Comparative Bone Morphometry between Mini Rats and Wistar Rats

Mini rats, a Wistar-derived transgenic rat strain harboring a rat growth hormone (GH) antisense gene, show lower plasma GH levels than Wistar rats. In our previous study, Mini rats showed a smaller femur size with lower mineral density and a reduction of the metaphyseal and diaphyseal bone mass at 8 and 22 weeks of age. In the present study, we examined the femur and tibia from Mini rats and Wistar rats at 10 to 60 weeks of age, and we performed cancellous and cortical bone histomorphometry on the tibia to examine the status of bone turnover. We found that the bone size, mineral content, and mineral density of the femur and tibia were significantly lower in Mini rats and tended to increase in both strains until 38 or 60 weeks of age. The longitudinal growth rate was significantly higher in Wistar rats until 26 weeks of age, and at 38 weeks of age that in Wistar rats is comparable to that in Mini rats. The periosteal bone formation rate tended to be higher in Wistar rats until 60 weeks of age. The longitudinal growth rate in both strains and the periosteal bone formation rate in Wistar rats tended to decrease with age. In contrast, bone turnover in the secondary spongiosa of the proximal tibial metaphysis showed no apparent difference between the two strains and seemed to remain unchanged until 60 weeks of age. Therefore, it is suggested that the reductions of bone mass in the metaphysis and diaphysis in Mini rats may be attributable to the lower longitudinal growth rate and periosteal bone formation rate, and that GH may influence bone modeling but not bone remodeling.