- 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.
Endocrine glands are collections of specialized cells that synthesize, store, and release their secretions directly into the bloodstream. They are sensing and signaling devices located in the extracellular fluid compartment that are capable of responding to changes in the internal and external environments and coordinating multiple activities that maintain homeostasis.
Normal Structure and Function
The pituitary gland (hypophysis) is divided into two major compartments: (1) the adenohypophysis (anterior lobe), composed of the pars distalis, pars tuberalis, and pars intermedia, and (2) the neurohypophyseal system, which includes the pars nervosa (posterior lobe), infundibular stalk, and hypothalamic nuclei (supraoptic and paraventricular) containing the neurosecretory neurons, which synthesize and package the neurohypophyseal hormones into secretory granules. The pars intermedia forms the thin cellular zone between the adenohypophysis and neurohypophysis. The arterial blood supply to the pituitary gland forms a capillary plexus that drains into the hypophyseal portal veins, which supply the adenohypophysis. The hypothalamic–hypophyseal portal system transports the hypothalamic-releasing and release-inhibiting hormones directly to the adenohypophysis, where they interact with their specific populations of trophic hormone-producing cells.
The pars distalis of the adenohypophysis is composed of multiple populations of endocrine cells that secrete the pituitary trophic hormones. The secretory cells are surrounded by abundant capillaries derived from the hypothalamic–hypophyseal portal system. The pars tuberalis functions primarily as a scaffold for the capillary network of the hypophyseal portal system during its course from the median eminence to the pars distalis.
Secretory cells in the adenohypophysis can be classified functionally into somatotrophs that secrete growth hormone (GH; somatotrophin), luteotrophs that secrete luteotropic hormone (LTH; prolactin), gonadotrophs, which secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH), thyrotrophs, which secrete thyroid-stimulating hormone (TSH), and chromophobes, which are involved with the synthesis of adrenocorticotropin hormone (ACTH) and melanocyte-stimulating hormone (MSH) in some species.
Each type of endocrine cell in the adenohypophysis is under the control of a specific releasing hormone from the hypothalamus (Figure 21–1). These releasing hormones are small peptides that are synthesized and secreted by neurons of the hypothalamus and are 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 TSH, FSH and LH, ACTH, and GH. Prolactin secretion is stimulated by a number of factors, the most important of which appears to be thyrotropin-releasing hormone (TRH). Dopamine serves as the major prolactin-inhibitory factor and suppresses prolactin secretion and ACTH production. Somatostatin (somatotrophin-release-inhibiting hormone, SRIH) ...