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Veterinary Medicine Notes Endocrinology and Integument 1 Notes

Adrenal Glands Notes

Updated Adrenal Glands Notes

Endocrinology and Integument 1 Notes

Endocrinology and Integument 1

Approximately 39 pages

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Adrenal Glands

Adrenal glands are paired organs that lie against the roof of the abdomen cranial to the kidneys (more medial in the horse). They are retroperitoneal. They are closely associated with the aorta on the left and the caudal vena cava on the right. They consist of two layers, the outer cortex and inner medulla.

The adrenal glands are elongated and often asymmetrical, as they are moulded to surrounding vessels. Their size is variable – in juveniles they are larger, and they are also larger in lactating or pregnant animals. The adrenal glands of a medium-sized dog measure around 2.5x1x0.5cm.

The adrenal glands develop from the neural crest ectoderm and the intermediate mesoderm. The medulla originates from the neural crest cells migrating from the sympathetic ganglion. Mesodermal cells then surround the neural crest cell, and the adrenal cortex develops from these. The gland is fully developed by 4 months of age.

The glands are supplied by the superior, middle and inferior suprarenal arteries, which branch before entering the adrenal capsule. Within the capsule, they branch again to give capsule capillaries, cortical sinusoidal capillaries and medullary arteries. The fenestrated sinusoidal capillaires supply the cortex then drain into fenestrated medullary capillary sinusoids. The medulla there receives a dual blood supply, with venous blood from the cortex and a direct arterial supply from the medullary arteries.

After perfusion, deoxygenated blood drains into adrenomedullary collecting veins and then pools in a central adrenomedullary vein, which empties in the caudal vena cava.

  1. The adrenal cortex

The adrenal cortex represents 80-90% of the gland and is made up of 3 layers:

  • Zona glomerulosa (15%) – secretes mineralocorticoids.

  • Zona fasciculata (60%) – secretes glucocorticoids.

  • Zona reticularis (25%) – secretes sex steroids or androgens.

All 3 of these zones produce hormones derived from cholesterol.

The cortex is red to light brown in appearance and radially striated. It is derived from the mesoderm, from a patch of celomic epithelium close to the gonadal ridge.

The cortex produces adrenocorticoid hormones from esterified cholesterol. Most cholesterol is actively taken up from the plasma by receptor mediated endocytosis of LDLs. Cholesterol is first converted to pregnenolone by the P450scc enzyme. This is regulated by ACTH and is the rate determining step of adrenocorticoid hormone production.

Pregnenolone is converted to different hormones by hydroxylation reactions in the mitochondria and endoplasmic reticulum. The location within the cortex determines which hormone is produced.

  1. Mineralocorticoids

Mineralocorticoids regulate the metabolism of inorganic ions such as sodium, potassium and chloride. The most important mineralocorticoid is aldosterone. It is produced from the conversion of pregnenolone to cortisol then corticosterone and finally aldosterone. It circulates bound to albumin or cortisol-binding globulin. It is eventually inactivated in the liver, where it binds to glucuronic acid and is excreted in bile and urine. It is regulated by renin and extracellular K+ ion concentrations. It mostly acts on renal regulation and electrolyte and fluid balance.

Secretion of aldosterone from the adrenal glands is controlled by angiotensin II as part of the RAAS and extracellular concentrations of potassium. Extracellular potassium acts directly on aldosterone producing cells. It depolarises these cells, opening voltage-gated calcium channels which stimulates aldosterone synthesis.

Aldosterone increases the amount of sodium present in the body. In the distal tubules and collecting ducts of the kidneys, it increases the number of Na+/K+ pumps in the basolateral membrane and the number of sodium ion channels in the apical membrane.

  1. Glucocorticoids

Glucocorticoids regulate glucose metabolism. They cause hyperglycaemia and reduced utilisation of glucose in the cells. The most important glucocorticoid is cortisol. This is formed in the zona fasciculate by the conversion of...

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