Morphological Bases for Evolution of Functions

The morphological and morphometrical changes in the renal proximal tubular cells of the salmonOncorhynchus gorbuscha during spawning migration are described for the first time. The evaluation of the ultrastructural changes in the kidney of the adult humpback salmon allows concluding that the proximal tubular cells are affected by significant osmotic loads during spawning migration. Various destructive processes in the proximal tubular cells indicate a decrease of their functional activity in the fresh water. The analysis of the results of the study of the humpback salmon kidney ultrastructure during the anadromous migration allow suggesting that reduction of the amount of the most active cell organelles in the proximal tubule, such as mitochondria and the basolateral membrane system, is associated with a decrease of the number of ion pumps that operate less intensively in the fresh water. Such mechanism of the kidney functioning in the euryhaline fish appears to be formed in the course of evolution as an adaptation to the freshwater environment. The morphological and morphometrical changes in the proximal tubules of nephrons were described for the first time in Pacific salmon smoltsOncorhynchus keta, O. gorbuscha,O. Kisutch, andO. masou in environments with different salinity. The most marked response to the transfer to the sea water or to the calcium-enriched sea water was observed in the chum salmon smolts. In 48 h, the tubular diameter reached its peak, more than 72 μm, the tubular lumen also was very large, while the epithelium height was only slightly greater than that in the fresh water control. In spite of the considerable morphological changes, as soon as in 24 h (the Ca²⁺-enriched sea water) and in 48 h (the sea water) the ion content in smolt carcasses was within the physiological norm for the environment with enhanced salinity. These results indicate a high efficiency of the osmoregulatory system in smolts during their transfer from the fresh to the sea water.