 Lactotropes (PRL cells, mammotropes) constitute 15% to 20% of the parenchymal cells in the anterior lobe of the pituitary gland. These are large, polygonal cells with oval nuclei. They produce prolactin (PRL). In their storage phase, lactotropes exhibit numerous acidophilic vesicles (the histologic feature of an acidophil). When the content of these vesicles is released, the cytoplasm of the lactotrope does not stain (the histologic feature of a chromophobe). Secretion of PRL is under inhibitory control by dopamine, the catecholamine produced by the hypothalamus. However, thyrotropin-releasing hormone (TRH) and vasoactive inhibitory peptide (VIP) are known to stimulate synthesis and secretion of PRL. During pregnancy and lactation, these cells undergo hypertrophy and hyperplasia, causing the pituitary gland to increase in size. These processes account for the larger size of the pituitary gland in multiparous women.
 Corticotropes (ACTH cells) also constitute 15% to 20% of the parenchymal cells in the anterior lobe of the pituitary gland. These polygonal, medium-sized cells with round and eccentric nuclei produce a precursor molecule of adrenocorticotropic hormone (ACTH), known as proopiomelanocortin (POMC). Corticotropes stain as basophils and also exhibit a strong positive reaction with periodic acid–Schiff (PAS) reagent, because of the carbohydrate moieties associated with POMC. POMC is further cleaved by proteolytic enzymes within the corticotrope into several fragments, namely ACTH, -lipotrophic hormone (-LPH), melanocyte-stimulating hormone (MSH), -endorphin, and enkephalin. ACTH release is regulated
FIGURE 21.7 • Pars distalis. This specimen of the pars distalis is stained with brilliant crystal scarlet, aniline blue, and Martius yellow to distinguish the various cell types and connective tissue stroma. The cords of cells are surrounded by a delicate connective tissue stroma stained blue. The sinusoidal capillaries are seen in close association with the parenchyma and contain erythrocytes stained yellow. In the region shown here, the acidophils (Ac) are the most numerous cell type present. Their cytoplasm stains cherry red. The basophils (Bas) stain blue. The chromophobes (Ch), although few in number in this particular region, are virtually unstained. 640.
by corticotropin-releasing hormone (CRH) produced by the hypothalamus.
 Gonadotropes (FSH and LH cells) constitute about 10% of the parenchymal cells in the anterior lobe of the pituitary gland. These small, oval cells with round and eccentric nuclei produce both luteinizing hormone (LH) and follicle-stimulating hormone (FSH). They are scattered throughout the pars distalis and stain intensely with both basic stains (thus classifying them as the basophil cell type) and PAS reagent. Many gonadotropes are capable of producing both FSH and LH. However, immunocytochemical studies indicate that some gonadotropes may produce only one hormone or the other. The release of FSH and LH is regulated by gonadotropin-releasing hormone (GnRH) produced by the hypothalamus. Both FSH and LH play an important role in male and female reproduction, which is discussed in Chapters 22 and 23.