Adrenal glands

Also called the suprarenal glands, the adrenal glands lie retroperitoneally on the superior poles of the kidneys and are enclosed by the renal fascia. The right adrenal gland is pyramidal, while the left is semilunar (crescent-shaped). Each adrenal gland has two anatomically and functionally distinct parts:

• An outer cortex
• An inner medulla

The cells of the adrenal cortex produce steroid hormones. Because steroid synthesis depends on lipid processing, these cells have a well-developed smooth endoplasmic reticulum and triglyceride droplets. The cortex is divided into three zones (Mnemonic GFR, from outside to inside):

1. Zona glomerulosa: This outermost zone contains aldosterone-secreting cells. The cells have prominent nuclei and pale cytoplasm and are arranged in irregular, ovoid, glomerulus-like clusters.

2. Zona fasciculata: This is the largest cortical layer and contains cortisol-secreting cells. The cells have foamy, pale, lipid-rich cytoplasm with prominent nuclei, abundant smooth ER and mitochondria, and are arranged in cords.

3. Zona reticularis: This innermost cortical zone secretes sex steroids (DHEA and androstenedione). It forms a network of cells arranged in cords. It stains darker, contains large amounts of lipofuscin pigment, and has fewer lipid droplets.

The adrenal medulla is technically a peripheral sympathetic ganglion. It secretes epinephrine and norepinephrine and is composed of chromaffin cells. Preganglionic sympathetic axons synapse on chromaffin cells; when stimulated, these cells release epinephrine and norepinephrine. The medulla has a rich vascular supply with prominent venous sinuses. Medullary cells are basophilic with granular cytoplasm and do not store lipids.

There are two types of chromaffin cells:

• Epinephrine-containing cells: more numerous; contain small, homogeneous, pale-staining granules.
• Norepinephrine-containing cells: fewer; contain large, electron-dense granules.

The adrenal gland is supplied by the superior, middle, and inferior suprarenal arteries.

• The superior suprarenal arteries arise from the inferior phrenic arteries.
• The middle suprarenal artery is a branch of the abdominal aorta.
• The superior and middle suprarenal arteries divide into capillaries that connect to the medullary venous sinuses.
• The inferior suprarenal arteries arise from the respective renal arteries and supply only the adrenal medulla.

Venous drainage occurs through a central vein that drains the medullary venous sinuses and then joins the renal vein on the left and the IVC on the right. Venous blood from the adrenal glands carries all adrenal hormones. For this reason, during an adrenalectomy for pheochromocytoma, the adrenal vein should be ligated first; otherwise, large amounts of adrenal hormones may be released into the circulation.