50-1) in a bone cavity called the sella turcica (Turkish saddle). The gland is small—1 cm or less in height and width and weighs approximately 500 mg. As discussed previously, the gland is anatomically divided into the anterior (adenohypophysis) and the posterior (neurohypoph-ysis) lobes. A third lobe (the intermediate lobe) is present in most vertebrates and in the human fetus this lobe is rudimentary in the adult human. 

The hypothalamus is a small region of the brain in the ventral aspect of the diencephalon. In the adult human, it is about 2.5 cm in length and weighs about 4 g. Ventromedi-ally, it surrounds the third ventricle and is continuous with the infundibular stalk of the pituitary (hypophysis). This cone-shaped region of the hypothalamus, the median eminence, consists mainly of axonal fibers from hypothalamic neurons, which either terminate in the median eminence or continue down into the posterior lobe of the pituitary, and it is perfused by a capillary network (primary plexus) derived from the carotid arteries. Blood from the primary plexus is transported by portal vessels (hypophyseal portal vessels) to another capillary network (secondary plexus) in the anterior lobe of the pituitary (adenohypophysis) (Figure 31-1). 

The pituitary gland, also referred to as the hypophysis, is located at the base of the brain in a cavity of the sphenoid bone known as the sella turcica. The pituitary is separated from the brain by an extension of the dura mater known as the diaphragma sella. The pituitary is a very small gland, weighing between 0.4 and 1 g in adults. It is divided into two distinct regions, the anterior lobe, or adenohypophysis, and the posterior lobe, or the neurohypophysis (see Fig. 75-1). 

Thyrotropin-releasing factor (TRF) is produced in the hypothalamus and arrives at the pituitary via the hypophyseal portal blood system. TRF mediates the release of thyrotropin from the anterior hypophysis. The thyrotropin release is inhibited by thyroxine, presumably free thyroxine, and the inhibition is dose dependent. Thus excessive levels of thyroxine depress thyrotropin release, and lower thyroxine levels result in increased thyrotropin release from the anterior pituitary. TSH, in turn, stimulates thyroid hormone synthesis and secretion by the thyroid gland. Thyroxine and triiodothyronine are bound to specific binding proteins in the blood. The amounts and binding constants of the specific thyroid-binding proteins, together with the rate of thyroid hormone release from the thyroid, determine the amount of free thyroxine in the blood. Free thyroxine levels are determined not only by the rate of... 

Lipotropin, lipotropic hormone, LPH, a polypeptide hormone from the hypophysis stimulating the mobilization of lipids from lipid depots. fi-LPH (91 aa Mr 10 kDa) is biosyntheticaUy formed from the precursor proopiomelanocortin (PMOC). S-LPH is released from PMOC in the anterior and intermediate lobes of the pituitary gland, whereas in the intermediate lobe only fi-LPH is split to y-LPH (corresponds to fi-LPH 1-58) and -endorphin [A. F. Bradbury et al., Biochem. Biophys. Res. Commun. 1976, 69,950 J. Bogard et al.,/. Biol. Chem. 1995, 270, 23038]. 

Biosynthesis of TRH occurs in a wide area of the hypothalamus and appears to be under the control of a nonribosomal (soluble) enzyme system, TRH synthetase (R5), which is activated by norepinephrine (G15). TRH is stored in the median eminence, from which is secreted into the hypophysial venous portal system to be transported to the anterior pituitary gland (R2). There it is specifically bound to membrane receptors (G13, W3) and activates adenyl cyclase, leading to increased production of cyclic adenosine monophosphate (K1). 

It is now firmly established that the synthesis and release of the hormones of the adenohypophysis are controlled by substances released from nerve endings in the hypothalamus and conveyed to the anterior pituitary gland via the portal vessels. The evidence which demonstrated the fundamental importance of the hypothalamus and the hypothalamo-hypophysial portal vessels came mainly from experiments which involved transection of the pituitary stalk, transplantation of the pituitary gland to a site remote from the sella turcica or electrical stimulation of the hypothalamus. 

The simple experiment of removing the pituitary and from the sella turcica to another site of the body also demonstrated the role of the hypothalamus in the control of anterior pituitary function. When the transplanted tissue is placed in a site remote from the sella turcica, for example, the anterior chamber of the eye, the kidney capsule or temporal lobe of the brain, partial or complete atrophy of the ovaries, testes, reproductive tracts, thyroid gland and adrenal cortices is observed together with a reduction in the rate of body growth. When, however, the transplanted tissue is placed in the vicinity of the hypothalamus and pituitary stalk, regeneration of the hypophysial portal vessels occurs and anterior pituitary function is restored as shown by the maintenance of reproductive, thyroid and adrenocortical activity [5]. 

Muscular degeneration and mild hyalinization of the muscles have also been described. In addition to the alteration described in the nervous system and muscles, changes in the endocrine glands frequently observed in experimental thiamine deficiency include hypertrophy of the thyroid, anterior hypophysis, adrenal, and islet of the pancreas. 

The hypophysis is a small, bean-shaped organ (0.6-0.65 g) located in the sella turcica at the base of the skull. On close examination of the gland, an anterior yellowish and a posterior more fibrous and whitish part can be distinguished. The two parts are separated by a pars intermedia, a lamellar structure, grayish and fibrous in appearance. The gland is attached to the floor of the third ventricle by a hollow pedicle, the stalk of the hypophysis. 

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