Dental Papilla Cells Synthesize But Do Not Deposit Fibronectin in Culture

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Abstract
The dental papilla cells play a major regulatory role during tooth morphogenesis, and they are the only mesenchymal cells capable of differentiating into odontoblasts secreting dentin. In this paper, we have extended our studies on the behavior of cultured dental papilla cells which have been disaggregated from 17-day mouse embryo teeth. Quite unexpectedly, we observed that these cells, which in vivo are embedded in a fibronectin-rich extracellular matrix, lose all surface-associated fibronectin when cultured as monolayers. Fibronectin was, however, detected intracellularly, and metabolic labeling and immunopreciopitation studies indicated that the dental papilla cells continued to synthesize fibronectin in culture. Furthermore, when purified plasma fibronectin was added at 50 μl/mL to the culture medium, it became incorporated as fibrillar matrix on the surfaces of dental papilla cells. This indicates that the cells are not deficient in cell-surface receptors or other surface-associated molecules which bind fibronectin. When pieces of dental palpillae were cultured as explants, an abundant matrix containing fibronectin was deposited on their surfaces. This matrix was gradually lost as the cells migrated from the explants. Furthermore, when the cells were disaggregated and cultured at high cell density, the cells in the central area of the pellet were covered by fibronectin containing fibrillar structures which were lost as the cells spread out. This indicates that the maintenance of close contacts between the dental papilla cells is required for the assembly of fibronectin into the extracellular matrix. Three other lineages of mesenchymal cells from the jaws of 11-, 13-, and 17-day-old embryos all developed a fibronectin-containing fibrillar network typical of fibroblastic cells in culture. When the ultrastructure of dental papilla cells was compared with that of gingival mesenchymal cells of 17-day-old embryos, both showed electron-dense material in the cisternae of rough endoplasmic reticulum. On the other hand, only minor amounts of extracellular material could occasionnally be found associated with the dental cells, whereas the gingival cells regularly showed abundant deposits of fibrillar extracellular material.