Direct ultrastructural validation of endocrine cell identity in zebrafish pancreatic islets

The zebrafish is widely used to study glucose homeostasis, diabetes, and endocrine cell plasticity, yet a definitive ultrastructural characterization of adult pancreatic endocrine cell types has remained incomplete. Here, we combined transmission electron microscopy with hormone-specific immunogold labeling to define the ultrastructural characteristics of the major endocrine cell populations in adult zebrafish pancreatic islets. Three principal endocrine cell types were distinguished based on secretory granule morphology, electron density, and intracellular organization. Glucagon-positive α-cells contained electron-dense granules with a characteristic halo and eccentric dense core, insulin-positive β-cells displayed larger moderately electron-dense granules, and somatostatin-positive δ-cells possessed smaller, predominantly electron-dense granules of heterogeneous shape. Immunogold labeling for glucagon, insulin, and somatostatin linked granule ultrastructure to endocrine cell identity. To independently validate β-cell assignment, we performed nitroreductase-mediated β-cell ablation in juvenile zebrafish and observed a marked depletion of cells containing the characteristic medium electron-dense granules. Comparative analysis indicates that zebrafish endocrine granule ultrastructure differs in several respects from mammalian islets and exhibits similarities to selected teleost species, highlighting evolutionary diversity in endocrine granule organization. This study provides a foundational ultrastructural reference for the three major endocrine cell types of the adult zebrafish endocrine pancreas and establishes criteria for identifying these populations in studies of pancreatic disease, regeneration, and cell plasticity while also identifying an additional unassigned endocrine population that warrants further characterization.