Research in developmental biology, as in many other fields of basic biology, is carried out largely in model-systems. In these, relatively simple events are singled out from the complex situation in vivo and examined under controllable conditions. Such model-systems should be simple enough to allow a meaningful analysis, but sufficiently complex to warrant extrapolation of the results to the normal condition in vivo. The vertebrate kidney is one such model-system for cell differentiation and organogenesis. It is my hope that the observations and conclusions quoted in this book could be of wider interest to developmental biologists, and that the use of the kidney system could unravel general developmental principles applicable to other cells and organs.

My work on kidney development was initiated during a postdoctoral stay in the laboratory of Clifford Grobstein at Stanford University. The research was continued in Helsinki in close collaboration with my late friend Tapani Vainio. Never, since then, have I worked with a more stimulating scientist, and many of Tapani's ideas guided our research long after his accidental death in 1965.

The ‘Wetterkulla Medical Center’ (WMC), an informal biannual working symposium, has met regularly during the last 25 years. These occasions have been of great inspiration to me, and they have also provided time for me to write this book. I thank my friends at the WMC: Sulo Toivonen, Osmo Järvi, Kari Penttinen, Erkki Saxén, Harri Nevanlinna, Esko Nikkilä, Kari Cantell and the late Bo Thorell.

I wish to express my deep gratitude to all my students, research fellows and colleagues, whose results and ideas I have quoted in this monograph and whose material I have used extensively for illustration.