 Thyrotropes (TSH cells) constitute about 5% of the parenchymal cells in the anterior lobe of the pituitary gland. These large, polygonal cells with round and eccentric nuclei produce thyrotropic hormone called thyroid-stimulating hormone (TSH), which acts on the follicular cells of the thyroid gland to stimulate production of thyroglobulin and thyroid hormones. Thyreotropes exhibit cytoplasmic basophilia (basophils) and stain positively with PAS reagent. Release of TSH is under the hypothalamic control of thyrotropin-releasing hormone (TRH), which also stimulates secretion of PRL.
Distinctive characteristics of the five cell types of the anterior lobe of the pituitary gland are readily seen with transmission electron microscopy (TEM). These characteristics are summarized in Table 21.3.
In addition to the five types of hormone-producing cells, anterior lobe of the pituitary gland contains folliculostellate cells.
Folliculo-stellate cells present in the anterior lobe of the pituitary gland are characterized by a starlike appearance with their cytoplasmic processes encircling hormone-producing cells. They have the ability to make cell clusters or small follicles, and they do not produce hormones. Folliculostellate cells are interconnected by gap junctions containing connexin-43 protein. Based on immunocytochemical and electrophysiological studies, it is hypothesized that the network of folliculo-stellate cells interconnected by gap junctions transmits signals from the pars tuberalis to pars distalis. These signals may regulate hormone release throughout the anterior lobe of the pituitary gland. Thus, the folliculo-stellate network may appear to function in addition to the hypophyseal portal vein system. Recent in vitro findings of gap junctions interconnecting not only folliculo-stellate cells but also hormone-producing cells support this proposed signaling mechanism in the anterior lobe of the pituitary gland.
Pars Intermedia. The pars intermedia surrounds a series of small cystic cavities that represent the residual lumen of Rathke’s pouch. The parenchymal cells of the pars intermedia surround colloid-filled follicles. The cells lining these follicles appear to be derived either from folliculo-stellate cells or various hormone-secreting cells. TEM reveals that these cells form apical junctional complexes and have vesicles larger than those found in the pars distalis. The nature of this follicular colloid is yet to be determined; however, often cell debris is found within it. The pars intermedia contains basophils and chromophobes (Fig. 21.8). Frequently, the basophils and cystic cavities extend into the pars nervosa.
The function of the pars intermedia cells in humans remains unclear. From studies of other species, however, it is known that basophils have scattered vesicles in their cytoplasm that contain either or -endorphin (a morphine-related compound). In frogs, the basophils produce MSH, which stimulates pigment production in melanocytes and pigment dispersion in melanophores. In humans, MSH is not a distinct, functional hormone but is a byproduct of LPH posttranslational processing. Because MSH is found in the human pars intermedia in small amounts, the basophils of the pars intermedia are assumed to be corticotropes.