Adrenal glands

Structure/function relationship

Innervation

The action of the suprarenal gland is regulated both by neuronal and hormonal stimulation. The cortex is activated by adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. ACTH subsequently stimulates the respective cortical zones to produce corticosteroids. On the other hand, the suprarenal medulla is innervated by type B (medium diameter, myelinated) preganglionic nerve fibers. These nerve fibers leave the intermediolateral cell column of the lateral horn of the spinal cord from the T5 – T8 segments of the spinal cord and bypass the paravertebral sympathetic ganglion chain. The fibers converge after bypassing the sympathetic trunk, forming the greater splanchnic nerve.

Some of these fibers will synapse at the celiac ganglion and post-synaptic fibers of the celiac ganglion will innervate the blood vessels supplying the suprarenal gland. Other fibers of the greater splanchnic nerve will bypass the celiac ganglion and enter the suprarenal gland to synapse on the membranes of chromaffin cells.

For this reason, the suprarenal medulla is sometimes considered as a neuroendocrine bridge between the two physiological segments. The chromaffin cells act as enlarged post-ganglionic fibers that release their neurochemicals directly into the blood stream instead of at a neuroeffector junction.

Blood supply and lymphatic drainage

The suprarenal glands receive arterial supply directly from the abdominal aorta as well as from the inferior phrenic arteries and the renal arteries. The inferior phrenic arteries leave the anterior surface of abdominal aorta just superior to the celiac trunk. Each inferior phrenic artery travels superolaterally and at the upper medial borders of the suprarenal glands, they branch into several superior suprarenal arteries that enter the gland.

The middle suprarenal arteries branch directly from the anterolateral surface of the abdominal aorta, adjacent to the celiac trunk. These vessels travel horizontally and pierce the lower medial border of the suprarenal glands by way of their branches.

Also, each renal artery provides an inferior suprarenal artery that enters the inferior aspect of the gland. The left inferior suprarenal artery travels vertically and cranially towards the gland, while the right vessel travels more horizontally and obliquely towards its destination. Both glands return deoxygenated blood to the systemic circulation by way of a right and a left suprarenal vein.

The right suprarenal vein takes a short horizontal course to drain directly into the inferior vena cava. However, the left suprarenal vein travels vertically to the renal vein, which then drains to the inferior vena cava. Lymphatic drainage is achieved by para-aortic lymph nodes.

Embryology

The suprarenal glands have two embryonic origins, and consequently produce two different types of signalling chemicals. The outer cortex is of mesodermal origin and releases steroidal hormones, while the inner medulla is derived from ectoderm and secretes adrenergic neurotransmitters (also known as catecholamines). In the 5th developmental week, mesothelial cells enter the mesenchymal layer. Subsequently, large acidophilic cells differentiate, forming a primitive cortex. Smaller cells then migrate and engulf the acidophilic cells; these will go on to form the definitive cortex.

In the 7th developmental week, neural crest cells that are formed at the apex of the neural folds migrate (after the neural tube is completely closed) via a ventral pathway. These cells enter the gland from the medial face and subsequently differentiate into chromaffin cells that are organized centrally, in cords and masses. The chromaffin cells are so named because of the yellow-brown stain they produce after reacting with chromium based salts.