Secretory cells of endocrine glands release their products, signaling molecules called hormones, into the neighboring vascularized compartment for uptake by capillaries and distribution throughout the body. There is no secretory duct as in exocrine glands. Endocrine cells are typically epithelial, at least in origin, and aggregated as cords or clusters. Besides the specialized endocrine glands discussed in this chapter, many other organs specialized for other functions, such as the heart, thymus, gut, kidneys, testis, and ovaries, contain various endocrine cells (Figure 20–1).
Locations of the major endocrine glands.
In addition to the major endocrine glands shown at the left here, there are widely distributed endocrine cells as well as various other tissues in organs (right) throughout the body with endocrine functions. Not shown are adipocytes, which exert important endocrine functions, and the many tissues in which paracrine signaling is important. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).
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. As discussed briefly in Chapter 2, other endocrine cells produce hormones that act on target cells only a short distance away. 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. Juxtacrine signaling is particularly important in embryonic and regenerative tissue interactions. In autocrine secretion, cells may produce molecules that act on themselves or on cells of the same type. For example, insulin-like growth factor (IGF) produced by several cell types may act on the same cells that produced it. 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.
Hormones, like neurotransmitters, are frequently hydrophilic molecules such as proteins, glycoproteins, peptides, or modified amino acids with receptors on the surface of target cells. Alternatively, hydrophobic steroid and thyroid hormones must circulate on transport proteins but can diffuse through the cell membranes and activate cytoplasmic receptors in target cells (see Chapter 2).
PITUITARY GLAND (HYPOPHYSIS)
The pituitary gland, or hypophysis (Gr. hypo, under + physis, ...