Adrenal gland

The suprarenal glands (adrenal glands), are paired (right and left suprarenal glands) endocrine organs situated cranio-medial to the kidneys in domestic mammals. Despite species-specific anatomical differences, their general structure and function are similar across taxa. They are essential for hormonal regulation, including stress responses, metabolism, and electrolyte balance.

1. General Structure and Location

The adrenal glands are generally flattened or oval structures positioned cranio-medially to the kidneys. They are retroperitoneal and embedded in fat. Their size, shape, and relative position vary among species:

• Dog and Cat: The glands are elongated and located close to the aorta and caudal vena cava. In dogs, the left adrenal is more caudal than the right.

• Ruminants (Cattle, Sheep, Goat): The glands are more rounded or ovoid and are closely associated with major vessels. In cattle, they may lie more medial to the kidneys.

• Pig: The glands are large and oval with an apparent lobulated surface. They are well encapsulated and lie close to the aorta and renal vessels.

• Horse: The glands are crescent-shaped, and the right is positioned near the caudal vena cava and the liver, while the left is near the aorta.

2. Histological Zonation

The adrenal gland is divided into two main parts: the cortex and the medulla, each with distinct embryological origins and functions.

• Cortex: Derived from mesoderm and divided into three zones:

  • Zona glomerulosa: Outermost layer, secretes mineralocorticoids (aldosterone).

  • Zona fasciculata: Middle layer, secretes glucocorticoids (cortisol).

  • Zona reticularis: Innermost layer, produces androgens.

  Notably, in some species like the pig, the zona glomerulosa and zona fasciculata are not clearly demarcated.

• Medulla: Derived from ectoderm (neural crest) and composed of chromaffin cells that secrete catecholamines (adrenaline and noradrenaline). It is innervated by sympathetic preganglionic fibers.

3. Vascularization and Innervation

The adrenal glands have a rich blood supply derived from three groups of arteries:

• Cranial, middle, and caudal suprarenal arteries. They form a subcapsular plexus, giving rise to cortical and medullary sinusoids.

Venous drainage typically occurs through a central adrenal vein:

• In most species, the right adrenal vein drains directly into the caudal vena cava, while the left adrenal vein may drain into the renal vein.

Innervation is primarily sympathetic, with direct innervation of the medulla facilitating rapid hormone release in response to stress.

4. Species-Specific Differences

• In ruminants, especially the cow, accessory adrenal tissue may be found along the aorta or in the retroperitoneum.

• In the pig, the gland's lobulation is externally visible, which is a unique feature.

• The horse adrenal is more curved and elongated than in ruminants.

5. Functional Implications

These glands are vital for life; bilateral adrenalectomy leads to death without hormone replacement. Cortical hormones regulate electrolyte and glucose metabolism, while medullary hormones mediate sympathetic stress responses.