Coordinated induction of cell proliferation and syndecan expression in dental mesenchyme by epithelium: evidence for diffusible signals

Abstract

Epithelial-mesenchymal interactions induce the expression of syndecan, a cell surface proteoglycan, and tenascin, an extracellular matrix glycoprotein in the mesenchymal component of many organ rudiments including the tooth. Experimental recombination cultures of early dental epithelium and mesenchyme were analysed by double immunostaining to compare the distribution of syndecan, tenascin, and proliferating cells (BrdU incorporation) in the induced dental mesenchyme. After 5-9 hr in culture expression of syndecan and tenascin as well as an increase in BrdU incorporation were evident in the mesenchymal cells adjacent to the epithelium and the positive area enlarged with time. Syndecan and tenascin were colocalized only partially in some explants. The expression of syndecan and tenascin in the recombinants correlates with their stage-dependent expression pattern during early tooth development in vivo (Vainio and Thesleff, 1992). The area of increased cell proliferation in the mesenchyme correlated closely with syndecan expression. In none of the explants was increased BrdU incorporation observed in syndecan negative areas. Epithelium induced also condensation of the mesenchymal cells. Induction and spread of the syndecan-positive zone in the dental mesenchyme required close and continuous contact with the epithelium. The mechanism by which the induction of syndecan expression spreads in the mesenchyme was studied in rat-mouse interspecies recombination cultures, using syndecan antibodies that recognize mouse but not rat syndecan. The rat mesenchyme and epithelium were first cultured in contact for 24 hr. Then the epithelium was removed and freshly dissected, "uninduced" mouse mesenchyme was placed in contact with different aspects of the rat mesenchyme. The rat mesenchymal cells that had located next to the epithelial tissue stimulated syndecan expression in adjacent mouse mesenchyme. The induction potential was gradually lost toward the periphery of the rat mesenchyme. Based on these findings we suggest that diffusible signal molecules mediate the spread of syndecan induction in the mesenchyme and that syndecan plays a role in the regulation of cell proliferation.