Adrenal glands

Also called as the suprarenal glands, they are located retroperitoneally on the superior aspect of the kidneys. They are enclosed by the renal fascia. The right adrenal gland is pyramidal while the left is semilunar or crescent shaped. Each adrenal gland is divided into an outer cortex and an inner medulla that are anatomically and functionally distinct from each other.

The cells of the adrenal cortex produce steroid hormones and hence, they have a well developed smooth endoplasmic reticulum and triglyceride droplets. The cortex is divided into three zones as follows ( Mnemonic GFR from outside to inside):

1. Zona Glomerulosa: It is made of aldosterone secreting cells with prominent nuclei, pale cytoplasm, arranged in irregular, ovoid glomerulus-like structures.

2. Zona Fasciculata: It is the largest layer and consists of cortisol secreting cells. The cells have foamy and pale, lipid rich cytoplasm with prominent nucleus, abundant smooth ER and mitochondria, and are arranged in cords.

3. Zona Reticularis: It is the innermost layer made of cells secreting sex steroids DHEA and androstenedione. It shows a network of cells arranged in cords. It stains darker, has large amounts of lipofuscin pigment and fewer lipid droplets.

The adrenal medulla is technically a peripheral sympathetic ganglion. It secretes epinephrine and norepinephrine. It is made of chromaffin cells. Preganglionic axons of the sympathetic nervous system synapse on the chromaffin cells which then secrete epinephrine and norepinephrine when stimulated. It has a rich vascular supply and prominent venous sinuses. The cells are basophilic with a granular cytoplasm and do not show any stored lipids. There are two types of chromaffin cells - epinephrine containing cells and norepinephrine containing cells. The epinephrine containing cells are more in number and show small, homogenous, pale staining granules. The norepinephrine containing cells are fewer in number and have 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. They supply only the adrenal medulla. Blood from the adrenal glands is drained by a central vein which drains the medullary venous sinuses and then joins the renal vein on the left and the IVC on the right. The venous blood from the adrenals carries all of the adrenal hormones. Because of this reason, it is important to ligate the adrenal vein before an adrenalectomy for pheochromocytoma as otherwise large amounts of adrenal hormones will be released into the circulation.