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. Have powerful effects when present in very. Constitutes the endocrine system and includes the hypothalamus, pancreas, thymus, ovaries, testes, pituitary, thyroid, parathyroid, adrenal, and pineal glands. 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. Secrete their products (hormones) into the interstitial fluid which defuses into capillaries to be carried away. 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.
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