Melatonin is released in the dark and regulates reproductive function in mammals by inhibiting the steroidogenic activity of the gonads (Table 21.6). Production of gonadal steroids is decreased by the inhibitory action of melatonin on neurosecretory neurons located in the hypothalamus (arcuate nucleus) that produce GnRH. Inhibition of GnRH causes a decrease in the release of FSH and LH from the anterior lobe of the pituitary gland. In addition to melatonin, extracts of pineal glands from many animals contain numerous neurotransmitters, such as serotonin, norepinephrine, dopamine,and histamine, and hypothalamic-regulating hormones, such as somatostatin and TRH. Clinically, tumors that destroy the pineal gland are associated with precocious (early-onset) puberty.
Animal studies demonstrate that information relating to the length of daylight reaches the pineal gland from photoreceptors in the retina. The pineal gland thus influences seasonal sexual activity. Recent studies in humans suggest that the pineal gland has a role in adjusting to sudden changes in day length, such as those experienced by travelers who suffer from jet lag. In addition, the pineal gland may play a role in altering emotional responses to the reduced length of day during winter in temperate and subarctic zones known as seasonal affective disorder (SAD).
The thyroid gland is located in the anterior neck region adjacent to the larynx and trachea.
The thyroid gland is a bilobate endocrine gland located in the anterior neck region and consists of two large lateral lobes connected by an isthmus, a thin band of thyroid tissue. The two lobes, each approximately 5 cm in length, 2.5 cm in width, and 20 to 30 g in weight, lie on either side of the larynx and upper trachea. The isthmus crosses anterior to the upper part of the trachea. A pyramidal lobe often extends upward from the isthmus. A thin connective tissue capsule surrounds the gland. It sends trabeculae into the parenchyma that partially outline irregular lobes and lobules. Thyroid follicles constitute the functional units of the gland.
The thyroid gland develops from the endodermal lining of the floor of the primitive pharynx.
The thyroid gland begins to develop during the fourth week of gestation from a primordium originating as an endodermal thickening of the floor of the primitive pharynx. The primordium grows caudally and forms a ductlike invagination known as the thyroglossal duct. The thyroglossal duct descends through the tissue of the neck to its final destination in front of the trachea, where it divides into two lobes. During this downward migration, the thyroglossal duct undergoes atrophy, leaving an embryologic remnant, the pyramidal lobe of the thyroid, which is present in about 40% of the population. About the ninth week of gestation, endodermal cells differentiate into plates of follicular cells that become arranged into follicles. By week 14, well-developed follicles lined by the follicular cells contain colloid in their lumen. During week 7, epithelial cells lining the invagination of the fourth branchial pouches (sometimes called the fifth branchial pouches), known as the ultimobranchial bodies, start their migration toward the developing thyroid gland and become incorporated into the lateral lobes. After fusing with the thyroid, ultimobranchial body cells disperse among the follicles, giving rise to parafollicular cells that become incorporated into the follicular epithelium.
The thyroid follicle is the structural and functional unit of the thyroid gland.