The spatiotemporal expression of TGF-β1 and its receptors during periosteal chondrogenesis in vitro

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Abstract
Transforming growth factor‐β1 (TGF‐β1) has been shown to stimulate chondrogenesis in periosteal explants cultured in agarose suspension. TGF‐βs exert their cellular effects through a heteromeric cell membrane receptor complex consisting of TGF‐β type I and type II receptors. In this study, the spatial and temporal expressions of the type I receptor (TβR‐I), type II receptor (TβR‐II) and endogenous TGF‐β1 in periosteal explants cultured in vitro were examined using reverse transcription polymerase chain reaction (RT‐TCR) and immunohistochemistry. The temporal changes in the expression of the TβR‐I and TβR‐II mRNAs correlated with that of TGF‐β1. Exogenous administration of TGF‐β1 upregulated the expression of both receptors and of the TGF‐β1 ligand in a biphasic pattern. The earlier peak of upregulation was observed at 7 days in culture. A later peak of upregulation was seen at 42 days, at which time cartilage formation reached a maximum. Immunohistochemical studies demonstrated co‐localization and TβR‐II simultaneously among the same cells expressing TGF‐β1. TGF‐β1 treatment increased the expression of TGF‐β1, I and TβR‐II in mesenchymal cells in the cambium layer at 7 days in culture. Small round chondrocytes showed widely distributed immunoreactivity of TGF‐β1, TβR‐I and TβR‐II in the 42‐day explants treated with TGF‐β1. These observations support the hypothesis that TGF‐β1 regulates the initiation and formation of cartilage during periosteal chondrogenesis. © 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.