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Secretory cells of endocrine glands release their products, signaling molecules called hormones, into neighboring vascularized compartments for uptake by capillaries and distribution throughout the body. Unlike exocrine gland secretions, hormone release does not involve secretory ducts. Most 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). As described in earlier chapters, release of hormone-like factors also occurs in certain cells developing and functioning also as neurons, smooth and cardiac muscle fibers, leukocytes, and fibroblasts.
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Distribution by the circulatory system allows hormones to act on target cells with their cognate receptors at a distance from the site of their secretion. As discussed briefly in Chapter 2, some 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 signaling molecules remain on the secreting cell’s surface or adjacent extracellular matrix and affect target cells when the cells make contact. Juxtacrine signaling is particularly important in embryonic and regenerative tissue interactions. In autocrine secretion cells 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 target the same cells that produced it. Endocrine glands are often also target organs for other hormones that can establish feedback mechanisms controlling hormone secretion that keep blood hormonal levels within strict limits.
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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 circulate bound to transport proteins but can diffuse through the cell membrane bilayer to be taken up and activate cytoplasmic receptors in target cells (see Chapter 2).
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PITUITARY GLAND (HYPOPHYSIS)
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The pituitary gland, or hypophysis (Gr. hypo, under + physis, growth), weighs about 0.5 g in adults and has dimensions of about 10 × 13 × 6 mm. As described ...