The effects of age on the response of rabbit periosteal osteoprogenitor cells to exogenous transforming growth factor-β2

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Abstract
Additional bone and cartilage are formed if transforming growth factor-β is injected into the periosteum of calvariae or long bones. To investigate this further, transforming growth factor-β2 was injected into the periosteum of the tibia of 3-day-old, 3-month-old and 2-year-old rabbits. In all instances, there was an increase in proliferation of the cells of the cambial layer of the periosteum, that is, the osteoprogenitor cells, and breakdown of the fibrous layer. Oedema was induced in the surrounding connective tissues. Over the experimental period the normal neonatal tibia is undergoing rapid growth; there is periosteal bone formation and endosteal resorption. In the experimental neonatal tibiae, an increase in periosteal bone formation is seen after three injections of 20 ng of transforming growth factor-β2, which is accompanied by cartilage after five injections; the amounts of induced bone and cartilage increase with the number of injections. The chondrocytes hypertrophy after 4 days and the cartilage is replaced by bone endochondrally. In contrast, after seven injections of 20 ng transforming growth factor-β2, there is only a small amount of new bone on the 3-month-old tibia and none on the 2-year-old tibia. One day after seven injections of 200 ng transforming growth factor-β2, there is a small amount of bone formation, while seven days after cartilage is found as small discrete nodules on the 3-month-old tibia, but as small areas within the bone on the 2-year-old tibia. It is concluded that the primary effect of transforming growth factor-β2 in this experimental model is to increase the proliferative rate of the osteoprogenitor cells in the periosteum. It is argued that transforming growth factor-β2 does not initiate osteoblastic or chondrocytic differen-tiation of osteoprogenitor cells. It is suggested that their differentiation is controlled by the local environment, in particular, the vascularity and locally circulating growth factors.