Most of the blood from the pituitary gland drains into the cavernous sinus at the base of the diencephalon and then into the systemic circulation. Some evidence suggests, however, that blood can flow via short portal veins from the pars distalis to the pars nervosa and that blood from the pars nervosa may flow toward the hypothalamus. These short pathways provide a route by which the hormones of the anterior lobe of the pituitary gland could provide feedback directly to the brain without making the full circuit of the systemic circulation.
The nerves that enter the infundibulum and pars nervosa from the hypothalamic nuclei are components of the posterior lobe of the pituitary gland (see the section that follows on the posterior lobe). The nerves that enter the anterior lobe of the pituitary gland are postsynaptic fibers of the autonomic nervous system and have vasomotor function.
Structure and Function of the Pituitary Gland
Anterior Lobe of the Pituitary Gland (Adenohypophysis)
The anterior lobe of the pituitary gland regulates other endocrine glands and some nonendocrine tissues.
Most of the anterior lobe of the pituitary gland has the typical organization of endocrine tissue. The cells are organized in clumps and cords separated by fenestrated sinusoidal capillaries of relatively large diameter. These cells respond to signals from the hypothalamus and synthesize and secrete a number of pituitary hormones. Four hormones of the anterior
FIGURE 21.6 • 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 hypothalamus. The system is activated in response to low thyroid hormone levels or in response to metabolic needs. TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone (thyrotropin).
lobe—adrenocorticotropic hormone (ACTH), thyroid-stimulating (thyrotropic) hormone (TSH, thyrotropin), follicle-stimulating hormone (FSH), and luteinizing hormone (LH)—are called tropic hormones because they regulate the activity of cells in other endocrine glands throughout the body (Fig. 21.6). 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. The general character and effects of the pituitary hormones of the anterior lobe are summarized in Table 21.1.
Pars Distalis. 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. 21.7), histologists 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.