Wednesday 9 February 2011
The two adrenal glands are located immediately anterior to the kidneys, encased in a connective tissue capsule and usually partially buried in an island of fat. Like the kidneys, the adrenal glands lie beneath the peritoneum (i.e. they are retroperitoneal). The exact location relative to the kidney and the shape of the adrenal gland vary among species.
Despite their organization into a single gland, the medulla and cortex are functionally different endocrine organs, and have different embryological origins. The medulla derives from ectoderm (neural crest), while the cortex develops from mesoderm. The utility, if any, of having them together in one discrete organ is not obvious. In some species, amphibians and certain fish, for example, two separate organs are found.
The adrenal gland is encased in a connective tissue capsule that extends septae into the substance of the gland.
The organ is richly vascularized and capsular blood vessels, nerves and lymphatics penetrate along with the connective tissue septae.
The most distinctive feature of the adrenal is its partitioning into cortex and medulla.
The medulla is fairly homogeneous, but even when viewed a low power, three concentric zones can be distinguished in the cortex:
zona glomerulosa - thin, outermost zone
zona fasiculata - thick, middle zone
zona reticularis - thin, inner zone
Based on embryologic origin and type of hormones produced, the cortex and medulla are best though of as separate endocrine organs.
The medulla produces catecholamines and the cortex produces several steroid hormones.
There is some overlap in hormones synthesized by the zonae fasiculata and reticularis (i.e. cells in the fasiculata produce a small amount of androgens and cells in the reticularis secrete some cortisol).
Histologic examination of the adrenal reveals a rich vasculature. Numerous small arteries from several sources ramify over the surface of the gland and penetrate into the gland into two ways:
Cortical arteries and arterioles branch into capillary beds within the cortex to supply that area, then coalesce into veins at the corticomedullary junction.
Medullary arteries and arterioles penetrate the cortex without branching, then form capillary beds in the medulla.
Blood from both cortical and medullary veins empties through a single large central vein, which leaves the adrenal and anastomoses with either the vena cava or renal vein.
Cells in the adrenal cortex are arranged into three concentric zones. At both low and high magnification, one can readily differentiate these zones based on the pattern produced by cords of cells. However, the boundaries between zones are indistinct.
The outermost zone is the zona glomerulosa. Cells within this zone tend to be columnar in shape and are arranged in irregular cords. In some species, cells adjacent to the capsule are are arranged in quite regular "arcades".
The zona fasiculata is the middle and largest of the three zones in the cortex. Cells in the fasiculata are polyhedral and usually have a foamy appearance due to abundant lipid droplets. They also are arranged in distinctively straight cords that radiate toward the medulla.
Cortical capillaries are usually prominent within the fasiculata.
The innermost zone of the cortex is the zona reticularis. Cells within this zone are arranged in cords that project in many different directions and anastomose with one another.
The most abundant cell in the adrenal medulla is the chromaffin cell. That name derives from the phenomenon, observed long ago, that if adrenal gland is fixed in a solution containing chromium salts, it takes on a brownish appearance due to oxidation of catecholamines to melanin. Chomaffin cells are also referred to by some as pheochromocytes.
Chromaffin cells are columnar in shape and rather basophilic. At higher magnification, they are seen to have a granular cytoplasm due to hormone-containing granules. They are arranged in clusters, usually around medullary veins.
The adrenal medulla is richly innervated by preganglionic sympathetic fibers. Additionally, small numbers of sympathetic ganglion cells are commonly observed in the medulla. Ganglion cells are round or polygonal with prominent nuclei.