Bone changes occurring early after cessation of ovarian function in beagle dogs: A histomorphometric study employing sequential biopsies

The beagle dog model has been established by our laboratory as a useful animal model to study bone loss after cessation of ovarian function. Previously we demonstrated bone loss associated with an osteoblastic insufficiency at 4 months after ovariohysterectomy (OHX). This study was designed to evaluate by four sequential monthly bone biopsies the development and course of the histologic bone abnormalities after OHX. We found cancellous bone volume, trabecular density, and wall thickness to be decreased (p < 0.05) and trabecular separation increased (p < 0.05) as early as 1 month after OHX. After 2 months, there was a decrease in mineralizing surface and mineral apposition rate (p < 0.05). Volume and surface of osteoid were increased after 3 months (p < 0.05), and there was an increase in the number of osteoblasts (p < 0.01). No histologic signs of increased resorption were observed during the experiment. However, the findings of low bone volume with decreased trabecular density and increased separation without a change in trabecular plate thickness 4 weeks after OHX suggest that a dramatic increase in resorption must have taken place soon after OHX. These results point to an early phase of initiation of bone loss related to hyperresorption followed by a maintenance phase of low bone mass ascribable to an osteoblastic insufficiency. The events that stimulate the early initiating phase after cessation of ovarian function, the factors contributing to it, and the direct demonstration of hyperresorption await further studies.