Pronephros and mesonephros characterization during the embryonic development of the giant South American river turtle, Podocnemis expansa (Podocnemididae: Testudines)

This study aimed to describe pronephros and mesonephros morphology during the embryonic development of Podocnemis expansa. Eggs were collected on an artificial beach at Balbina, Amazonas, Brazil, during the entire incubation period (mean of 59 days). The kidney-gonad complex was processed using light microscopy and the mesonephros using transmission electron microscopy. The pronephros was present for the first time on stage 4, composed of external glomeruli devoid of a capsule, protruding into the coelomic cavity, and internally composed of a capillary network. The pronephros degenerated after development stage 15. The first sign of the appearance of the mesonephros occurred around stage 8, indicated by the early formation of renal corpuscles. The mesonephros comprised an renal corpuscles, neck segment, proximal tubule, intermediate segment, distal tubule, collector tubule, and collector duct. Ultrastructural analysis of the mesonephros brush border was done in the proximal tubule, and the presence of cells with structural characters indicative of secretory activity was detected in the juxtatubular region. Renal corpuscles and proximal tubules were the main components that underwent morphological alterations during mesonephros degeneration. The pronephros is a transient kidney, and the mesonephros became the functional embryonic kidney in P. expansa. Mesonephros degeneration occurs in the cranial-caudal direction, and histologically, the degeneration is identified by changes in the morphology of the renal corpuscle and proximal tubule. However, the mesonephros is still present after hatching.