Kidney development and regeneration: An introduction to this volume in Current Topics in Developmental Biology.

Mechanistic studies of renal development arguably began 70 years ago, in 1955 when Clifford Grobstein identified an inductive interaction between ureteric bud and metanephric mesenchyme. As an introduction to a special volume of Current Topics in Developmental Biology, this review looks back over the decades since Grobstein's paper to ask how well we have now answered the mechanistic questions raised in his 'pre-molecular' age, and to highlight new questions that have emerged from an increasing understanding of how kidneys develop. I consider that some old questions, such as lineage, have been answered fairly comprehensively. Some questions such as the nature of inductive signalling have become much more complicated, as a notion of 'the signal' has been replaced by hundreds, or possibly thousands, of communications that coordinate renal development. Some old questions, particularly about morphogenesis, remain open. Others, such as metabolism, were ignored for decades but are now being studied again, very profitably. New topics, such as stem cell behaviour, self-organization, epigenetics and congenital abnormalities, join work on the old ones. We have undoubtedly learned much over the last 70 years but, strangely perhaps, the number of questions still to be answered now seems much larger than it did in decades long past.