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KEY POINTS

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  • 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.

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INTRODUCTION

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Higher animals have developed the ability to regulate their internal environment, independent of wide external fluctuations via the endocrine system. An endocrine system consists of an endocrine gland that secretes a hormone, the hormone itself, and a target tissue that responds to the hormone. A hormone is a chemical substance produced by a ductless endocrine gland that is secreted into the blood. The hormone-producing glands include the pituitary, the thyroid and parathyroids, the adrenals, the gonads, and the pancreas. There are primarily three chemical classes of hormones: amino acid derivatives (catecholamines and thyroid hormones), peptide hormones (pancreatic), and steroids (derivatives of cholesterol). Endocrine glands are sensing and signaling devices that are capable of responding to changes in the internal and external environments and coordinating multiple activities that maintain homeostasis.

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PITUITARY GLAND

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Anatomy and Physiology

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The pituitary may be divided into two major subdivisions: the pars distalis and the pars nervosa (Figure 21–1). The pars distalis, adenohypophysis or anterior pituitary, is the largest subdivision and it receives peptides from the hypothalamus through a capillary portal system (hypothalamo–hypophyseal vessels). The pars nervosa, neurohypophysis or posterior pituitary, has its cell bodies in the hypothalamus with their axons stretching to the posterior lobe of the pituitary; therefore, functionally and anatomically, the posterior pituitary is an extension of the hypothalamus.

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FIGURE 21–1

Control of trophic hormone secretion from the adenohypophysis by hypothalamic-releasing hormones (RH) and release-inhibiting hormones (RIH). The releasing and release-inhibiting hormones are synthesized by neurons in the hypothalamus, transported by axonal processes, and released into capillary plexus in the median eminence. They are transported to the adenohypophysis by the hypothalamic–hypophyseal portal system, where they interact with specific populations of trophic hormone-secreting cells to govern the rate of release of preformed hormones, such as growth hormone (GH), somatotropic hormone (STH), luteotropic hormone (LTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyrotropic hormone (TTH), adrenocorticotropic hormone (ACTH), and melanocyte-stimulating hormone (MSH). There are RIHs for those trophic hormones (e.g., prolactin and growth hormone) that do not directly influence the activity of target cells and result in production of a final endocrine product (hormone) that could exert negative feedback control.

Graphic Jump Location
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The releasing and release-inhibiting hormones are synthesized by neurons in the hypothalamus, transported by axonal processes, and released into capillary plexus. They are transported to the adenohypophysis by the hypothalamic–hypophyseal portal system, where they interact with specific populations of trophic hormone-secreting cells ...

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