An update on the evolutionary origin of aglomerular kidney with structural and ultrastructural descriptions of the kidney in three fish species.

The kidney of fish contains numerous nephrons, each of which is divided into the renal corpuscle and renal tubules. This glomerular structure is the filtration unit of the nephron and is important for the kidney function, but it has been reported that the renal corpuscle was lost in at least four independent linages of fish (i.e., aglomerular kidney). In this study, the authors newly described renal structures for three species by histological and ultrastructural observations: two aglomerular kidneys from a seahorse Hippocampus barbouri and a toadfish Allenbatrachus grunniens and a glomerular kidney from a snake eel Pisodonophis boro. The renal development of H. barbouri was also described during 1-35 days after birth. In all species tested, the anterior kidney was comprised of haematopoietic tissues and a few renal tubules, whereas the posterior kidney contained more renal tubules. Although the glomerular structure was present in P. boro, light microscopic observations identified no glomeruli in the kidney of H. barbouri and A. grunniens. Ultrastructurally, abundant deep basal infoldings with mitochondria in the renal tubules were observed in A. grunniens compared to H. barbouri and P. boro, suggesting the possible role of basal infoldings in maintaining the osmotic balance. By integrating the results from the three species and comprehensive literature search, the authors further showed that 56 species have been reported to be aglomerular, and that the aglomerular kidney has evolved at least eight times in bony fishes.