Physiology of Finfish and Shellfish (2+1)

5.2.8. Adrenal cortical tissue or Inter-renal tissue

There is no true adrenal gland present in most fish (exception is sculpins- The sculpin is a spiny fish in the order Scorpaenidae, family Cottidae. They are relatives of scorpion fish, and common in North American waters. The sculpin can be found in both salt and fresh water, and commonly in tide pools and other shallow water, where it clings close to the bottom). In a large number of teleosts, the inter-renal and chromaffin cells are found in the head kidney which is composed of haemopoietic or lymphoidal tissue.

Only a few renal tubules are seen in the head kidney. The inter-renal cells may be located in 1 or several layers along the post cardinal vein. In Puntius ticto, these cells form a thick glandular mass, but in Channa punctatus, they are arranged in the form of cords of cells along the vein. Considerable variations have been seen in the structure and distribution of inter-renals in teleosts and fishes can be divided into 4 groups (Nandi, 1962).

Group I: Inter-renal cells surround the post cardinal vein or its large branches

Group II: Inter-renal cells are diffused throughout the head kidney, along small and medium sized veins.

Group III: Inter-renal cells are associated with venous sinuses in the head kidney

Group IV: Inter-renal tissue forms a solid mass in a localised area in the head kidney.

The inter-renal cells may be columnar, polygonal, and cuboidal or spindle shaped, each having a large round nucleus and are stained with eosin. These cells may be closely associated with chromaffin cells. In fishes treated with drugs and hormones or subjected to hypophysectomy (Surgical removal of the pituitary gland) or salinity changes, the inter-renal cells exhibit changes in shape and structure. These cells show vacuolisation (Vacuolization is the formation of vacuoles within or adjacent to cells) and reduction in size, in a hypophysectomised fish. Temperature shock, treatment with heavy metals and maturation of gonads cause histological alterations in the inter-renal cells of the fish.

Two kinds of secretions are produced by the inter-renal cells. These are

1. Glucocorticoids that influence blood sugar
2. Mineral corticoids effecting osmoregulation

The inter-renals of teleosts are homologous with the adrenal cortex of mammals and secrete corticosteroids. But, their level in the blood varies considerably depending upon the species, season and physiological state of the fish. The corticosteroids are important for the regulation of water and electrolyte balance and in the metabolism of CHO and proteins. The inter-renals play an important role in the osmotic regulation of the fish. They are involved in gluco-genesis (The formation of glucose through the breakdown of glycogen). Experiments have shown that inter-renal ectomy in eels produces hypoglycaemia (Hypoglycemia also called low blood glucose or low blood sugar, occurs when blood glucose drops below normal levels) and loss of liver glycogen. It has also been shown that inter-renals are directly related to gonad development and maturation and are also involved in stress and spawning migration.