The changes in the volumes of three of the principal islet cell types in the developing rat pancreas were quantitated from day 10 to day 210 of life. The insulin-, glucagon-, and somatostatin-positive cell populations of the islets were identified by immunocytochemical staining, and their volumes were determined by linear scanning. The insulin and glucagon content of the pancreata, and the concentrations of these hormones in plasma, were also determined at each age by radioimmunoassays. The volumes of the A- and D-cells reached their maximum values by day 50 and day 35, respectively. The content of glucagon within the pancreas reached adult levels by day 50. In contrast, the volume of B-cells and the insulin content were highest on day 210. The concentrations of both insulin and glucagon in the plasma reached adult levels by approximately day 50. There were no differences in the pancreatic parameters between male and female rats until day 25, when the wet weight of the male gland became significantly greater. The concentrations of the islet hormones and the percentages of the islet cell types within the pancreas did not differ between the sexes at any age. However, the greater weight of the pancreas resulted in a greater total content of islet cells and hormones in the male gland after day 25. The data suggest that, in the rat, the B-cell volume may continue to increase with age, while the A- and D-cells apparently do not. The physiologic consequences of these changes remain to be determined.

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