Imaging human pancreatic endocrinogenesis during early prenatal life

Murine pancreatic endocrinogenesis has been extensively studied, but human data remain scarce due to limited sample availability. Here, we first built a large collection of human embryonic and fetal pancreases covering the first trimester of pregnancy to explore human endocrinogenesis. Using an experimental pipeline combining in toto staining, tissue clearing, and light-sheet fluorescence microscopy, we show that insulin⁺, glucagon⁺, and somatostatin⁺ cells appear simultaneously at Carnegie Stage (CS) 16. This contrasts with rodents where glucagon⁺ cells appear first, followed by insulin⁺ and finally somatostatin⁺ cells and highlights interspecies differences. We also detected bi-hormonal endocrine cells in 7 out of 9 human pancreases between CS16-18, which were no longer detected at later stages. We observed that cell distribution within human fetal islets resembles adult mouse islets, with a core of beta cells surrounded by alpha and delta cells, differing from a more complex arrangement in adult human islets. This, in connection with the small size of human fetal islets when compared to adult, suggests that adult human islets may form by fusion of pre-existing islets, in contrast to the mouse fission model. Together, our study provides a detailed and comprehensive description of the spatiotemporal dynamics of human pancreatic endocrinogenesis.