Contribution of postnatally formed small beta cell aggregates to functional beta cell mass in adult rat pancreas

Neogenesis of beta cells and their clustering to small aggregates is a key process in prenatal development of beta cell mass. We investigated the contribution of postnatally formed small aggregates to functional beta cell mass in adult rats. Conditions were defined for (1) counting total beta cell number in pancreases with relative error of < 10% and (2) determining their distribution over aggregates of different size and over functionally different subpopulations. Pancreases of 10-week-old male Wistar rats contained 2.8 +/- 0.2 x 10(6) beta cells, of which > 90% was generated postnatally, involving: (1) neo-formation of 30,000 aggregates with diameter < 50 mu m including single cells; and (2) growth of 5,500 aggregates to larger sizes, accounting for 90% of the increase in cell number, with number of growing aggregates in the tail 50% greater than elsewhere. At 10 weeks, 86% of aggregates were < 50 mu m; compared with aggregates > 200 mu m, their beta cells exhibited a higher basal insulin content that was also resistant to glibenclamide-induced degranulation. The pool of Ki67-positive beta cells was sixfold larger than at birth and distributed over all aggregate sizes. We describe a method for in situ counting of beta cell numbers and subpopulations with low relative error. In adult rats, > 90% of beta cells and beta cell aggregates are formed after birth. Aggregates < 50 mu m are more than 100-fold more abundant than aggregates > 200 mu m, which are selected for isolated islet studies. Their topographic and functional properties contribute to the functional heterogeneity of the beta cell population; their growth to larger aggregates with characteristic beta cell functions may serve future metabolic needs.