THE ROLE OF TRANSFORMING GROWTH-FACTOR-ALPHA IN RAT CRANIOFACIAL DEVELOPMENT AND CHONDROGENESIS

To explore the possible role of transforming growth factor alpha (TGF- alpha) in craniofacial development, its expression in the craniofacial region of rat embryos from embryonic day (d) 9 to d 20 was examined b y in situ hybridisation and immunostaining. The TGF-alpha transcripts were first detected in the neural fold of embryonic d 9 and 10 embryos . In the craniofacial region, the TGF-alpha transcripts were not detec ted until embryonic d 16 in mesenchyme surrounding the olfactory bulb, within the olfactory bulb, the nasal capsule, vomernasal organ, and v ibrissal follicle. Tn addition, TGF-alpha message was detected in mese nchyme in the vicinity of Meckel's cartilage, and in the dental epithe lium and lamina. This expression pattern of TGF-alpha transcripts pers isted until embryonic d 17 but disappeared by d 18. The presence of TG F-alpha protein largely coincided with TGF-alpha message although, unl ike the message, it persisted throughout later embryogenesis in the cr aniofacial region. The possible function of TGF-alpha in chondrogenesi s was explored by employing the micromass culture technique. Cartilage nodule formation in mesenchymal cells cultured from rat mandibles in the presence of TGF-alpha was significantly inhibited. This inhibitory effect of TGF-alpha on chondrogenesis was reversed by addition of ant ibody against the EGF receptor, which crossreacts with the TGF-alpha r eceptor. The inhibitory effect of TGF-alpha on chondrogenesis in vitro was further confirmed by micromass culture using mesenchymal cells fr om rat embryonic limb bud. Taken together, these results demonstrate t he involvement of TGF-alpha in chondrogenesis during embryonic develop ment, possibly by way of a specific inhibition of cartilage formation from mesenchymal precursor cells.