Neurohypophysis

^ Components

• Infundibulum (hypophyseal stalk)

♦ Extension from the hypothalamus; continuous with the pars nervosa

♦ Contains the hypothalamo-hypophyseal tract which consists of axons from neurons whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus

♦ Contains axons and axon terminals of the neurons forming the hypothalamo-hypophyseal tract

♦ Herring bodies. Expanded axon terminals which accumulate secretory granules containing oxytocin or antidiuretic hormone

(vasopressin)

■ Oxytocin causes smooth muscle and myoepithelial cell contraction.

■ Antidiuretic hormone (ADH) acts on the kidney tubules to prevent water loss.

♦ Also contains "astrocyte-like" cells, called pituicytes; no secretory cells are present. ^ Regulation of neurohypophyseal secretion

• Oxytocin and vasopressin are synthesized by neurons in the hypothalamus, transported down the axons and stored in axons terminals (Herring bodies) in the pars nervosa.

• Activity in these neurons, in response to physiological signals, causes hormone release (neurosecretion) in a manner similar to release of neurotransmitters.

Thyroid Gland

^ The thyroid gland consists of two unique structural and functional subdivisions, the thyroid follicles and the parafollicicular cells.

^ Thyroid follicles

• Spheres composed of a single layer of follicle cells; the follicle cells form an epithelium (follicular epithelium) and, thus, these cells have apical and basal surfaces and demonstrate cellular polarity.

• Follicle cells secrete thyroglobulin, a glycoprotein that is stored in the center of the follicle.

• Thyroglobulin contains modified tyrosine amino acids that constitute the thyroid hormones, thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3). Follicle cells take up the stored thyroglobulin and release the hormones into the blood stream.

• Thyroid hormones regulate the basal metabolic rate.

^ Parafollicular cells (C cells, clear cells)

• Occur within the follicular epithelium and in small clusters between follicles

• Possess secretory granules containing the hormone calcitonin, which acts to inhibit bone resorption, lowering calcium levels

• Belong to the diffuse neuroendocrine system (DNES)

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