[1] Once a more advanced kidney forms, the previous version typically degenerates by apoptosis or becomes part of the male reproductive system.
It is present at the embryo of more advanced fish and at the larval stage of amphibians where it plays an essential role in osmoregulation.
In pronephroi the glomerulus (or glomus if it extends over multiple body segments) projects into the coelom rather than into the proximal tip of the nephron.
These thin epithelial tubes are densely packed with cilia and have a distinct morphology to the other tubular epithelia of the kidney.
Older anatomical texts describe the pronephros as condensing from nephrotomes, but modern visualization techniques have shown that this represents a histological artifact.
While this transient primordium never forms functional nephrons, the duct derived from it is essential to the development of the more complex later kidneys.