The Xenopus Pronephros: A Kidney Model Making Leaps toward Understanding Tubule Development

The Xenopus embryonic kidney, the pronephros, serves as a useful model for the study of nephron development as well as tubulogenesis in general. The history of embryological studies on pronephric development through transplantation, explant, and animal cap experiments provides a valuable body of knowledge on which to base current studies. Continued investigation of the molecular underpinnings of nephric formation and the sustained development of innovative techniques complement the fundamental embryological advantages of this system. Because the Xenopus pronephros is composed of a single functional nephron that lies just beneath the ectoderm through all stages of development, it can be easily manipulated via microdissection techniques and visualized both in living embryos and in fixed tissue. The ability to target molecular alterations specifically to the pronephric anlagen through the application of kidney-specific transgenic promoters along with cell fate-based microinjections of mRNAs or morpholinos provides the ability to manipulate gene expression in a spatially restricted manner. Recently, light-inducible morpholinos have provided a means to temporally regulate gene expression in Xenopus, as have transgenic promoters. Application of targeted nuclease technologies should also soon provide the ability to manipulate amphibian models at the level of genomic DNA. Because the Xenopus pronephros progresses into a functional nephron in 3-4 days within an embryo that develops outside the mother, it provides unique advantages for observations of nephrogenesis. Additionally, recent work suggests that the Xenopus kidney will facilitate studies on nephron regeneration. In this chapter, we will review key studies in Xenopus pronephric development and highlight current innovations that will accelerate our understanding of kidney formation and tubulogenesis in the near future.