An anterior pituitary hormone is secreted (in micro gram quantities) into the circulation from which it reaches its specific receptor in a target cell. A signal transduction process, specific to the hormone, its receptor, and the target cell, ensues which culminates in the release of the target cell hormone, usually in milligram or high microgram quantities. As one example shown in Figure 1, ACTH stimulates the adrenal cortex to secrete the glucocorticoid that is active in humans, cortisol. Another example is TSH, which stimulates the secretion of thy roid hormone from the thyroid gland. The mechanisms by which the pituitary hormones stimulate the secretion of the peripheral hormones will be dis cussed in more detail in the chapters that focus on the peripheral hormones.
Fig1. Summary of hypothalamic-pituitary-hormonal systems. Specific systems that operate according to the principles outlined in Figure 2 are shown. For each, the predominant hypothalamic releasing hormone (in green) or release-inhibiting factor (red) is shown. The main target tissues of the anterior pituitary hormones are indicated, along with the hormones they produce and, in the green boxes, major biological actions.
Fig2. Overview of hypothalamic-pituitary-peripheral systems. An archetypal cascade of hormonal events is shown, beginning with an external or internal environmental signal. These signals (green boxes) are transmitted to the central nervous system and then propagated to the hypothalamus, the anterior pituitary (adenohypophysis), and the peripheral target gland (pink boxes) for generating the appropriate biological response. The order of magnitude of the amount of hormone released in each of the three final steps is given in parentheses. The increasing amounts of hormones secreted at each step provide for amplification and the cascade is kept in check by both short (e.g., anterior pituitary to hypothalamus) and long (peripheral gland to hypothalamus) negative feedback loops.
Finally, it is important to understand that, in several cases, an anterior pituitary (trophic) hormone, in addition to stimulating hormone secretion from its distal target cell, may also be a growth factor for these cells and therefore the gland. For example, ACTH, in addition to causing the secretion of cortisol from the adrenal gland, is also required for the survival of the cells of the adrenal (zona fasciculata cells) that produce cortisol. And, as discussed in Chapter 5, the growth and development of the thyroid gland is highly dependent on TSH from the pituitary gland.
