Osteoporosis after oophorectomy in the mature female rat and the effect of oestrogen and-or progestogen replacement therapy in its prevention

Fifty-two mature female rats on a controlled diet were studied to compare the effects of oophorectomy, and hormone replacement therapy after oophorectomy, on femoral morphology and mineral content. Oophorectomy was followed by the development of osteoporosis after 11 months of observation. This was characterized by a reduction in ash per unit length of bone and a diminution of mid-shaft femoral cortical width. The administration of a progestogen (9 μg ethynodiol diacetate/rat/day) for 10 months after oophorectomy prevented the reduction in ash per unit length from occurring, whereas an oestrogen (0·9 μg mestranol/rat/day) had no significant effect on either parameter of osteoporosis. The progestogen appeared to produce this effect by a relative increase in periosteal new bone formation at the expense of increased loss of bone from the endosteal surface.