Fig. 2: Coalescence and stochastic proliferation of islet progenitors underlies islet size heterogeneity.

a (Left) Schematic depicting specification of islet progenitors from bipotent ductal–islet progenitors: If islet progenitor production were a rare event, islet clone size would be independent of ductal clone size. If the rate were high, islet clone size would scale in approximate proportion to ductal clone size. Representative images from 3 experiments based on >15 recorded images each showing examples of ductal–islet clones with islet representation being b majority, c minority and d approximately equal. b, c show snapshots from different Z-sections. Arrows indicate ductal cells, arrowheads indicate islet cells. DBA shown white, Chromogranin A in grey. e Dot plot of ductal to islet size within individual clones showing weak correlation (R² = 0.012). f Plot of islet size of unipotent (islet-only) and multipotent clones showing no statistically significant difference in average size (n = 13 unipotent and n = 53 multipotent clones from N = 3 mice, P = 0.76, two-tailed Mann–Whitney test). Error bars show average ± SD. g Schematic depicting possible modes of islet expansion: (top) endocrine progenitors expand uniformly within islets; (bottom) progenitors have variable proliferative capacity. h Cumulative distribution of islet clone sizes shows exponential-like dependence (“Methods”, n = 69 clones from N = 3 mice). Error bars show average ± SD. i Schematic depicting hypothesis that islet size heterogeneity is associated with local islet-to-islet variability in proliferative activity of constituent progenitors. Representative images (based on >10 recorded images each) from 3 experiments showing variable labelling fraction within islets with j large clone fraction and k small clone fraction. DBA shown white, Chromogranin A in grey. l Dot plot showing that there is no correlation between islet clone size and total size of host islet (R² = 0.001). m Summary schematic of islet growth whereby the final size of islets is dictated by degree of polyclonality arising from coalescence of islet progenitors combined with stochastic proliferation. Smaller islets are, on average, composed of a smaller number of founding progenitors that merge. n Fraction of islets occupied by individual islet clones traced from E9.5 to P14 and E12.5 to P14 (n = 93 clones from N = 5 mice at E9.5 and n = 69 clones from N = 3 mice at E12.5). Error bars show average ± SD. Source data provided as Source Data file.