Somatotropin (growth hormone, GH), a 22-kDa protein New York, NY: McGraw-Hill 2008 McKinley M, O'Loughlin VD.

(Reproduced, with permission, from McKinley M, O'Loughlin VD. The primary plexus surrounds the infundibulum and median eminence, and the second is throughout the pars distalis and drains into the hypophyseal veins. (b) The hypothalamic-hypophyseal portal system, with blood from the superior hypophyseal artery, consists of two capillary networks connected by the hypophyseal portal vein. (a) The hypothalamic-hypophyseal tract consists of axons extending from the hypothalamic supraoptic and paraventricular nuclei, through the infundibulum and into the pars nervosa of the posterior pituitary, where peptide hormones they carry are released for capillary uptake. Endocrine glands are often also target organs for other hormones that can establish a feedback mechanism to control hormone secretion and keep blood hormonal levels within strict limits. For example, insulin-like growth factor (IGF) produced by several cell types may act on the same cells that produced it. In autocrine secretion, cells may produce molecules that act on themselves or on cells of the same type.

Juxtacrine signaling is particularly important in embryonic and regenerative tissue interactions. This may involve paracrine secretion, with localized dispersal in interstitial fluid or through short loops of blood vessels, as when gastrin made by pyloric G cells reaches target cells in the fundic glands, or juxtacrine secretion, in which a signaling molecule remains on the secreting cell’s surface or adjacent extracellular matrix and affects target cells when the cells make contact. As discussed briefly in Chapter 2, other endocrine cells produce hormones that act on target cells only a short distance away. Distribution by the circulation allows hormones to act on target cells with receptors for those hormones at a distance from the site of their secretion.

Position of adrenal gland in human body