Fig. 3

a–d View onto the interstitial situation at the a mature, b extending, c extended renal vesicles (RV), and the d early comma-shaped body (CSB) in the fetal human kidney during advanced pregnancy by the optical microscope. a While the mature RV arises, at its distal pole, a transverse but only partial separation (dashed black line) from the overlying pretubular aggregate (PTA) occurs. This is visible at the end of the clear interface (black asterisks) and the beginning of the adhesion (white asterisk) next to the section border between the head (H) and conus (CO) of the CD ampulla (A). The lateral part of the distal pole at the RV remains connected with the proximal end of the PTA via a two-layered progenitor cell strand (arrow head). b In the extending RV, the interjacent cleft transversely expands. The distal pole of the RV stays near the proximal end of the PTA so that a cone is formed by the subcapsular interstitium (yellow line with crosses) and the perivascular interstitium (white boxes with red line) of a perforating radiate artery (PRA). The future connecting tubule (CNT) starts to invade the CD ampulla, while the tubule anlage (TA) elongates. c Due to the vertical elongation of the CD ampulla and the internal folding in the extended RV, the interstitial cleft (white arrow) between the CNT and the progenitor cell strand turns direction from transverse to vertical, increases in length, and forms a pocket. In parallel, the narrow vertical interstitial cleft between the conus of the CD ampulla and the medial aspect of the renal vesicle expands in vertical direction. It originates from the cone, which is formed by the peritubular interstitium of a previously developed CNT (black line) and the perivascular interstitium of the perforating radiate artery. d While the early comma-shaped body is forming, the progenitor cell strand, which earlier connected the distal pole of the extended renal vesicle with the proximal end of the PTA, dissolves. This causes that the interstitial pocket yet fuses with the perivascular interstitium next to the PTA. As a consequence, between the tubule anlage and the developing lateral fold, a vertical cleft (white boxes with red line and arrow head) arises. This is a morphological sign that the glomerulus defoliates at the proximal pole. + Lumen