Kidney development: two tales of tubulogenesis

The mammalian kidney may well be one of the most complex organs of postnatal life. Each adult human kidney contains on average more than one million functional filtration units, the nephrons, residing within a specialized cellular interstitium. Each kidney also contains over 25 distinct cell types, each of which must be specifically aligned with respect to each other to ensure both normal development and ultimately, normal renal function. Despite this complexity, the development of the kidney can be simplistically described as the coordinate formation of two distinct sets of tubules. These tubules develop cooperatively with each other in time and space, yet represent two distinct but classical types of tubulogenesis. The first of these tubules, the ureteric bud, forms as an outgrowth of another epithelial tube, the nephric duct, and undergoes extensive branching morphogenesis to create the collecting system of the kidney. The second tubules are the nephrons themselves which arise via a mesenchyme-to-epithelial transition induced by the first set of tubules. These tubules never branch, but must elongate to become intricately patterned and functionally segmented tubules. The molecular drivers for these two tales of tubulogenesis include many gene families regulating tubulogenesis and branching morphogenesis in other organs; however, the individual players and codependent interrelationships between a branched and non-branched tubular network make organogenesis in the kidney unique. Here we review both what is known and remains to be understood in kidney tubulogenesis.