Fig. 3

View onto the a mature, b extending, and c extended renal vesicle (RV) in the fetal human kidney by optical microscopy. a In a mature renal vesicle, the medial part of the distal pole (DP, small black/white arrow head) is fixed on the CD ampulla (A) at the border between its head (H) and conus (CO). In the middle of the distal pole, the separation (flash) persists. The lateral part of the distal pole remains connected with the pretubular aggregate (PTA) via a two-layered progenitor cell strand (larger black/white arrow head). The proximal pole (PP) is orientated towards the medulla. The lumen (cross) expands unevenly. Between the conus of the CD ampulla and the medial aspect of the renal vesicle, the interphase (white asterisks) lines in vertical direction. b The extending renal vesicle is fixed via the future connecting tubule (CNT) on the CD ampulla. At the proximal pole, the lumen is slightly rounded. At the distal pole (DP) the epithelium invaginates (black/white arrow head) to generate the tubulus anlage (TA). The progenitor cell strand (white arrow head) at the lateral part of the distal pole is connected with the PTA. The interphase between the conus of the CD ampulla and the medial aspect of the renal vesicle is transforming into to a vertically lining cleft (white arrows). c In the extended renal vesicle, the fixing on the CD ampulla at the border between its head and conus is consolidating by invasion of the future connecting tubule. At the lateral part of the distal pole, the renal vesicle is connected with the pretubular aggregate via a two-layered progenitor cell strand (white arrow head). The epithelium at the distal pole further invaginates towards the underlying lumen, resulting in an inner fold (black spot). The medial leg of the inner fold lines to the part of the CNT that is ending at the head of the CD ampulla. The lateral leg lines to the cell strand (white arrow head) connected with the PTA. Between the legs of the inner fold, a vertically lining cleft (dashed white arrow) arises. The cleft (white arrows) between the medial aspect of the renal vesicle and the conus of the CD ampulla elongates and changes its direction from vertical to slightly transverse. The lumen exhibits a supine C-form. C renal capsule