Endocrine glands are collections of specialized cells that synthesize, store, and release their secretions directly into the bloodstream.
Each type of endocrine cell in the adenohypophysis is under the control of a specific releasing hormone from the hypothalamus.
Toxicants can influence the synthesis, storage, and release of hypothalamic-releasing hormones, adenohypophyseal- releasing hormones, and the endocrine gland-specific hormones.
Higher animals regulate their internal environment, independent of wide fluctuations in external factors, via an endocrine system that consists of (1) an endocrine gland that secretes hormone, (2) the hormone itself, and (3) a target tissue. A hormone produced by a ductless endocrine gland is secreted into the blood, which carries it to a target organ. The hormone-producing glands include the pituitary (hypophysis), thyroid and parathyroids, adrenals, gonads, and pancreas. In response to a stimulus, endocrine cells synthesize and secrete hormones into the blood. Hydrophilic hormones (peptides and proteins) are freely dissolved in plasma. Hydrophobic hormones (steroids, amino acid derivatives) bind specialized serum binding proteins or albumin. Hormones elicit biological responses by binding to a receptor in target tissue. Receptors can be located on the plasma membrane (peptides, proteins, and catecholamines) or in the cellular nucleus (steroids, thyroid, and vitamin D hormones). The interaction of the hormone with its receptor initiates a chain of intracellular events leading to its physiological response.
The pituitary may be divided into four major subdivisions: the pars distalis (adenohypophysis or anterior pituitary), the pars intermedia between the adenohypophysis and neurohypophysis, the pars tuberalis, and the pars nervosa (neurohypophysis or posterior pituitary). There are two basic types of communication between the hypothalamus and pituitary (Fig. 20–1).
Communication between the hypothalamus and the pituitary. The anterior pituitary receives endocrine input from the hypothalamus in the form of peptide releasing hormones, which are transported in portal vessels to stimulate release of hormones in the anterior pituitary cells. The posterior pituitary is innervated by axons extending from magnocellular neurons with their cell bodies located in the hypothalamus. Stimulation of these cell bodies results in hormonal secretion in the posterior pituitary. (Adapted from Hedge GA, Colby HD, Goodman RL, eds. Clinical Endocrine Physiology. Philadelphia, PA: W.B. Saunders; 1987.)
Each type of endocrine cell in the adenohypophysis is under the control of a specific releasing hormone from the hypothalamus (Fig. 20–2). These releasing hormones are small peptides synthesized, secreted by the hypothalamus into capillaries, and conveyed by the hypophyseal portal system to specific trophic hormone-secreting cells in the adenohypophysis. Each hormone stimulates the rapid release of preformed secretory granules containing a specific trophic hormone. Specific releasing hormones have been identified for thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), and growth hormone (GH). Prolactin (PRL) secretion is stimulated by a number of factors ...