A feedback system regulates endocrine function at two levels: hormone production in the pituitary gland and hypothalamic releasing hormone production in the hypothalamus.
The circulating level of a specific secretory product of a target organ, a hormone or its metabolite, may act directly on the cells of the anterior lobe of the pituitary gland or the hypothalamus to regulate the secretion of hypothalamic releasing hormones (see Fig. 21.6). The two levels of feedback
FIGURE 21.10 • Electron micrograph of rat posterior lobe.
Neurosecretory granules and small vesicles are present in the terminal portions of the axonal processes of the hypothalamohypophyseal tract fibers. Capillaries with fenestrated endothelium are present in close proximity to the nerve endings. 20,000. (Courtesy of Drs. Sanford L. Palay and P. Orkland.) allow exquisite sensitivity in the control of secretory function. The hormone itself normally regulates the secretory activity of the cells in the hypothalamus and pituitary gland that regulate its secretion.
In addition, information from most physiologic and psychologic stimuli that reach the brain also reaches the hypothalamus. The hypothalamohypophyseal feedback loop provides a regulatory path whereby general information from the CNS contributes to the regulation of the anterior lobe of the pituitary gland and, consequently, to the regulation of the entire endocrine system. The secretion of hypothalamic regulatory peptides is the primary mechanism by which changes in emotional state are translated into changes in the physiologic homeostatic state.
The pineal gland (pineal body, epiphysis cerebri) is an endocrine or neuroendocrine gland that regulates daily body rhythm. It develops from neuroectoderm of the posterior portion of the roof of the diencephalon and remains attached to the brain by a short stalk. In humans, it is located at the posterior wall of the third ventricle near the center of the brain. The pineal gland is a flattened, pine cone–shaped structure, hence its name (Fig. 21.11). It measures 5 to 8 mm high and 3 to 5 mm in diameter and weighs between 100 and 200 mg.
The pineal gland contains two types of parenchymal cells: pinealocytes and interstitial (glial) cells.
Pinealocytes are the chief cells of the pineal gland. They are arranged in clumps or cords within lobules formed by connective tissue septa that extend into the gland from the pia mater that covers its surface. These cells have a large, deeply infolded nucleus with one or more prominent nucleoli and contain lipid droplets within their cytoplasm. When examined with the TEM, pinealocytes show typical cytoplasmic