Adrenal Gland

These are a pair of glands, one associated with each kidney.

They have an inner medulla and outer cortex, and there are different types of endocrine tissue in each.

The embryological origin of the cortex, is similar to that of the gonads. The embryological origin of the medulla is the same as that of the sympathetic nervous system.

Adrenal cortex

All the hormones secreted by this region are steroid hormones, which are all based on cholesterol. Secretory cells, secrete steroid based hormones, and contain triglyceride droplets. The cortex can be divided into three regions:

• Zona glomerulosa
• Zona fasciculata
• Zona reticularis

Different hormones are secreted from each of these regions

1. Zona glomerulosa, the outermost zone of the adrenal cortex secretes mineralcorticoids. These hormones are important for fluid homeostasis. These include aldosterone, which regulates absorption/uptake of K+ and Na+ levels in the kidney.

The secretory cells are arranged in irregular ovoid clusters that are surrounded by trabeculae which contain capillaries. Can you identify them?

The nuclei stain strongly, and the cytoplasm is less pale than that of the next zone, the zona fasciculata, as there are fewer lipid droplets in these cells.

2. Zona fasciculata, the middle zone of the adrenal cortex secretes glucocorticoids which are important for carbohydrate, protein and lipid metabolism. An example is cortisol which raises blood glucose and cellular synthesis of glycogen. Its secretion is controlled by a hormone from the pituitary - ACTH.

The secretory cells are arranged in cords, often one cell thick, surrounded by fine strands of supporting tissue. Can you identify them?

The nuclei of these cells stain strongly, and the cytoplasm is rich in sER, mitochondria and lipid droplets. The cytoplasm looks pale and 'foamy' due to the presence of lipid droplets.

3. Zona reticularis, the innermost layer of the cortex, secretes sex hormones (androgens). and small amounts of glucocorticoids. These hormones are secreted by the inner zone of the cortex, which is called the zona reticularis.

Some brown pigment is seen in some of these cells - this is lipofuscin, probably an insoluble degradation product of organelle turnover - an 'age' pigment. The cytoplasm of the cells in this region stains more darkly, and contains fewer lipid droplets.

Can you identify the secretory cells and pigment?

Adrenal medulla:

This region of the adrenal glands contains basophilic staining cells, with a granular cytoplasm and no stored lipid. It also contains many venous channels which drain blood from the sinusoids of the cortex, pass through the medulla, and drain into the medullary vein.

This is because these cells are actively secreting the peptide based hormones - nor-adrenaline and adrenalin (catecholamines), which are stored in the granules.

Can you identify veins and secretory cells?

Secretion of these hormones is controlled by the sympathetic nervous system. The targets of these hormones are the adrenergic receptors in the heart, blood vessels, bronchioles, visceral muscle, skeletal muscle, and in the liver, where they promote glycolysis (breakdown of glycogen).