Physiology of Finfish and Shellfish (2+1)

2.3.5. Internal structure of kidney

Fish kidney consists of a large number (hundreds to 10,000) of tubular nephrons or uriniferous tubules each of which produces urine. Each nephron is made up of or consists of renal corpuscle or malpighian capsule and the tubule. The head kidney differs from the trunk kidney as the renal corpuscles and tubules are generally absent and it is composed of lymphoid, hematopoietic, interrenal and chromaffin tissue (Fig). A few nephrons may be seen in the head kidney of some species but the renal corpuscles are usually absent, so that it is not excretory in function. The head kidney does not show excretory function.

(interrenal tissue and chromaffin tissue are the endocrine glands and is composed of lymphoid tissue that is particularly rich in lymphocytes , hematopoietic tissue means Blood-forming tissue, consisting of reticular fibers and cells ).

A typical nephron of freshwater teleosts is called glomerular nephron and generally consists of

1. A renal corpuscle containing a well developed vascularized glomerulus.
2. A ciliated neck region connecting the corpuscle with the tubule.
3. A proximal ciliated segmented which is further divided into 1^st and 2^nd parts.
4. An intermediate ciliated segment is present between the 2^nd proximal segment and the distal segment.
5. A distal segment with clear cells consists of several mitochondria.
6. A collecting duct tubule or system

In the marine teleosts, the distal segment is probably absent. In the freshwater teleosts, the kidney has a large number of renal corpuscles each containing a highly vascular glomerulus, hence called glomerular nephron. However, the renal corpuscles are reduced in number and size in the marine teleosts. The kidneys of marine fishes generally contains few, poorly developed vascularized glomeruli than the freshwater teleosts hence called aglomerular nephron. Eg. Therapon, Arius etc.