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The mechanism by which oestrogen and hormone replacement therapy (HRT) maintain bone mass in women is still unclear. It has previously been shown that cells of osteoblast lineage in vivo, particularly osteocytes, express oestrogen receptor α (ERα). Nevertheless, it is still debatable whether oestrogen and the ovarian steroids have a direct affect on osteocytes. If they could regulate osteocyte ERα expression, this would be strong evidence for the involvement of these cells in the hormonal regulation of bone mass. This study therefore aimed to compare bone biopsies from women who were replete with ovarian steroids (pre‐ovariectomy or post‐HRT) with those from the same women when hormone‐deficient (post‐ovariectomy or pre‐HRT) for cellular localization of ERα protein or mRNA expression by indirect immunofluorescence, or by in situ hybridization combined with reverse transcriptase‐polymerase chain reaction (IS‐RT‐PCR) respectively. Image analysis showed that proportions of osteocytes positive for immunodetectable ERα were higher in hormone‐replete than in hormone‐deficient women (25 ± SEM 3 per cent, 12 ± SEM 4 per cent, respectively; n = 5), with similar but non‐statistically significant changes in osteoblasts. This was observed even when HRT was commenced 18 years after menopause. In contrast, grain volume/unit cell area of osteoblast mRNA signal was markedly higher when hormone‐deficient (0·055 ± 0·01) than when hormone‐replete (0·016 ± 0·004), with similar but non‐significant differences in osteocytes. This preliminary study indicates up‐regulation of osteocyte ERα protein by ovarian steroids in these patients, which is accompanied by decreased osteoblast ERα mRNA expression, providing further evidence for the involvement of osteocytes in the regulation of skeletal structure by ovarian steroids. Copyright © 1999 John Wiley & Sons, Ltd.

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