Extended Data Figure 9: Functional assay of proximal tubule maturation within kidney organoids.

a, Fluorescent microscopy showing the dextran uptake in both the kidney organoids and E14 mouse embryonic kidneys organ culture after 24 h presence of dextran–Alexa488 (10 μg ml⁻¹) in the culture medium (24 h dextran–Alexa488). 1 h incubation was insufficient for either organoids or mouse kidney explants to uptake dextran from the culture media (1 h dextran–Alexa488). No background signals were detected in a control without dextran (no dextran). Dashed line circles the organoids and kidneys. Scale bars, 1 mm. b, Endocytosis mediator cubilin (CUBN) was present on apical surface of the proximal tubules in kidney organoids (left panel). The same staining without detergent during the process showed the complete absence of CUBN staining on apical surface (right panel), demonstrating that the tubules within the organoids are intact. This explains the requirement for a 24 h incubation with dextran before evidence of apical uptake. Dashed line circles LTL⁺ proximal tubules. Scale bars, 50 μm. c, Low power immunofluorescence microscopy of day 18 kidney organoids after being treated by cisplatin for 24 h. No apoptosis was observed in proximal tubules in the absence of cisplatin (0 μM, left panel). LTL⁺ECAD⁺ proximal tubular cell-specific apoptosis was observed only in response to either 5 μM (not shown) or 20 μM cisplatin (arrowheads in middle panel). Global cell death was observed after culture in 100 μM cisplatin (right panel). Scale bars, 100 μm.