Megakaryocyte population in human bone marrow increases with estrogen treatment: a role in bone remodeling?

Skeletal effects of conventional hormone replacement therapy (HRT) are predominately antiresorptive, while high doses of estrogen have anabolic effects. The mechanisms mediating these effects are unclear but may involve cells in the bone marrow. We have investigated the in vivo effects of estrogen on the megakaryocyte (MK) population in bone marrow in 10 postmenopausal women before and after 2 years of conventional HRT, in 11 women after long-term, high-dose estradiol therapy, and in 2 premenopausal and 4 postmenopausal women who had received no previous estrogen treatment. Transiliac crest biopsies were halved and either decalcified and paraffin wax embedded for immunolocalization studies or dehydrated and embedded in LR White resin for histology. MKs were identified morphologically, and the bone marrow cell population and MK number quantified by cell counting in a defined area of view (1 mm(2)) from 5 randomly selected fields of bone marrow. Compared with pretreatment values, significantly higher MK numbers were found after conventional HRT treatment (before treatment, mean +/- SEM; 7.3 +/- 1.1 vs. after treatment, 18.0 +/- 1.6/5 mm(2); p < 0.0001), while the greatest MK number was associated with long-term, high-dose estradiol treatment (32.8 +/- 2.1/5 mm(2); p < 0.0001). Total bone marrow cell number did not differ significantly between groups. Immunolocalization studies revealed more intense estrogen receptor (ER)beta expression in MKs in the high-dose estradiol-treated group but similar levels of weak ERalpha staining in MKs in the control and high-dose estrogen-treated groups. Positive immunoreactivity for transforming growth factor (TGF)beta1, 2, and 3 and TGFbeta receptor I, II, and III was detected in MKs, with more intense staining being demonstrated in the high-dose estradiol-treated group, particularly for TGFbeta2 and TGFbetaRI and II. Our results demonstrate an increase in the MK population in bone marrow from women treated with estrogen. The ability of MKs to express ERs and synthesise TGFbeta, a potent mitogen in osteoblast differentiation, suggests that these cells may play a role in mediating estrogen-induced effects on bone.