Expression of a High Molecular Weight Cell Surface Glycoprotein (LETS Protein) by Preimplantation Mouse Embryos and Teratocarcinoma Stem Cells

The expression of a high molecular weight cell surface glycoprotein (LETS, fibronectin) by preimplantation mouse embryos as well as cultured teratocarcinoma stem cells was detected by using indirect immunofluorescent staining. When each stage of preimplantation embryonic development was tested for the presence of LETS protein, none was observed on two-cell, four-cell, or eight-cell embryos, or on the morula or the outer cell layer (trophectoderm) of the early or late blastocyst. However, when the inner cell mass was isolated by immunosurgery, positive staining was observed. The intensity of the staining was significantly greater on the inner cell mass isolated from the expanded (day 4) blastocyst than on that from the early (day 3) blastocyst. Certain established cell lines of teratocarcinoma stem cells (embryonal carcinoma cells) also express cell surface LETS protein. "Nullipotent" (Nulli-SCC-1) as well as pluripotent (PSA 1) embryonal carcinoma cell lines have deposits of LETS protein concentrated in areas of cell-cell contact. In addition, a teratocarcinoma-derived endodermal cell line (PYS) was found to be capable of depositing LETS onto the substratum in a fibrillar network. Taken together, our results indicate that LETS protein is synthesized at a specific stage of preimplantation mouse embryonic development. In particular, they suggest that LETS protein is a product of the embryonic ectoderm, and that some types of embryonic endoderm are also capable of synthesizing this protein.