Hypophysis Anterior lobe

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. 

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). 

The anterior lobe of the hypophysis (70-80% of the total weight of the gland) is made of several types of cells that are arranged in long columns forming a complex network. The meshes of the network are filled with small amounts of connective tissue and large capillaries. In some areas of the anterior lobe close to the pars intermedia, these cells are arranged to form pedicles that contain a colloid material. 

Acromegaly and gigantism result from (1) hypersecretion of some hormones of the anterior lobe of the hypophysis (especially the growth hormone), (2) decreased secretion of some other anterior lobe hormones, particularly gonadotropins, and (3) compression of the surrounding tissue at the base of the skull. 

Hypopituitarism may involve all the hormones of the anterior hypophysis, or it may be restricted to one or a few hormones of the anterior lobe, depending on the patient s age. General hypopituitarism of the anterior lobe results in pituitary dwarfism, infantilism, and Simmonds cachexia. The restricted forms of hypopituitarism lead to well-known clinical syndromes such as Frohlich s and the Laurence-Moon-Biedl syndrome. 

Tumors of the anterior lobe of the hypophysis and the midbrain are responsible for the development of 30-40% of the cases of diabetes insipidus. Chronic encephalitis causes diabetes insipidus in 8-15% of the cases, but in more than 20% of the cases the anatomical reasons for the development of diabetes insipidus are not known. In an idiopathic, hereditary form of diabetes insipidus, no anatomical changes can be recognized. Occasionally, Hand-Schiiller-Christian disease is responsible for the development of diabetes insipidus. 

ACTH is secreted in the anterior lobe of the hypophysis and, although it is not clear which cells secrete ACTH, the basophils are considered the source of ACTH because the cytoplasm of these cells is degranu-lated in stress, and basophilic tumors of the hypophysis develop in Cushing s disease. These arguments are not accepted by all it has been suggested that the hyperplasia of the cells of the anterior hypophysis that develops after adrenalectomy involves either chromophobe or acidophilic cells. The treatment of a Cushing s adenoma preparation with fluorescent anti-ACTH serum yielded fluorescence in the chromophobic cells. In adrenalectomized rats injected with tritiated glycine, the isotopes accumulate in the chromophobic cells therefore, it has been concluded that these cells synthesize ACTH. 

Control of ACTH Secretion. While the neurohypophysis is technically part of the brain, the anterior lobe of the hypophysis is disconnected from the central nervous system except for the existence of a common circulatory system, the portal system of the hypophysis. These differences between the anatomical structures of the anterior and posterior lobes are at the origin of the differences between the mechanisms controlling ACTH secretion. The secretion of ACTH is probably not triggered by any neurosecretory mechanism, like that triggering vasopressin secretion. 

In fact, ACTH secretion is regulated by a neuro-hormonal mechanism and a humoral feedback mechanism. The neurohormonal control of ACTH secretion involves what has been referred to as a third-order neuroendocrine mechanism. Three steps are involved in this control. A stimulus (stress, for example) brings the central nervous system to secrete a hormone (CRF), which acts on another endocrine structure in the anterior lobe of the hypophysis where it induces the secretion of second hormone (ACTH), which ultimately acts on the target endocrine gland—the adrenal cortex [40, 56-60]. 

The portal vessels of the hypophysis play a considerable role in the physiology of the hypophysis. They constitute an important link between the hypothalamus and the anterior lobe of the hypophysis. When the hypophysis is disconnected from its stalk and its normal vascular system and is transplanted at a dis-... 

The pituitary body, or hypophysis, is double in origin and multiple in functions. From the stomodceal pouch of Rathke is developed the anterior lobe and the -pars intermedia from the floor of the third ventricle are formed the pars nervosa of the posterior lobe, the infundibulum and the tuber cinereum. The pituitary autacoids are of two types secretions acting directly on other tissues, and hormo-kinetic secretions acting indirectly by stimulating other endocrine glands. 

The hypophysis (pituitary gland) is an accessory gland of the brain. It consists of two anatomically distinct organs, the anterior lobe and the posterior lobe, and each lobe elaborates several different hormones. A middle lobe (pars intermedia developed to different extents in different animals), produces another hormone. The second part of Table XVIII (Section 1) lists all the different hypophyseal hormones. Many hormones of the anterior lobe are master hormones (Section 1). 

Hormones of the Anterior Lobe of the Hypophysis. Table XVIII in Section 2 shows that the anterior lobe produces six different hormones somatotropin, thyrotropin, corticotropin, luteotropic hormone, follicle-stimulating, and interstitial cell-stimulating hormones. The last three hormones are grouped together under the generic term gonadotropic hormones. 

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. 

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]. 

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]. 

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