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We investigated bone's osteogenic response to loading in mice lacking functional ER-α. The normal peak locomotor strains (0.0026; length change as a proportion of original length) and strain rates (0.1 s−1) in the mouse ulna shaft were determined from surgically implanted strain gauges. We then loaded the ulnae of 20–24-week-old skeletally mature female mice through their olecranon and flexed carpus for three days a week for two weeks6. Each loading session comprised 40 repetitions of a 3.4-newton axial compressive load, which engendered peak strains (0.0028) and maximum strain rates (0.1 s−1) within the high physiological range.

In mice with normal ER-α function (ERα+/+ mice), this loading regimen stimulates sufficient new bone formation on the periosteal and endosteal surfaces to increase the cortical area at the midshaft by 8 ± 0.8% (P < 0.001; Fig. 1a). In their ERα−/− littermates, however, this response was diminished threefold (2.4 ± 0.4%; P < 0.001; Fig. 1b, c).

Figure 1: Absence of ER-α limits bone's adaptive response to mechanical loading.
figure 1

a, b, Transverse sections from control (left) and loaded (right) ulnae from ERα+/+ (n = 7) (a) and ERα−/− (n = 8) (b) mice. New bone formation is labelled with fluorochrome calcein (shown in white), given on days 3 and 12 of the 2-week experiment. Scale bars, 100 μm. c, In ulnae from ERα−/− mice, the adaptive increase in cortical bone area in response to loading was only one-third of that in ulnae from ERα+/+ littermates. d, Mechanical strain stimulates proliferation in osteoblast-like cells derived from ERα+/+ mice but not from ERα−/− mice. Transfecting osteoblast-like cells derived from ERα−/− mice with competent human ER-α confers strain-induced proliferative responsiveness in comparison with non-strained controls (asterisk denotes P ≤ 0.05; in five experiments with three cultures per experiment, transfection efficiency was 20 ± 2% using 'Effectene' (Qiagen), as assessed by β-galactosidase expression in 1,057 cells).

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A single period of dynamic strain applied to monolayer cultures of osteoblast-like cells stimulates their proliferation, a response that is inhibited by ER blockers and increased by transfection with additional ER-α (ref. 7). We derived separate primary cultures of osteoblast-like cells from the ulnae of ERα−/− and ERα+/+ littermates and exposed them to a single 10-min period of mechanical strain (600 cycles, 1 Hz, 0.0034). Over the next 24 h, the number of ERα+/+ cells increased by 58 ± 34%, P = 0.050, whereas the number of ERα−/− cells did not increase. ERα−/− mice have fully functional oestrogen receptors of the β-form, indicating that ER-β does not compensate for incompetent ER-α in this response. However, a proliferation response to strain was conferred on ERα−/− cells by transfecting them with a functional human wild-type ER-α expression vector (pRST7-ER; ref. 8) (Fig. 1d).

These results obtained in vivo and in vitro indicate that strain-related responses by differentiated cells of the osteoblast lineage require ER-α activity. This might explain why postmenopausal women no longer maintain adequate bone mass — their bone cells are less responsive to mechanical stimulation owing to decreased ER-α activity.

The oestrogen receptor is the ancestral steroid receptor9, with one of its possible early reproductive functions being to induce skeletal remodelling to release calcium for egg-laying or embryonic development. The strain-sensitive mechanisms that now enable mammalian and avian skeletons to adapt to load-bearing might have developed later, exploiting this receptor's ability to influence bone (re)modelling activity.