Fig. 4: Islet cells originate from bipotent islet progenitors that become sublineage-restricted.

a Experimental schedule. Representative images from 4 experiments of b bipotent, c unipotent β-cell, and d unipotent α-cell clone (based on >60, 45, >20 recorded images, respectively). Insulin is grey, glucagon is pink. Arrows (arrowheads) indicate insulin+ (glucagon+) cells. e Fraction of β-cells (normalised to α+β-cell number) at P14 in islets (n = 36 islets from N = 4 mice) and clones traced from E12.5 to P14 (n = 138 clones from N = 4 mice; mouse with low ratio contained only 7 clones), quantified volumetrically from 100 μm section. Error bars show average ± SD. f Percentage of clone types (n = 138 clones from N = 4 mice). Error bars show average ± SEM. g Typical model lineage (“Methods”): following endocrine commitment, progenitors undergo average of n rounds of stochastic division after which they become sublineage-restricted choosing randomly between α- or β-cell fate in ratio 1:2 (cf. “loaded dice”). Between E12.5 and P14, islet progenitors and progeny undergo average of N rounds of division. For simplicity, model excludes differentiation into minority sublineages. h Cumulative size distribution of islet clones traced from E12.5 to P14 disaggregated by composition (total black, α-cell purple and β-cell blue). Data shown as dots (n = 138 clones from N = 4 mice, error bars show average ± SD) and fitted model predictions as lines. i Joint distribution of α- and β-cell numbers in islet clones from E12.5 to P14 tracings from data (left) and model (right) (main text and Methods). Cumulative size distribution of islet clones (key as in h) traced from j E15.5 to P14 (n = 40 clones from N = 3 mice) and k E18.5 to P14 (n = 65 clones from N = 3 mice). Joint distribution of α- vs. β-cell numbers in clones traced from l E15.5 and m E18.5 (cf. model prediction in Supplementary Fig. 5a, b). n Percentage of clone types traced from E15.5 to P14 (n = 40 clones from N = 3 mice) and E18.5–P14 (n = 65 clones from N = 3 mice). Error bars show average ± SD. o Percentage of clones contributing to insulin+ or glucagon+ lineage from E12.5 to E14.5, E12.5 to E15.5 and E12.5 to E18.5 tracings, co-stained with DBA and glucagon, or DBA and insulin, showing no statistically significant difference for all timepoints (P > 0.2, Mann–Whitney). n = 105 (104) clones from N = 3 mice for E14.5 insulin/DBA (glcg/DBA) (P = 0.40, two-tailed Mann–Whitney test). n = 140 (137) clones from N = 5 mice for E15.5 insulin/DBA (glcg/DBA) (P = 0.30, two-tailed Mann–Whitney test). n = 58 (65) clones from N = 3 mice for E18.5 insulin/DBA (glcg/DBA) (P = 0.80, two-tailed Mann–Whitney test). Error bars show average ± SD. Source data provided as Source Data file.