Fig. 4

Graphical sketches show the structural relations between the CD ampulla and the developing nephron in the fetal human kidney during late gestation. The distal end of a collecting duct (CD) dilates to form a CD ampulla (A), which expands radially. It consists of a tip (T) pointing to the renal capsule (C), head (H), conus (CO), and neck (N). There are only 2 to 3 layers of mesenchymal progenitor cells (MES) separating the renal capsule from the tip. a A striking interface (black asterisk) separates the epithelial progenitor cells in the tip of a CD ampulla from the innermost layer of nephrogenic mesenchymal progenitor cells. This special arrangement indicates the center of a nephrogenic niche. The forming pretubular aggregate (PTA) is first found close to the tip and then head of the CD ampulla. b The mesenchymal to epithelial transition and the initial separation of the primitive renal vesicle occurs at the border between the head and conus of the CD ampulla. c At this point, its medial aspect adheres so that the future connecting duct tubule (CNT) can form. Although separated by an interstitial cleft, the medial aspects of the d mature, e extending, f extended renal vesicles, the g early, h mid, and i late comma-shaped bodies (CSB) as well as the j early, k mid, and l late S-shaped bodies (SSB) are near the concave conus of the CD ampulla. The proximal pole (PP) of the S-shaped body is near its neck and faces the underlying maturation zone. In contrast, the lateral aspects of the renal vesicles, the comma- and the S-shaped bodies are covered by the loose mesenchyme and face a vertically lining perforating radiate artery (PRA, short black arrow). Collecting duct (CD) tubule, CD ampulla (A) composed of a tip (T), head (H), conus (CO) and neck (N), and lumen of the arising nephron +