Spatial and temporal changes in the expression of fibroglycan (syndecan-2) during mouse embryonic development

Fibroglycan (syndecan-2) is a member of a family of cell surface heparan sulfate proteoglycans that interact with adhesion molecules, growth factors and a variety of other effector systems that support the shaping, maintenance and repair of an organism. To investigate this apparent redundancy of proteoglycans at the cell surface, we have studied the expression of fibroglycan in the mouse embryo and compared this expression with that of syndecan-1. The characterisation of mouse embryo cDNA clones that crosshybridized to human fibroglycan-cDNA predicted that murine and human fibroglycan were highly similar in structure. Consistently, the analysis of transfectant cells, murine cell lines and embryo extracts indicated that the murine proteoglycan reacted specifically with monoclonal antibody 10H4 developed against the human protein. Fibroglycan, as detected by monoclonal antibody 10H4 in sections of embryonic tissues, occurred exclusively on mesenchymal cells that represented the putative precursors of the hard and connective tissue cells. No fibroglycan was detected in epithelia or in muscle cells. Areas where fibroglycan was particularly abundant were sites of high morphogenetic activity where intense cell-cell and cell-matrix interactions are known to occur (e.g. the epithelial-mesenchymal interfaces, the prechondrogenic and preosteogenic mesenchymal condensations). The expression of fibroglycan was weak in the early embryo, culminated during the morphogenetic phase and at the moment of cell lineage differentiation, and persisted in the perichondrium, periosteum and connective tissue cells. Syndecan-1, in contrast, was primarily detected in epithelia, and transiently in some mesenchymal cells, with mesenchymal localisations that did not or only partially overlap with those of fibroglycan. In situ hybridization analyses confirmed these expression patterns at the transcriptional level, identifying mesenchymal cells as the major source of fibroglycan production. These data indicate that the expression of fibroglycan occurs along unique and developmentally regulated patterns, and suggest that fibroglycan and syndecan-1 may have distinctive functions during tissue morphogenesis and differentiation.