Y

thyroid gland thyroid hormones FIGURE 20.5

Interaction of the hypothalamus, anterior lobe of the pituitary gland, and thyroid gland. Production of thyroid hormones is regulated through a negative feedback system. The thyroid hormone can feed back on the system and inhibit further release of thyroid hormones. Such inhibition occurs at the level of the anterior lobe and the hypothalmus. The system is activated in response to low thyroid hormone levels or in response to metabolic needs. TRH, thyrotropin-re-leasing hormone; and TSH, thyroid-stimulating hormone.

The two remaining hormones of the anterior lobe, growth hormone (GH) and prolactin (PRL), are not considered tropic because they act directly on target organs that are not endocrine in nature. The general character and effects of the pituitary hormones of the anterior lobe are summarized in Table 20.1.

Pars Distal is

The cells within the pars distalis vary in size, shape, and staining properties. The cells are arranged in cords and nests with interweaving capillaries. Early descriptions of the cells within the pars distalis were based solely on the staining properties of secretory vesicles within the cells. Using mixtures of acidic and basic dyes (Fig. 20.6), histol-ogists identified three types of cells according to their staining reaction, namely, basophils (10%), acidophils (40%), and chromophobes (50%). However, this classification contains no information regarding the hormonal secretory activity or functional role of these cells.

Histochemical, physiologic, and immunocytochemical methods better define the functions of the cell types in the pars distalis

The three methods used to define the functions of the different cell types of the pars distalis are

• Histochemistry. The use of histochemical stains for specific chemical groups, such as the periodic acid-Schiff (PAS) reaction for carbohydrates of glycoproteins, trichrome stains (Mallory's, Cleveland-Wolfe, etc.), and other empirically derived combinations of stains (e.g., aldehyde-fuchsin), enables investigators to characterize the cells further. Acidophils and basophils have been subdivided into smaller groups that more closely correlate staining properties with function, as summarized in Table 20.2.

• Histophysiologic studies. The cells in Table 20.2 are characterized as acidophils and basophils on the basis of their staining characteristics. The role of the cells, however, was identified by combining information on staining properties with changes in the number, size, and staining intensity of the cells as the target organs of the pituitary hormones were physiologically manipulated. Pathologic conditions were also correlated with the absence or overgrowth of specific hypophyseal cells. These physiologic and pathologic observations have led to the recognition that many of the cells originally identified as chromophobes are actually transiently degranulated forms of secretory cells.

• Electron microscopy and immunocy to chemistry. Ultra-structurally, the cells of the anterior lobe of the pituitary gland show relatively distinctive characteristics based on comparison of cell size and shape, degree of development of cytoplasmic organelles, and secretory vesicle size, density, and distribution (Table 20.3).

Five functional cell types are identified in the pars distalis on the basis of immunocytochemical reactions

All known hormones of the anterior lobe of the pituitary gland are small proteins or glycoproteins. This important

TABLE 20.1. Hormones of the Anterior Lobe of the Pituitary Gland

Hormone

Composition

MW (kDa)

Major Functions

Growth hormone (somatotropin, GH)

Straight-chain protein (191 aa)

21,700

Stimulates liver and other organs to synthesize and secrete insulin-like growth factor I (IGF-I), which in turn stimulates division of progenitor cells located in growth plates and in skeletal muscles, resulting in body growth

Prolactin (PRL)

Straight-chain protein (198 aa)

22,500

Promotes mammary gland development; initiates milk formation; stimulates and maintains secretion of casein, lactalbumin, lipids, and carbohydrates into the milk

Adrenocorticotropic hormone (ACTH)

Small polypeptide (39 aa)

4,000

Maintains structure and stimulates secretion of glucocorticoids and gonadocorticoids by the zona fasciculata and zona reticularis of the adrenal cortex

Follicle-stimulating hormone (FSH)

2-chain glycoprotein" («, 92 aa; fi, 111 aa)

28,000

Stimulates follicular development in the ovary and spermatogenesis in the testis

Luteinizing hormone (LH)

2-chain glycoprotein" (a, 92 aa; y3,116 aa)

28,300

Regulates final maturation of ovarian follicle, ovulation, and corpus luteum formation; stimulates steroid secretion by follicle and corpus luteum; in males, essential for maintenance of and androgen secretion by the Leydig (interstitial) cells of the testis

Thyrotropic hormone (TSH)

2-chain glycoprotein" (a, 92 aa; p, 112 aa)

28,000

Stimulates growth of thyroid epithelial cells; stimulates production and release of thyroglobulin and thyroid hormones

'"The u chains of FSH, LH, and TSH are identical and encoded by a single gene; the ß chains are specific for each hormone.

'"The u chains of FSH, LH, and TSH are identical and encoded by a single gene; the ß chains are specific for each hormone.

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