Delayed Puberty Results in Decreased Percentage of Lamellar Bone and Decreased Mechanical Strength in Female Rats

Abstract

The failure to attain peak bone mass during childhood has been linked to bone fracture in adulthood. An increased age at puberty correlates with low bone mass in young women and an increased incidence of stress fracture. A model of delayed puberty has reported deficits in mechanical strength yet increases in the size of the cortex. Furthermore, there were increases in periosteal labeled surface. It is hypothesized that the microarchitecture of the bone differs between the delayed puberty and control animals and results in decreased bone strength in the delayed puberty group. PURPOSE: The goal of this study was to determine the relative amounts of lamellar and non-lamellar bone in the femoral cortex of animals with delayed pubertal onset. METHODS: At 23 days of age, female Sprague Dawley rats (Charles River) were randomly assigned into a control group (n=15) and a delayed puberty group that received injections of Gonadotropin releasing hormone antagonist (GnRH-a) (Zentaris GmbH) intraperitoneally (0.2ml) for a 29 day period (500 ug/dose, 5 days per week). Left femora were mechanically tested under 3-point bending. The right femora were dehydrated, embedded in polymethylmethacrylate, cut and ground to 100 μm thickness. Bones were analyzed under polarized light using Stereo Investigator Software. The proportion of the cortex with primary lamellar versus woven/other primary tissue type were measured and expressed as percent of the total cortical bone area.. RESULTS: There was a significant increase (p<.05) in the mean cortical area between the control group bones and the dose-response bones (3.6 mm2 versus 4.4 mm2). However, mechanical properties did not increase; peak moment was significantly less (-17%) and stiffness was reduced -23% in the delayed puberty animals compared to controls. There was also a statistically significant decrease (p<. 05) in the relative percentage of lamellar tissue when comparing the means of the control bones (41.4%) to the delayed puberty bones (16.8%). CONCLUSION: The decrease in lamellar bone in the delayed puberty animals may be a factor in the decreased mechanical strength in that group. The percentage of lamellar versus non-lamellar tissue contributes to a bone's overall mechanical tendencies.