Development of the zebrafish mesonephros.

The vertebrate kidney plays an essential role in removing metabolic waste and balancing water and salt. This is carried out by nephrons, which comprise a blood filter attached to an epithelial tubule with proximal and distal segments. In zebrafish, two nephrons are first formed as part of the embryonic kidney (pronephros) and hundreds are formed later to make up the adult kidney (mesonephros). Previous studies have focused on the development of the pronephros while considerably less is known about how the mesonephros is formed. Here, we characterize mesonephros development in zebrafish and examine the nephrons that form during larval metamorphosis. These nephrons, arising from proliferating progenitor cells that express the renal transcription factor genes wt1b, pax2a, and lhx1a, form on top of the pronephric tubules and develop a segmentation pattern similar to pronephric nephrons. We find that the pronephros acts as a scaffold for the mesonephros, where new nephrons fuse with the distal segments of the pronephric tubules to form the final branching network that characterizes the adult zebrafish kidney.