Islet size and spatial organization govern endocrine cell composition throughout the human pancreas

Stem cell-derived (SC) islets hold promise as a transformative therapy for type 1 diabetes (T1D). To enhance graft uniformity, SC-islet organoids are often dissociated and reaggregated into standardized sizes. However, how endocrine cell composition varies with islet size in native human islets remains largely unknown. Defining this relationship is essential for establishing physiological benchmarks for islet engineering for transplantation. In this study, we examined the relationships among islet size, endocrine cell composition, and spatial distribution throughout whole pancreata from 10 age-matched, non-diabetic male subjects. Insulin-, glucagon-, and somatostatin-positive endocrine cells were identified as single-, double-, and triple-hormone populations ranging from non-islet single endocrine cells and small clusters (niSSCs) to large islets. Approximately 80% of the total endocrine area was contained in islets positive for all three hormones. Islet size strongly correlated with endocrine composition. Specifically, larger islets had increased alpha-cell fractions and reduced beta-cell fractions, while delta-cell fractions were consistent across sizes. Spatial analysis revealed that niSSCs, mono-, and tri-hormone islets frequently exhibit a clustered distribution. Additionally, tri-hormone islets were positioned near the core of the pancreas, whereas mono-somatostatin islets were often localized in the periphery. These findings were consistent across all three pancreatic regions (head, body and tail).