Adrenal gland

The adrenal glands are two small endocrine organs triangular to semilunar shape, located right on top of the kidneys and are enveloped by a fibrous capsule surrounded by adipose tissue. Each gland consists of two parts, the adrenal cortex and the adrenal medulla.

The adrenal cortex secretes corticosteroids and androgens and consists of three layers, namely:

  • Zona glomerulosa the outer layer, produces aldosterone which is important for the control of blood pressure. The cells are cuboidal to columnar in shape and arranged in rounded clumps, glomeruli, surrounded by sinusoidal capillaries. The round cell nucleus is darkly stained and the cytoplasm is acidophilic.

  • Zona fasciculata the middle and broadest layer and consists of glucocorticoids producing cells which are arranged in parallel cords, surrounded by sinusoid capillaries. The shapes of the cells are cuboidal or polygonal with a poorly stained cytoplasm. Lipid droplets give the cells a foamy appearance. The nucleus is round and darkly stained.

  • Zona reticularis the inner layer, consists of cells arranged in anastomosing cords, giving it a reticular pattern. These cells produce weak androgens and some glucocorticoids. The cells of the zona reticularis are smaller and darker stained than the ones in zona fasciculata.

The adrenal medulla is made up of nervous tissue and secretes catecholamines in response to neuronal signals. The most abundant cells in the medulla is the chromaffin cells. Cells of the adrenal medulla are innervated by presynaptic sympathetic neurons and release the catecholamines norepinephrine and epinephrine on direct response to nerve impulses. These chromaffin cells in the medulla are lightly stained basophilic cells that are arranged in ovoid clusters in close proximity to capillaries. A small number of sympathetic ganglion cells are commonly observed in the medulla, a large nucleus with prominent nucleoli.