Fig. 6

Graphical sketches depict the expanding interstitium, which envelops the developing nephron in the fetal human kidney during late gestation. In the district of progenitor cell recruitment, the interstitium is recognized at the inner side of the renal capsule, and between the mesenchymal progenitor cells. a In the nephrogenic niche, an interface is seen between the innermost layer of nephrogenic mesenchymal progenitor cells and the tip of the CD ampulla. This extends at the lateral aspect of the pretubular aggregate (PTA), b the mesenchymal to epithelial transition, and c the primitive renal vesicle (RV). In the area of nephron shaping, the interstitium is between the medial aspect of the d mature, e extending, and f extended renal vesicle. Furthermore, it covers the conus of the CD ampulla, where a vertical interstitial cleft forms. In parallel, between the progenitor cell strand (white arrow head) connecting the renal vesicles with the pretubular aggregate and the tubule anlage (TA) including the connecting tubule (CNT) a narrow interstitial cleft becomes visible. During the development of g the early comma-shaped body, the progenitor cell strand dissolves (black arrow head). As a result, during the formation of the h mid and i late comma-shaped body (CSB), the cleft opens near the lateral fold (black/white point). Due to the winding of the tubule anlage, the interstitial cleft turns from a vertical to transverse direction during the development of the j early, k mid, and l S-shaped body. Yet, it opens at the deep lateral aspect. In contrast, the interstitial cleft between the medial aspect of the S-shaped body lines first along the conus of the CD ampulla and then to the connecting and finally to the distal tubule segments before reaching the geometrical center. Renal capsule (C), collecting duct (CD) tubule, CD ampulla (A), perforating radiate artery (PRA, short black arrow), and lumen of the arising nephron +