Formation of a human β-cell population within pancreatic islets is set early in life.

Insulin resistance can be compensated by increased functional pancreatic β-cell mass; otherwise, diabetes ensues. Such compensation depends not only on environmental and genetic factors but also on the baseline β-cell mass from which the expansion originates. Little is known about assembly of a baseline β-cell mass in humans. Here, we examined formation of β-cell populations relative to other pancreatic islet cell types and associated neurons throughout the normal human lifespan. Human pancreatic sections derived from normal cadavers aged 24 wk premature to 72 yr were examined by immunofluorescence. Insulin, glucagon, and somatostatin were used as markers for β-, α-, and δ-cells, respectively. Cytokeratin-19 marked ductal cells, Ki67 cell proliferation, and Tuj1 (neuronal class III β-tubulin) marked neurons. Most β-cell neogenesis was observed preterm with a burst of β-cell proliferation peaking within the first 2 yr of life. Thereafter, little indication of β-cell growth was observed. Postnatal proliferation of α- and δ-cells was rarely seen, but a wave of ductal cell proliferation was found mostly associated with exocrine cell expansion. The β-cell to α-cell ratio doubled neonatally, reflecting increased growth of β-cells, but during childhood, there was a 7-fold change in the β-cell to δ-cell ratio, reflecting an additional loss of δ-cells. A close association of neurons to pancreatic islets was noted developmentally and retained throughout adulthood. Negligible neuronal association to exocrine pancreas was observed. Human baseline β-cell population and appropriate association with other islet cell types is established before 5 yr of age.