The postnatal growth of the beta-cell mass in pigs.

Studies of the postnatal growth of the β-cell mass in rats have revealed some unexpected and apparently paradoxical results, the most prominent being a β-cell mass plateau in the early phase of life. We have studied the postnatal growth of the β-cell mass in the domestic pig to investigate its development in a larger mammal. The pancreases from a total of 86 male pigs from 5 to 100 days of age were studied. The β-cell mass increased linearly from day 5 to day 40, reached a plateau from day 40 to day 60, and then increased further into adulthood. The relative β-cell mass (β-cell mass per body mass) was increased in the early postnatal period but reached a constant level from day 60, after which there was a linear relationship between the β-cell mass and the body mass. There were high rates of both β-cell apoptosis and mitosis at 50 and 60 days of age, while the volume-weighted mean islet volume increased from birth and reached a plateau at approximately 60 days of age. A β-cell mass plateau early in life accompanied by a wave of β-cell apoptosis coinciding with the relative β-cell mass decreasing to reach a constant level, and a linear relationship between the β-cell mass and the body mass in later life is exactly what has previously been reported in rats. The coincidence of these events in both rats and pigs, although occurring at different ages in the two species, suggests a causal relationship as previously suggested in a proposed explanatory model for postnatal β-cell growth.