Estradiol-17 beta-induced morphometric changes in chick embryo femurs

The influence of exogenous estradiol-17 beta (E2) on chick embryo femur morphology was assessed. Fertile chicken eggs were air cell injected with 18.75, 37.5, or 75 micrograms E2/.1 ml distilled water: ethanol (7:3, v/v) on Days 10, 12, and 14 of incubation. Controls received vehicle only (.1 ml) on the same days. On Day 16 of incubation, right femurs were excised, and midshaft cross sections were examined by light microscopy. Structural differences were quantified using a computerized histomorphometric technique. Injection of E2 produced significant (P less than .01) dose-dependent elevations in mean femur total osseous tissue area, bone matrix area, percent of matrix surfaces covered by osteoblasts, and percent of matrix area occupied by osteocytes when compared with controls. Estradiol was ineffective in inducing medullary bone formation in chick embryo femurs. We conclude that E2 is involved in bone matrix formation processes in chick embryos. The lack of medullary bone formation upon E2 treatment in chick embryos is perhaps an indication that, as in growing (immature) birds, it is likely that endogenous testosterone levels in Day 16 chick embryos are insufficient to support medullary bone growth.