Correlation between volume fraction and volume-weighted mean volume, and between total number and total mass of islets in post-weaning and young Wistar rats.

The aim of this study was to estimate the number and volume distribution of islets of Langerhans in post-weaning young Wistar rat pancreas and their variation with age. Four groups of six normal Wistar Kyoto rats, at 3, 6, 9 and 12 weeks of age, were used. The whole pancreas was weighed (W), fixed in buffered formaldehyde and embedded in JB4 resin, and 1.5-microm serial sections were obtained. A fraction of whole tissue was obtained in accordance with the multistage fractionator principle and used to estimate total number of islets (N(is)). Volume fraction (V(f)) of islets and volume-weighted mean volume (Vv) of islets were estimated using point-counting and point-sampled intercept methods, respectively. The numbers of islets increased steadily with age (P < 0.001), whereas the volume-weighted mean volume of individual islets was not significantly different among all age groups of rats (P > 0.05). There was a strong positive correlation between total islet number and islet mass (r = 0.96, P = 0.001), and between volume fraction and islet mass (r = 0.969, P = 0.001). However, there was a weak positive correlation between volume fraction and volume-weighted mean islet volume (r = 0.6, P = 0.002) in the age window investigated. These findings indicate that there was an increase in the number and volume fraction of islets with age in post-weaning young rats but that individual islet volume did not change significantly. It appears the mechanism of alteration in islet morphology in the rat is mainly by the formation of new islets while keeping their individual volume distribution unchanged.