Adult nephrogenesis and persistence of Six2 expressing progenitor cells in the Reptilian kidney

Abstract

Kidney formation proceeds in reptiles, birds, and mammals through the sequential development of the pronopheros, mesonephros, and finally the metanephros. Metanephric nephron formation, termed nephrogenesis, ceases in mammals around birth and is completely absent in adults. We investigated the presence of adult nephrogenesis in reptiles by examining adult kidneys from several species including T. scripta, C. picta, B. constrictor, T. tegu, A. carolinensis, and A. mississipiensis. We found that all major reptilian groups (Testudines, Archosauromorpha and Squamates) showed the presence of adult nephrogenesis. The total amount of nephrogenesis varied greatly between species with turtles and alligators displaying the highest density of nephrogenesis. The nephron is derived from progenitor cells which express the transcription factor Six2. In mammals, Six2 expression is downregulated as the progenitor cell population is exhausted and nephrogenesis ends. Since we detected nephrogenesis in adult reptiles, we postulated that Six2 expression persists in these species. Unlike in mammals, we were able to detect Six2 protein expressing condensing mesenchyme in adult alligator kidneys, correlating with areas of nephrogenesis. This suggests that reptiles have maintained the ability to continually grow new nephrons during adulthood due to the presence of a Six2‐expressing progenitor cell population.