Pre-pattern in the pronephric kidney field of zebrafish

Vertebrate embryos use a series of transient kidneys to regulate fluid bala nce, osmolarity and metabolic waste during development. The first kidney to form in the embryo is the pronephros. This kidney is composed of several c ell types with very different functions and is organized into discrete segm ents: glomerulus, tubules and nephric duct. The site of origin of these cel ls is poorly understood, as are their lineage relationships. We have define d regions of the intermediate mesoderm as candidates for the pronephric fie ld by expression patterns of the Wilms' Tumor suppressor gene (wt1), single -minded 1 (sim1) and pax2.1, All of these potential kidney markers are expr essed in a stripe of intermediate mesoderm, with distinct, overlapping ante ro-posterior borders. We labeled small groups of cells in this area by lase r uncaging of a fluorescent dextran, and then tracked their fates. We found that there was a bounded contiguous region of the intermediate mesoderm th at provides pronephric progenitors. As is true for other organ fields, the pronephric field regulates after focal destruction, such that a normal pron ephros forms after laser-mediated removal of the wt1 domain, The progenitor s for podocytes, tubular cells and duct are restricted to subdomains within the pronephric field. The most anterior cells in the pronephric field give rise to podocytes, This corresponds to the wt1-expressing region. The next more posterior cells contribute to the tubule, and express both wt1 and pa x2.1. The most posterior cells contribute to the nephric duct, and these ex press pax2.1 and sim1, but not wt1. Thus, there is a field for the pronephr ic kidney with classical attributes of defined border, pre-pattern and regu lation. The pattern of the fate map reflects particular combinations of tra nscription factors.