Kidney Morphology in Marine and Terrestrial Birds and Its Phylogenetic Links to Mammals via Aquaporin (AQP) Genome Sequences

The kidney is the main osmoregulatory organ in terrestrial vertebrates and depends on water conservation for survival. Only birds, among vertebrates other than mammals, can concentrate their urine as an adaptation to live in terrestrial environments. The water inflow of collecting duct epithelial cells is controlled by aquaporin 2 (AQP2) and an arginine vasotocin (AVT)-sensitive water channel expressed along cortical and medullary collecting ducts. This study aims to examine the anatomical, histological, histochemical, and ultrastructural variations in the kidneys of two avian species: marine birds, mallard duck (Anas platyrhynchos), and terrestrial birds, domestic chicken (Gallus gallus domesticus). Furthermore, phylogenetic relationships have been used to compare the AQP2 genome sequences of these two species with those of mammalian vertebrates. Histologically, the kidneys of the two studied species did not differ in structure. In the mallard, the renal lobule was comparatively divided into an elongated medulla (30%) and cortex (70%) and had more abundant cortical reptilian nephrons. However, the chicken renal lobule was a larger cortical region (85%) than the medullary region (15%) and had more mammalian nephrons. The brush border of the proximal convoluted tubules and the basement membrane of the nephron lining cells were positive for PAS stain but negative for AB, whereas the collecting ducts and the luminal surfaces of the distal convoluted tubules were positive for AB. Glomeruli and proximal convoluted tubules were more likely to react with bromophenol blue stain in chicken than in duck. TEM revealed that the cells lining the proximal, distal, and collecting renal tubules, as well as collecting ducts in duck, contained active and hypertrophied mitochondria, so the kidneys were more active in duck than in chicken. Multiple sequence alignment and phylogenetic tree analysis revealed that the anticipated genomic sequences of AQP2 are relatively conserved among avian species. In conclusion, studies of nephrons have identified both interspecies similarities and intraspecies differences among avian species.