Unraveling the origin of the vertebrate kidney: Emergence from a somitic compartment

Somite compartmentalization and kidney development are considered as two distinct developmental processes. It is widely acknowledged that in amniotes as well as in frogs and bony fish, the pronephros and therefore the adult kidney develop from the intermediate mesoderm. Our study sheds light on the evolution of pronephros formation in vertebrates, revealing its ancient origin from a specific, segmented, somitic subterritory, the nephrotome, which buds off during early development to create the nephric duct. In nonvertebrate chordates, mesoderm markers are expressed in common progenitor cells of dermomyotome and nephrotome, whereas in cyclostomes and elasmobranchs, they segregate into distinctive domains within the somite. Two main discoveries are thus described here: (i) The intermediate mesoderm originated developmentally and evolutionarily from a specific domain within somitic tissues, and (ii) the pronephric progenitor cell domain is redefined as the fifth somitic compartment, the nephrotome, alongside the dermatome, myotome, syndetome, and sclerotome. Our results support the hypothesis that this fifth somitic domain, the nephrotome, is the evolutionary predecessor of the intermediate mesoderm, giving rise to the pronephros and hence to the adult kidney.