Fig. 4

a – d View onto the interstitial situation at the a early, b mid, c late S-shaped body, and the d maturing nephron in the fetal human kidney during advanced pregnancy by the optical microscope. a In the early S-shaped body, its peritubular interstitium at the connecting tubule (CNT; black line) faces the proximal end of the overlying pretubular aggregate (PTA). The elongating distal tubule (DT) is exposed at the lateral aspect to the perivascular interstitium (white boxes with red line) of the perforating radiate artery (PRA, black arrows). At the deep lateral aspect, the arising glomerulus defoliates along a transverse cleft (white boxes with red line and arrow head), which is bordered by the forming proximal tubule (PT). Between the conus (CO) of the CD ampulla (A) and the medial aspect of the S-shaped body, a vertical but narrow interstitial cleft (white arrow) ascends up to the CNT. b In the mid S-shaped body, interstitial cells invade the transverse cleft at the deep lateral aspect. At the same time, as well as the afferent and efferent arterioles as the intra- and extraglomerular mesangium develop. Underneath, the developing glomerulus shows the visceral layer including the podocytes (P) and the parietal layer representing the Bowman’s capsule (BC). This is in touch with the peritubular interstitium of a previously developed CNT. At the medial aspect, the vertical interstitial cleft lines along the conus of the CD ampulla and the meandering tubule portions. In this case, the constellation is shown at the left side of a CD ampulla. c The late S-shaped body shows at its proximal pole the developing glomerulus. The Bowman’s capsule is becoming flat, the podocytes acquire a cobblestone-like appearance, and capillaries and the mesangium are forming. At the medial aspect, the S-shaped body is facing beside the CD ampulla and also the differentiating collecting duct (CD) tubule. The proximal pole is opposite the peritubular interstitium of the CNT of a previously formed nephron, at the lateral aspect by the perivascular interstitium of the PRA. The forming proximal, intermediate, distal, and connecting tubule portions become covered by a faint peritubular interstitium. d The maturing nephron is positioned vertically underneath the head of a CD ampulla. Within the expanding glomerulus beside the podocytes, also numerous capillaries and the mesangium are visible. The lateral aspects are opposite the perivascular interstitium of the ascending PRAs. The meandering tubule portions are surrounded by a faint peritubular interstitium. + Lumen, yellow line with crosses subcapsular interstitium