232 E N D O C R I N E S Y S T E M

Pars Nervosa and Infundibular Stalk

•The pars nervosa does not present a very organized appearance. It is composed of pituicytes, cells believed to be neuroglial in nature that may fulfill a supporting function for the numerous unmyelinated axons of the pars nervosa.

•These axons, whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus, enter the pars nervosa via the hypothalamohypophyseal tract.

•Their axons possess expanded axon terminals, referred to as Herring bodies, within the pars nervosa.

Herring bodies contain oxytocin and antidiuretic hormone (ADH, vasopressin), two neurosecretory hormones that are stored in the pars nervosa but are manufactured in the cell bodies in the hypothalamus.

The release of these neurosecretory hormones (neurosecretion) is mediated by nerve impulses and occurs at the interface between the axon terminals and the fenestrated capillaries.

When the axon is ready to release its secretory products, the pituicytes withdraw their processes and permit the secretory product a clear access to the capillaries.

Pars Tuberalis

The pars tuberalis is composed of numerous cuboidal cells whose function is not known.

THYROID GLAND

The thyroid gland consists of right and left lobes that are interconnected by a narrow isthmus across the thyroid cartilage and upper trachea (see Table 10-2 and Graphic 10-2). It is enveloped by a connective tissue capsule whose septa penetrate the substance of the gland, forming not only its supporting framework but also its conduit for its rich vascular supply.

The parenchymal cells of the gland are arranged in numerous follicles, composed of a simple cuboidal epithelium lining a central colloid-filled lumen. The colloid, secreted and resorbed by the follicular cells, is composed of thyroid hormone that is bound to a large protein, and the complex is known as thyroglobulin.

To synthesize thyroid hormone

•Iodide from the bloodstream is actively transported into follicular cells at their basal aspect via iodide pumps.

•Iodide is oxidized by thyroid peroxidase on the apical cell membrane and is bound to tyrosine residues of thyroglobulin molecules.

•Within the colloid, the iodinated tyrosine residues become rearranged to form triiodothyronine (T3) and

thyroxine (T4).

To release thyroid hormone

•The binding of thyroid-stimulating hormone (TSH) released by the pituitary, to receptors on the basal aspect of their plasmalemma induces follicular cells to become tall cuboidal cells.

•They form pseudopods on their apical cell membrane that engulf and endocytose colloid.

•The colloid-filled vesicles fuse with lysosomes, and T3 and T4 residues are removed from thyroglobulin, liberated into the cytosol, and are released at the basal aspect of the cell into the perifollicular capillary network.

•Thyroid hormone (see Table 10-2) is essential for regulating basal metabolism and for influencing growth rate and mental processes and generally stimulates endocrine gland functioning.

An additional secretory cell type, parafollicular cells (clear cells), is present in the thyroid. These cells have no contact with the colloidal material.They manufacture the hormone calcitonin, which is released directly into the connective tissue in the immediate vicinity of capillaries. Calcitonin (see Table 10-2) helps control calcium concentrations in the blood by inhibiting bone resorption by osteoclasts (i.e., when blood calcium levels are high, calcitonin is released).

Parathyroid Glands

The parathyroid glands, usually four in number, are embedded in the fascial sheath of the posterior aspect of the thyroid gland. They possess slender connective tissue capsules from which septa are derived to penetrate the glands and convey a vascular supply to the interior. In the adult, two types of parenchymal cells are present in the parathyroid glands:

•numerous small chief cells and a smaller number of

•large acidophilic cells, the oxyphils.

Fatty infiltration of the glands is common in older individuals. Although there is no known function of oxyphils, chief cells produce parathyroid hormone (PTH see Table 10-2).

•Parathyroid hormone (PTH) is responsible for maintaining proper calcium ion balance.

•The concentration of calcium ions is extremely important in the normal function of muscle and nerve cells and as a release mechanism for neurotransmitter substance.

•A drop in blood calcium concentration activates a feedback mechanism that stimulates chief cell secretion.

•PTH binds to receptors on osteoblasts that release osteoclast-stimulating factor followed by bone