TABLE Hormones of the Anterior Lobe of the Pituitary Gland 21.1 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 gluco-corticoids and gonadocorticoids by the zona fasciculata and zona reticularis of the adrenal cortex Follicle-stimulating hormone (FSH) 2-chain glycoproteina (, 92 aa; , 111 aa) 28,000 Stimulates follicular development in the ovary and spermato-genesis in the testis Luteinizing hormone (LH) 2-chain glycoproteina (, 92 aa; , 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 glycoproteina (, 92 aa; , 112 aa) 28,000 Stimulates growth of thyroid epithelial cells; stimulates production and release of thyroglobulin and thyroid hormones aThe chains of FSH, LH, and TSH are identical and encoded by a single gene; the chains are specific for each hormone. kDa, kilodaltons; aa, amino acids.
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 fact has led to definitive identification of specific cell types by immunocytochemistry (Table 21.2). These studies have classified cells of the anterior lobe of the pituitary gland into five cell types:  Somatotropes (GH cells) are most commonly found within the pars distalis and constitute approximately 50% of the parenchymal cells in the anterior lobe of the pituitary gland. These medium-sized, oval cells exhibit round, centrally located nuclei and produce growth hormone (GH; somatotropin). The presence of eosinophilic vesicles in their cytoplasm classifies them as acidophils. Three hormones regulate the release of GH from somatotropes. Two of these hormones are opposing hypothalamic-releasing hormones: growth hormone–releasing hormone (GHRH), which stimulates GH release from the somatotropes, and somatostatin, which inhibits GH release from the somatotropes. Recently, a third hormone, a 28-amino acid peptide ghrelin, was isolated from the stomach. It is a potent stimulator of GH secretion and appears to coordinate food intake with GH secretion. Hormonally active tumors that originate from somatotropes are associated with hypersecretion of GH and cause gigantism in children and acromegaly in adults.