circumanal gland

Embryology

Each adrenal gland is a two-part organ composed of an outer cortex and an inner medulla. The cortex arises in the embryo from a region of the mesoderm that also gives rise to the gonads. The medulla arises from ectoderm, which also gives rise to the sympathetic nervous system.

Anatomy

The entire gland is enclosed in a tough connective tissue capsule from which trabeculae extend into the cortex. The cortex consists of cells arranged into three zones: the outer zona glomerulosa, the middle zona fasciculata, and the inner zona reticularis. The cells are arranged in cords. The medulla consists of chromaffin cells arranged in groups or in anastomosing cords. The two adrenal glands are retroperitoneal, each embedded in perirenal fat above its respective kidney. In an adult, the average weight of an adrenal gland is 5 g (range: 4 to 14 g).

Physiology

The adrenal medulla synthesizes and stores three catecholamines: dopamine, norepinephrine, and epinephrine. The chief effects of dopamine are the dilation of systemic arteries, increased cardiac output, and increased flow of blood to the kidneys. The primary action of norepinephrine is constriction of the arterioles and venules, resulting in increased resistance to blood flow, elevated blood pressure, and slowing of the heart. Epinephrine constricts vessels in the skin and viscera, dilates vessels in skeletal muscle, increases heart rate, dilates the bronchi by relaxing bronchial smooth muscle, increases the conversion of glycogen to glucose in the liver to increase the blood glucose level, and diminishes activity of the gastrointestinal system. The three catecholamines are also produced in other parts of the body.

The adrenal medulla is controlled by the sympathetic nervous system and functions in conjunction with it. It is intimately related to adjustments of the body in response to stress and emotional changes. Anticipatory states tend to bring about the release of norepinephrine. More intense emotional reactions, esp. those in response to extreme stress, tend to increase the secretion of both norepinephrine and epinephrine; epinephrine is important in mobilizing the physiological changes that occur in the “fight or flight” response to emergency situations.

The cortex synthesizes three groups of steroid hormones from cholesterol. These are 1) glucocorticoids (cortisol, corticosterone), which regulate the metabolism of organic nutrients and have an anti-inflammatory effect; 2) mineralocorticoids (aldosterone, dehydroepiandrosterone), which affect metabolism of the electrolytes sodium and potassium; and 3) androgens and estrogens (estradiol), which contribute to body changes at puberty. See: aldosterone; cortisol; steroid

Pathology

Hypersecretion of adrenal cortical hormones results in Cushing syndrome. Hypersecretion of aldosterone results in a surgically correctable form of hypertension (aldosteronism). Adrenocortical insufficiency may be acute or chronic; acute insufficiency of adrenal hormones produces circulatory shock, while chronic insufficiency results in Addison disease. See: Addison disease; aldosteronism; Cushing syndrome; pheochromocytoma

The hormones produced by endocrine cells regulate the body's salt, water, mineral, and glucose levels; they adjust the body's metabolic balances, growth rates, and reproductive cycles; and they maintain the body's stress responses. Like exocrine and paracrine cells, endocrine cells are stimulated and inhibited by autonomic axons; the activities of endocrine cells are also modulated by circulating hormones, especially pituitary hormones. Both the neural and the hormonal signals to the endocrine system are ultimately regulated by the hypothalamus of the brain, which is the integration center for the body's visceral homeostasis. See: hormone

Endocrine health disorders usually result from the production of either too much or too little of a hormone.

Abnormalities

An excess of parathormone results in hyperparathyroidism; a deficiency, in hypoparathyroidism, with neuromuscular hyperexcitability manifested by carpopedal spasm, wheezing, muscle cramps, urinary frequency, mood changes, and lassitude. Blood calcium falls and blood phosphorus rises. Other symptoms include blurring of vision caused by cataracts, poorly formed teeth if onset was in childhood, maldevelopment of hair and nails, and dry and scaly skin. Hyperparathyroidism or hypersecretion results in a rise in blood calcium and fall in blood phosphorus. Calcium is removed from bones, resulting in increased fragility. Muscular weakness, reduced muscular tone, and general neuromuscular hypoexcitability occur. Osteitis fibrosa cystica is associated with hyperplasia and resulting hypersecretion of the parathyroids.

Function

The pituitary is an endocrine gland secreting a number of hormones that regulate many bodily processes including growth, reproduction, and other metabolic activities. It is often referred to as the “master gland of the body.”

Hormones are secreted in the following lobes: Intermediate lobe: In cold-blooded animals, intermedin is secreted, influencing the activity of pigment cells (chromatophores) of fishes, amphibians, and reptiles. In warm-blooded animals, no effects are known.

Anterior lobe: Secretions here are the somatotropic, or growth hormone (STH or GH), which regulates cell division and protein synthesis for growth; adrenocorticotropic hormone (ACTH), which regulates functional activity of the adrenal cortex; thyrotropic hormone (TTH or TSH), which regulates functional activity of the thyroid gland; and prolactin, also called lactogenic hormone, which induces secretion of milk in the adult female. The gonadotropic hormones are as follows: in women, follicle-stimulating hormone (FSH) stimulates development of ovarian follicles and their secretion of estrogen; in men, it stimulates spermatogenesis in the testes. In women, luteinizing hormone (LH) stimulates ovulation and formation of the corpus luteum and its secretion of estrogen and progesterone. In men, LH also called interstitial cell-stimulation hormone (ICSH), stimulates testosterone secretion.

Posterior lobe: Hormones are secreted by the neurosecretory cells of the hypothalamus and pass through fibers of the supraopticohypophyseal tracts in the infundibular stalk to the neurohypophysis, where they are stored. Secretions here are oxytocin, which acts specifically on smooth muscle of the uterus, increasing tone and contractility, and antidiuretic hormone (ADH), which increases reabsorption of water by the kidney tubules. In large amounts, ADH also causes vasoconstriction, and is also called vasopressin.

Disorders

Hypersecretion of anterior lobe causes gigantism, acromegaly, and pituitary basophilism (Cushing disease). Hyposecretion of anterior lobe causes dwarfism, pituitary cachexia (Simmonds disease), Sheehan syndrome, acromicria, eunuchoidism, or hypogonadism. Posterior lobe deficiency or hypothalamic lesion causes diabetes insipidus. Anterior and posterior lobe deficiency and hypothalamic lesion cause Fröhlich syndrome (adiposogenital dystrophy) and pituitary obesity.

Salivary secretion is under nervous control, reflexly initiated by mechanical, chemical, or radiant stimuli acting on taste buds in the mouth, olfactory receptors, or the eyes. Secretion may also be due to conditioned reflexes as when one thinks about food or hears a dinner bell. The nerve supply of the salivary glands is from the facial and glossopharyngeal nerves, which increase secretion, and from the sympathetic nerves, which decrease secretion. The blood supply is from branches of the external carotid artery.