Info

Action

Stimulates increase in size and rate of division of body cells; enhances movement of amino acids through membranes; promotes growth of long bones Sustains milk production after birth; amplifies effect of LH in males

Source of Control

Secretion inhibited by somatostatin (SS) and stimulated by growth hormone-releasing hormone (GHRH) from the hypothalamus

Anterior Lobe

Hormone

Growth hormone (GH)

Prolactin (PRL)

Thyroid-stimulating hormone (TSH)

Adrenocorticotropic hormone (ACTH)

Follicle-stimulating hormone (FSH)

Luteinizing hormone (LH or ICSH in males)

Action

Stimulates increase in size and rate of division of body cells; enhances movement of amino acids through membranes; promotes growth of long bones Sustains milk production after birth; amplifies effect of LH in males

Controls secretion of hormones from the thyroid gland Controls secretion of certain hormones from the adrenal cortex Development of egg-containing follicles in ovaries; stimulates follicular cells to secrete estrogen; in males, stimulates production of sperm cells Promotes secretion of sex hormones; releases egg cell in females

Source of Control

Secretion inhibited by somatostatin (SS) and stimulated by growth hormone-releasing hormone (GHRH) from the hypothalamus

Secretion inhibited by prolactin release-inhibiting hormone (PIH) and may be stimulated by yet to be identified prolactin-releasing factor (PRF) from the hypothalamus Thyrotropin-releasing hormone (TRH) from the hypothalamus Corticotropin-releasing hormone (CRH) from the hypothalamus Gonadotropin-releasing hormone (GnRH) from the hypothalamus

Gonadotropin-releasing hormone (GnRH) from the hypothalamus

Posterior Lobe

Hormone Action Source of Control

Antidiuretic hormone (ADH) Causes kidneys to reduce water excretion; Hypothalamus in response to changes in blood water in high concentration, raises blood concentration and blood volume pressure

Oxytocin (OT) Contracts muscles in uterine wall and Hypothalamus in response to stretch in uterine and those associated with milk-secreting vaginal walls and stimulation of breasts glands with a clear viscous colloid, which consists primarily of a glycoprotein called thyroglobulin. The follicular cells produce and secrete hormones that may either be stored in the colloid or released into nearby capillaries (fig. 13.17). Other hormone-secreting cells, called ex-trafollicular cells (C cells), lie outside the follicles.

Thyroid Hormones

The thyroid gland produces three important hormones. The follicular cells synthesize two of these, which have marked effects on the metabolic rates (met"ah-bol'ik ratz) of body cells. The extrafollicular cells produce the third type of hormone, which influences blood concentrations of calcium and phosphate ions.

The two important thyroid hormones that affect cellular metabolic rates are thyroxine (thi-rok'sin), or tetraiodothyronine (also called T4, because it includes four atoms of iodine), and triiodothyronine (tri"i-o"do-thi'ro-nen), also called T3 because it includes three atoms of iodine. These hormones help regulate the metabolism of carbohydrates, lipids, and proteins. Specifically, thy-roxine and triiodothyronine increase the rate at which cells release energy from carbohydrates, enhance the rate of protein synthesis, and stimulate breakdown and mobi lization of lipids. These hormones are essential for normal growth and development and for maturation of the nervous system (fig. 13.18). TSH from the anterior pituitary gland controls their levels.

Follicular cells require iodine salts (iodides) to produce thyroxine and triiodothyronine. Such salts are normally obtained from foods, and after they have been absorbed from the intestine, the blood carries some of them to the thyroid gland. An efficient active transport protein called the iodine pump moves the iodides into the follicular cells, where they are concentrated. The iodides, with the amino acid tyrosine, are used to synthesize these thyroid hormones.

Follicular cells synthesize a protein called thy-roglobulin, which is rich in tyrosine molecules, many of which have already had iodine attached by an enzymatic reaction. As the thyroglobulin protein twists and coils into its tertiary structure, bonds form between some of the tyrosine molecules, creating potential thyroid molecules waiting to be released. The follicular cells take up molecules of thyroglobulin by exocytosis, break down the protein, and release the individual thyroid hormones into the bloodstream. When the thyroid hormone levels in the bloodstream drop below a certain

Blood Vessels Figures

Figure 13.16

(a) The thyroid gland consists of two lobes connected anteriorly by an isthmus. (b) Follicular cells secrete thyroid hormones (T3, T4).

Figure 13.16

(a) The thyroid gland consists of two lobes connected anteriorly by an isthmus. (b) Follicular cells secrete thyroid hormones (T3, T4).

Figure

Follicular cells Colloid

Extrafollicular cells

A light micrograph of thyroid gland tissue (40x micrograph enlarged to 240x). The open spaces that follicular cells surround are filled with colloid.

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