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 pe lvis has similarities to that of human iliac crest. Although we made n o direct comparisons between the estrogen deficiency-induced rat osteo penia model and postmenopausal histomorphometry of iliac crest, we att empted to determine whether the rat pelvis might be appropriate to stu dy changes in bone modeling and in situ changes in osteoblast protein expression Three groups of young, sexually mature rats (12 weeks of ag e, each group comprising six animals) were either ovariectomized (ovx) and treated with 17 beta-estradiol (ovx + E), vehicle (ovx), or sham- operated (sham), Histomorphometric variables were quantitated in the p elvis 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 ov x rats when compared with sham-operated animals (p < 0.01), although b one 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 mag reflect differences in longitudinal growth in you ng rats, but the other intersite differences in bone remodeling conseq uent to ovx were at least as well demonstrated in the pelvic trabecula r structure. Because ex vivo removal of the rat pelvis is simple, and provides a larger histomorphometric section with which to evaluate dyn amic changes in metabolic bone disease, we suggest that this site mag 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 th e rat pelvis is as accessible for histological study as the more conve ntional appendicular sites. When compared with the proximal tibial met aphysis, the rat pelvis (1) has a more homogeneous trabecular structur e; (2) has more than twice as much trabecular bone area to sample; (3) has no open epiphyseal growth cartilages; (4) loses trabecular bone h alf as rapidly after ovx; (5) displays a greater increase in bone turn over after ovx; and (6) is the same anatomic site that is sampled in h umans. We have also shown that the pelvis is a suitable site to demons trate immunocytochemistry for osteoblast-derived proteins. (Bone 23: 2 67-274; 1998) (C) 1998 by Elsevier Science Inc. All rights reserved.