Transforming Growth Factor-β Stimulates Bone Matrix Apposition and Bone Cell Replication in Cultured Fetal Rat Calvariae*

Abstract

Transforming growth factor-β (TGFβ) stimulates the expression of extracellular matrix proteins and may be a local regulator of bone growth. The aims of this research were to localize the effect of TGFjS on bone matrix formation and to determine if this effect was dependent on increased cell replication, using histomorphometry and autoradiography of bone organ cultures. Half-calvariae of 21-day-old fetal rats were cultured with native or recombinant TGFβ1 for 24 h and labeled either with [3H]proline for 0–24 or 24–48 h or with [3H]thymidine for the last 6 h of culture. Bones were fixed in glutaraldehyde, embedded in glycol methacrylate, and processed for autoradiography. Bone matrix formation was assessed as the matrix apposition rate per day and the percentage of [3H]proline-labeled bone surface. Cell replication was evaluated based on the number and percentage of [3H]thymidine labeled cells in the osteoblast cell zone, the osteoprogenitor cell zone, and the pericranial fibroblastic periosteum. Both native and recombinant TGFβ at 1-30 ng/ml increased bone matrix formation by 25-40% (P < 0.05). At 30 ng/ml, TGFβ had a generalized mitogenic effect as cell replication increased by approximately 2-fold in all cell zones of the pericranial periosteum. TGFβ had specific effects on bone cell differentiation. The number of unlabeled cells lining the bone surface increased, and the number of osteoclasts on bone decreased. Inhibition of cell replication by hydroxyurea only partially blocked the stimulatory effect of TGFβ on bone matrix formation, suggesting that TGFβ may have independent effects on cell replication and differentiated bone cell function. In summary, TGFβ had a generalized mitogenic effect on the pericranial periosteum and specific stimulatory and inhibitory effects on bone cell differentiation and function. (Endocrinology126: 421–426, 1990)