Defining the islet: a minimum size threshold and opportunities for standardization

Although islets play a central role in the pathogenesis of diabetes, they still lack a universally accepted definition beyond being large clusters of endocrine cells within the pancreas. Previous definitions have primarily relied on arbitrary size or cell-count thresholds rather than biologically meaningful criteria. As a result, this longstanding ambiguity has recently prompted the introduction of the term endocrine objects (EOs), which replaces the traditional distinction among islets, small endocrine cell clusters, and solitary endocrine cells with a unified, continuous size-based classification. This raises a fundamental question: what biologically meaningful features distinguish islets from their smaller endocrine counterparts? In our recent study of volumetric intraislet vascularization among various mammalian species including humans, we defined the smallest islet size that receives direct internal vascularization as being ~40 µm in diameter. Considering that the islet has been described as highly vascularized, the presence or absence of direct vascularization should fairly distinguish islets from non-islet single endocrine cells and small cell clusters. In addition to variability in islet size thresholds, the lack of standardized analytical approaches has further complicated meaningful comparisons across islet studies. This perspective aims to address these limitations through discussion of islet effective diameter, islet sampling, and comparing 2D and 3D analytical methods.