Review of Medical Physiology - William F. Ganong 2002

Endocrine System, Metabolism, and Reproduction
Pituitary Gland
Histology

In the posterior lobe, the terminals of supraoptic and paraventricular axons are located close to Blood Vessels. In some species, these terminals have a brush-like Structure. Pituicytes are also distinguished as stellate Cells containing lipid droplets; they were previously thought to secrete Posterior Pituitary Hormones, but today they are regarded as modified astroglia.

The intermediate lobe is vestigial in humans and other mammals: most of its cells are agranular, although basophilic elements, which are actually anterior lobe cells, are frequently found. Small thyroid-like follicles are arranged along the residual cleft, some containing a small amount of colloid. The function of this colloid remains unknown.

The anterior lobe of the Pituitary Gland is composed of interweaving cords of cells and a branching network of sinusoidal capillaries. The capillary endothelium is fenestrated and resembles that of other endocrine Organs. The cells contain hormone-laden granules that leave The Cell via exocytosis. Since the penetration of intact granules into capillaries has not been observed, it is likely that they disintegrate in the pericapillary zone.

Cell types of the anterior pituitary gland

Human anterior pituitary cells are traditionally classified based on their staining reactions into chromophobic and chromophilic cells. Accordingly, chromophilic cells include acidophils, which are stained by acidic Dyes, and basophils, which are stained by basic dyes. Many chromophobes are secretory cells that remain inactive and contain few secretory granules. Other cells are stellate (follicular) cells (see below). Using immunocytochemical and electron microscopic Methods, five distinct types of secretory cells have been identified: somatotrophs, which secrete Growth Hormone; lactotrophs (also called mammotrophs), which secrete prolactin; thyrotrophs, which secrete TSH; gonadotrophs, which secrete LH and FSH; and corticotrophs, which secrete ACTH and ß-LPH. The characteristics of these cells are summarized in Table 22-2. Their appearance and granule content may vary with changes in hormonal status. Notably, according to this Classification, two or more polypeptide hormones can be secreted by multiple cell types.

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Fig. 22-1. Diagram showing The Development of the pituitary gland and its various parts in adults.

The anterior pituitary also contains stellate cells that extend processes between the secretory cells. These cells have been shown to contain and secrete the cytokine interleukin-6 (IL-6) (see Chapter 27), although their physiological role remains unclear.

Two-subunit structure of FSH, LH, and TSH

All three pituitary glycoprotein hormones—FSH, LH, and TSH—are composed of two subunits. Each subunit (designated as a and ß) possesses some intrinsic activity, but they must combine to elicit maximum physiological action. Furthermore, the placental glycoprotein human chorionic gonadotropin (hCG) also contains a- and ß-subunits (see Chapter 23). The a-subunits of all these hormones are products of a single Gene, sharing an identical Amino Acid Composition yet differing in carbohydrate residues. The ß-subunits, produced by different genes, vary in structure and confer hormonal Specificity. The a-subunits are highly interchangeable, which can lead to The formation of hybrid molecules. The physiological and Evolutionary Significance of the unique two-subunit structure of these glycoprotein hormones is not yet fully understood.