17β-Estradiol Stimulates Mineralized Bone Nodule Formation when Added Intermittently to SaOS-2 Cells

It is now well established that estrogen inhibits bone resorption. However, its effect on bone formation remains controversial. We studied the effect of 17beta-estradiol (E2) on mineralized bone nodule formation in long-term cultures of osteosarcoma SaOS-2 cells. We showed that SaOS-2 cells formed mineralized nodules which under electron microscopy revealed a bone structure with active osteoblasts, entrapped osteocytes, extracellular collagen fibrils and hydroxyapatite deposits, making this system a valid model to study bone formation in vitro. Intermittent addition of E2 for 6 hours during a 48-hour cycle of changes of medium, starting from day 3, resulted in a dose-dependent stimulation of mineralized bone nodule number and area, as well as alkaline phosphatase activity. In conclusion, we report for the first time a stimulatory effect of E2 on mineralized bone nodule formation in human osteoblasts in culture.