Fig. 3

View onto the installation of the nephrogenic niche, the pretubular aggegate (PTA) and the mesenchymal to epithelia transition (MET) in the fetal human kidney during advanced pregnancy by the optical microscope. At the niche site (black asterisks), the innermost layer of the nephrogenic progenitor cells faces the epithelial progenitor cells, which are contained in the tip (T) of a CD ampulla (A). Between both cell types, a clear interface is visible. This extends up to the medial part of the pretubular aggregate (PTA). The lateral part of the pretubular aggregate is exposed to the subcapsular interstitium. The medial side of the proximal end (PE) at the PTA shows an adhesion (white asterisks) to the CD ampulla at the section border between its head (H) and conus (CO). The mid of the proximal end at the PTA indicates the proximal pole of the prospective nephron and its mounting (hatched) next to the connecting tubule (CNT) including the distal pole (DP) of a previously developed S-shaped body. Solely here, the mesenchymal to epithelial transition becomes active. The lateral side of the proximal end at the pretubular aggregate becomes positioned near a vertically lining perforating radiate artery. C renal capsule