The Effect of Estrogen Repletion on Osteoblast Differentiation and DNA Synthesis in Ovariectomized Rats

Abstract

Previous studies have demonstrated that estrogen plays a significant role in bone mass conservation. To investigate the role that estrogen plays in osteoblast differentiation, the fractional distribution of periodontal ligament (PDL) osteoblast precursor cells was determined. Twenty six-month old female rats (Charles River Co.) were divided into two groups. Both groups were ovariectomized (OVX). Thirty-five days after ovariectomy one group (OVX+E) received supplemental daily injections of estrogen (0. 1 mg/kg Ethinyl Estradiol) for three days. After sacrifice, PDL sections through the mesial root of the maxillary first molar were prepared for microscopic analysis. Using a nuclear morphometric assay, the fibroblast-like cells of the POL were identified as early osteoprogenitor (A+A'), preosteoblast (C+D) or nonosteogenic (B) cells (i.e., A+A'=40-79 μm3; 8=80-119 μm3; C+D>120 μm3). Comparison of the OVX and OVX+E groups showed that treatment with estrogen increased early osteoprogenitor (A+A') cell and decreased preosteoblast (C+D) cell fractional distributions. No changes were seen in the non-osteogenic (B) cell group (expressed as% cell type, mean±SEM for n=4-8 rats/group; *p<0.05). Specimens were also stained with 5-Bromo- 2'-deoxyuridine (BrdU) to localize cells undergoing DNA synthesis. Both OVX and OVX+E groups showed minimal random BrdU staining throughout the PDL. Group= OVX A+A’= 8.8 ± 1.8 B= 30.2 ± 2.3 C+D= 60.8 ± 2.6 Group= OVX+E A+A’= 21.9 ± 2.6* B= 36.0 ± 2.2 C+D= 41.6 ± 3.8* The data suggest a block in proliferation of both less-differentiated precursor cells and preosteoblasts in estrogen-deficient animals. Furthermore, they suggest that estrogen may be required for normal preosteoblast differentiation leading to osteoblast production.