Comparison of the response of pelvic and proximal tibial cancellous bone in rat to ovariectomy with estrogen replacement.

In this study, we found that the trabecular architecture of the rat pelvis has similarities to that of human iliac crest. Although we made no direct comparisons between the estrogen deficiency-induced rat osteopenia model and postmenopausal histomorphometry of iliac crest, we attempted to determine whether the rat pelvis might be appropriate to study changes in bone modeling and in situ changes in osteoblast protein expression. Three groups of young, sexually mature rats (12 weeks of age, each group comprising six animals) were either ovariectomized (ovx) and treated with 17beta-estradiol (ovx + E), vehicle (ovx), or sham-operated (sham). Histomorphometric variables were quantitated in the pelvis and compared with proximal tibial metaphysis in the three groups. Immunocytochemical localization of osteocalcin was also evaluated in the two skeletal sites. There was a greater reduction in bone volume of the proximal tibial metaphysis of ovx rats than in the pelvis of ovx rats when compared with sham-operated animals (p < 0.01), although bone formation rates were significantly higher at the pelvic site than tibial metaphysis (p < 0.01). The more rapid loss of bone between the tibia and pelvis may reflect differences in longitudinal growth in young rats, but the other intersite differences in bone remodeling consequent to ovx were at least as well demonstrated in the pelvic trabecular structure. Because ex vivo removal of the rat pelvis is simple, and provides a larger histomorphometric section with which to evaluate dynamic changes in metabolic bone disease, we suggest that this site may be useful in studies of osteopenia in the sexually mature female rat. Immunocytochemical demonstration of osteocalcin in trabecular surface osteoblasts was excellent in both sites. These results suggest that the rat pelvis is as accessible for histological study as the more conventional appendicular sites. When compared with the proximal tibial metaphysis, the rat pelvis (1) has a more homogeneous trabecular structure; (2) has more than twice as much trabecular bone area to sample; (3) has no open epiphyseal growth cartilages; (4) loses trabecular bone half as rapidly after ovx; (5) displays a greater increase in bone turnover after ovx; and (6) is the same anatomic site that is sampled in humans. We have also shown that the pelvis is a suitable site to demonstrate immunocytochemistry for osteoblast-derived proteins.