Fig. 2

a–d View onto the interstitial situation at the a nephrogenic niche and pretubular aggregate (PTA), b mesenchymal-to-epithelial transition, c arising, and d established primitive renal vesicle (RV) in the fetal human kidney during advanced pregnancy by the optical microscope. The transverse double line marks the border between the district of progenitor cell recruitment and the underlying area of nephron shaping. The dashed line indicates the border between the overlying nephrogenic zone and the subjacent maturation zone. a At the niche site, the nephrogenic progenitor cells are separated by a clear interface (black asterisks) from the epithelial progenitor cells, which are contained in the tip (T) of a CD ampulla (A). b During the mesenchymal-to-epithelial transition, the medial part of the PTA faces the clear interface, while its lateral part is exposed to the subcapsular interstitium (yellow line with crosses). Determining, the medial side at the proximal end of the PTA shows an adhesion (white asterisk) to the CD ampulla at the section border between its head (H) and conus (CO). The mid of the proximal end at the PTA faces the interstitium at the connecting tubule (black line) of a previously developed nephron. The lateral side of the proximal end at the PTA is positioned near the perivascular interstitium (white boxes with red line) of an ascending perforating radiate artery (PRA). c and d The arising primitive RV presents polarized epithelial cells first at the proximal end of the PTA. Here, also a small lumen ( +) becomes visible. The distal pole of the RV remains preliminarily connected with the PTA. The basal lamina at the proximal pole of the RV is part of a cone, which is formed by the perivascular interstitium of the PRA and the peritubular interstitium of the connecting tubule (CNT) at a previously formed nephron. SSB, S-shaped body