Specific Transforming Growth Factor-β Subtypes Regulate Embryonic Mouse Meckel's Cartilage and Tooth Development

Abstract

Members of the transforming growth factor-β (TGF-β) superfamily have emerged as critical regulators for cell growth and differentiation. Whereas the different TGF-β subtypes are equipotent in the majority of biological assays using cell lines cultured in vitro, there are indications that in more complex systems involving epithelial-mesenchymal interactions, the TGF-β subtypes differ in their biological activities. To test the hypothesis that TGF-β subtypes specifically regulate either Meckel's cartilage or tooth morphogenesis, we designed experiments to compare loss of function effects of TGF-β1, TGF-β2, and TGF-β3 subtypes using a serumless, chemically defined medium to culture embryonic mouse E10 (42-44 somite pairs) mandibular explants. The major effect of loss of function resulting from abrogation of TGF-β1 using antisense treatment resulted in a 20% increase (P < 0.05) in chondrocyte number, a decrease in extracellular matrix, and dysmorphology of the rostral region of Meckel's cartilage. Exogenous TGF-β1 provided indistinguishable recovery to the normal phenotype. TGF-β2 antisense treatment produced a threefold enlargement (P < 0.05) of tooth organs and advanced their development to the cap stage. TGF-β2 provided recovery to the normal phenotype (e.g., reduced tooth size and development to the bud stage), whereas TGF-β1 or TGF-β3 polypeptides had no effect. TGF-β3 antisense treatment resulted in a reduction of approximately 15% in the length of Meckel's cartilage. We interpret these results to suggest that TGF-β1 functions to regulate the number of chondrogenic cells, the amount of extracellular matrix, and the rate of developmental assembly of the rostral to posterior segments in forming Meckel's cartilage. TGF-β2 appears to regulate tooth size and stage of development without affecting cartilage. TGF-β3 appears to regulate Meckel's cartilage size without altering tooth size or shape. The results are discussed in terms of the regulatory functions of the TGF-β subtypes during embryonic craniofacial morphogenesis.