Fig. 2

View onto the outer cortex of the fetal human kidney during advanced pregnancy by the optical microscope. Covered by the renal capsule (C), one can recognize the nephrogenic zone (NZ), and underneath the beginning maturation zone (MZ). Both are separated by a transverse dashed line. The vertical and transverse black lines indicate that the nephrogenic zone can be divided into side by side aligned nephrogenic compartments. In each, a collecting duct (CD) ampulla (A) is visible, which represents the dilated ureteric bud-derived end of the CD tubule. The CD ampulla consists of a tip (T), head (H), conus (CO), and neck (N). In the space between the inner side of the renal capsule and the tip of a CD ampulla (vertical double arrow), an outer layer of interstitial/stromal progenitor cells and 2 to 3 subjacent layers of nephrogenic progenitor cells are located. The innermost layer faces a clear interface (black asterisks). The pretubular aggregate (PTA) is positioned opposite the head of the CD ampulla. When a transverse double line is drawn along the section border between the head and conus of the CD ampulla to the meeting between the proximal end of the pretubular aggregate and the underlying connecting tubule (CNT) of an earlier developed nephron, the nephrogenic compartment can be subdivided. The upper part represents the district of progenitor cell recruitment (DPCR). Dependent on the developmental progress, in the subjacent area of nephron shaping (ANS), either a renal vesicle, a comma-, or a S-shaped body (SSB) is visible. Most relevant for the installation is the adhesion (white asterisks) of the proximal end at the pretubular aggregate on the CD ampulla, and the mounting of the future proximal pole at the henceforth shaping renal vesicle (RV) next to the connecting tubule of an earlier developed S-shaped body. The distance between its proximal (PP) and distal (DP) poles reflects the maximal vertical width of the area of nephron shaping. PRA vertically lining perforating radiate artery