ADRENAL GLAND

SUPRARENAL GLAND

The adrenal (suprarenal) glands are located retroperitoneally at the upper pole of the kidneys. The parenchyma of the adrenal glands is divided into cortex and medulla. The cortex is larger and occupies 4/5 of the parenchyma. The cortex is composed of trabeculae of epithelial cells surrounded by a small amount of reticular connective tissue with blood sinusoids. The cortex can be divided into three zones according to the shape of the trabeculae. Just under the capsule there is the zona glomerulosa, whose arcuate trabecules resemble glomeruli. The endocrine cells of this layer produce mineralocorticoids, including aldosterone. The most striking and widest is the zona fasciculata; its cords run parallel like 'fascicles'. The fascicles are composed of large, pale, polyhedral cells with a centrally located nucleus. Because their cytoplasm contains numerous lipid droplets, these cells are referred to as spongiocytes. Spongiocytes produce and release glucocorticoids. The zona reticularis is the deepest layer of the cortex, whose anastomosing trabeculae run in different directions and resemble an intricate network (Lat. rete, network). The epithelial cells of this zone have a darker cytoplasm containing lipofuscin. The cells produce androgens, especially dehydroepiandrosterone. All of the above hormones are examples of steroid hormones. Therefore, the ultrastructure of the endocrine cells of the adrenal cortex also corresponds to that of steroid-synthesizing cells.

The adrenal medulla has a different structure. It contains chromaffin cells, which are formed by the transformation of sympathetic neurons. The chromaffin cells are polyhedral in shape and their body is without projections. Transmission electron microscopy detects secretory granules and rough endoplasmic reticulum in the body of these cells. Two subpopulations of chromaffin cells produce catecholamines: one produces epinephrine (adrenaline) and the other norepinephrine (noradrenaline).

A more detailed description of the adrenal glands and their hormones is provided in the e-course Endocrine System II.

The different histological structure of the cortex and medulla is explained by a different origin of both parts. The cortex is derived from the coelomic epithelium (i.e. it is of mesodermal origin), whereas the medulla develops from the neuroectoderm of the neural crest. During embryonic development, the two parts converge and the cortex subsequently surrounds the medulla.