Fig. 2

View onto a the pretubular aggregate (PTA), b the mesenchymal to epithelial transition, and c the primitive renal vesicle (RV) in the fetal human kidney by optical microscopy. a The upper frame in a nephrogenic compartment marks the district of progenitor cell recruitment. The lower frame shows the area of nephron shaping. The distal end (DE, renal capsule-orientated) of a pretubular aggregate is in contact with nephrogenic mesenchymal (MES) progenitor cells. Separated by a clear interface (black asterisks), it extends along the tip (T) and head (H) of the CD ampulla (A). Between the head and the proximal end (PE, medulla-orientated) of the pretubular aggregate, a close adhesion (white asterisks) is seen. b Interstices between cells and smoothening of the surface at the proximal end of the PTA indicate the mesenchymal to epithelial transition. Remarkable is the transition from the clear interface to the close adhesion, and a lumen (cross) becomes visible in the proximal end. The flash indicates the starting separation of the RV from the PTA. c A primitive renal vesicle is still part of the pretubular aggregate. At its distal pole (DP), a significant but only partial separation from the PTA is noticed (flash). The epithelium of the renal vesicle contacting the conus (CO) of the CD ampulla is more prominent than at its lateral aspect. The lumen (cross) extends and a basal lamina covers the proximal pole (PP) of the renal vesicle. C renal capsule, short black arrow vertically lining perforating radiate artery, SSB S-shaped body