Adrenal glands

Embryology

The adrenal (suprarenal) gland consists of two main parts, each with different embryonic origins. The inner part of the gland, the adrenal medulla, is originally derived from ectodermal neural crest cells. Also known as chromaffin cells for their staining properties, these cells migrate towards the adrenal cortex at approximately 7 weeks’ gestation and gradually invade the cortex to achieve their ultimate central location. Many of these islands of chromaffin cells spontaneously regress, except for those that make up the glands of Zuckerkandl at the level of the inferior mesenteric artery just lateral to the aorta. This migration of chromaffin cells also explains the existence of heterotopic adrenal glands and paragangliomas, especially along the vertebral column anterior to the aorta. Indeed, 10% of phaeochromocytomas occur in an extra-adrenal location.

The outer adrenal cortex begins to form at approximately the fourth week of gestation from the urogenital ridge of mesenchyme. Meso­nephric mesenchymal cells differentiate and proliferate to form the primitive steroid-producing cortex. By week 6, the fetal adrenal cortex is fully formed. As the fetus matures, the distinct cortical zones begin to differentiate. By birth, both the zona glomerulosa and zona fasciculata are present; the zona reticularis is not present until the third year of life. At birth, the adrenal glands are almost one-third of the size of the kidney but the cortex gradually shrinks in size, and the medulla grows only minimally after birth. This size regression continues into the second month, by which time the gland has reduced in size by almost 50%. From age 2 until puberty, the gland increases to its adult weight of approximately 5 g.

Adrenal rests represent accessory or ectopic islets of adrenal tissue. Those located close to the native adrenal gland are more likely to contain medullary tissue, while more distant rests typically contain only cortical tissue. They can be found anywhere but are most commonly located along the course of gonadal descent (that is, the spermatic cord or broad ligament). Most adrenal rests atrophy early in life and are clinically insignificant.

Surgical surface anatomy

The adrenal glands are located deep to the tenth and eleventh ribs, just lateral to either side of the vertebral column. Their position varies with respiration by approximately 3 cm with each movement of the diaphragm ( Fig. 65.1 ).

Clinical anatomy

The adrenal glands are located in the retroperitoneum at the superomedial aspect of the kidneys ( Fig. 65.2 ; Table 65.1 ). Each gland weighs 4–8 g (average 5 g, slightly larger in females), measures approximately 4 × 3 × 1 cm, is enclosed in Gerota's fascia, and is surrounded by periadrenal fat, except for a partition of connective tissue that separates it from the kidney. Both glands are firmly attached to the fascia of the abdominal wall and diaphragm by several vascular attachments and fibrous bands.