Location of Adrenal Cortex
Adrenal glands are located on the top of each kidney as small, yellowish-brown, flattened and wrinkled triangular cap-like structures, each weighing 3.5 - 4.5 g in humans. Each Adrenal gland has an outer cortex which produces steroid hormones and an inner medulla which is called Adrenal Cortex.
Structure of Adrenal Cortex
Each adrenal gland is enclosed in a connective tissue capsule and consists of two functionally distinct parts: a superficial yellowish part called Adrenal cortex, and a deeper brownish part known as adrenal medulla. The volume of the cortex is about ten times the medulla. The adrenal cortex secretes steroid hormones called adreno-corticosteroids or corticoids, while the medulla secretes catecholamines.
Histology of Adrenal Cortex
The adrenal cortex histologically comprises of the following three zones:
i) Zona glomerulosa
ii) Zona fasciculate
iii) Zona reticularis
It is the outermost zone of adrenal cortex, about five cell layer deep, the cells are densely set with their long axes parallel to the surface. This zone secretes aldosterone, the mineralocorticosteroid.
It is the intermediate zone of adrenal cortex, widest amongst the three zones. It consists of long cells arranged in radiating columns, perpendicular to the surface. The principal hormone secreted by this zone is
Cortisol and the gluco-corticosteroid.
It is the innermost zone of adrenal cortex, and named so because the cells are arranged in a somewhat net-like
fashion. The dominant secretions of this zone are adrenal androgens.
Hormones of Adrenal cortex
About fifty steroid compounds known as corticosteroids or adrenocorticoids, have been inaccessible from adrenal cortex. But only a few of these have been found to be biologically active as hormones. These can be separated into three groups, those are:
(iii) Sex hormones (androgens).
Mineralocorticosteroids are twenty-one carbon steroids secreted by the cells of the zona glomerulosa of the adrenal cortex. They are so named because their main function is to regulate the plasma sodium and potassium concentrations. The most important mineralocorticosteroid is aldosterone. It is a salt-retaining hormone. Its secretion is controlled by sodium level in the blood. A fall in plasma sodium concentration or a rise in plasma potassium concentration or a fall in circulatory volume of blood stimulates the secretion of it. The retention of Na+ and loss of K+ in the body causes by aldosterone. This is done by its action on the renal tubules. By retaining more sodium in the blood, it increases the re-absorption of water from the urine. As a result of excess aldosterone in the body, there is excess sodium retention, which in turn may lead to rise of blood pressure.
Glucocorticoids. These are also C-21 steroid hormones secreted by the cells of zona fasciculate of adrenal cortex. They are so named because their main action, when they were first discovered, appeared to be on glucose metabolism. Cortisol, and corticosterone and cortisone are the main glucocorticoids. The anterior pituitary hormone, called corticotropin or ACTH, stimulates glucocorticoid secretion, whereas glucocorticoids exert a feedback inhibitory effect on corticotropin secretion. Glucocorticoids play an important role in carbohydrate, lipid, protein, and purine metabolism and have influence on cardiovascular, skeletal muscle, central nervous system, lymphoid, connective, and other tissues.
Sex corticoids are C-19 steroid hormones secreted by the zona reticularis of the adrenal cortex. These adrenal androgens are androstenedione and dehydroepiandrosterone. They are weak androgens compared to testicular androgens and are most likely the major source of androgens in female. The secretion of these hormones is stimulated by ACTH of anterior pituitary. These hormones stimulate the development of external sex characters in males. In females, adrenal androgens cause restraint of feminine characters.