Quantitative analysis of pancreatic islet development and insulin storage in the foetal and newborn rat

Pancreatic islet development and insulin storage were studied in foetal rats during the last 4 days of gestation (day 19 to 22 post-coitum (p.c.)) and in 1 and 5 days old neonatal rats. Adult female virgin rats were also studied. The percentage of granulated B-cells per islet, the degree of B-cell granulation and the islet insulin concentration rose from low levels on day 19 to adult levels on day 22 and remained stable after birth. This indicates that the qualitative maturation of the pancreatic islets as insulin producing units is completed on the last day of gestation. The percentage of islet tissue slowly rose from 0.7% at day 19 to 1.5% on day 22. A further and much more rapid rise occurred during the first day of birth. At the 5th postnatal day the islets comprised 3.6% of the pancreas versus 1.1% in adult rats. Likewise, the neonatal pancreatic insulin concentration was about 3 times higher than in the adult pancreas. The foetal pancreas as a whole showed rapid exponential growth between day 18 and 21 p.c., but a sudden decline in growth rate occurred from day 21 onward. The total mass of islet tissue, on the other hand, continued to expand at its high initial rate up to the first day after birth, whereafter this high rate also declined. The high concentration of insulin in the neonatal rat pancreas therefore appears to be due to differential growth rates of the endocrine and exocrine tissue during the last day of pregnancy and the first day after birth.