Cell interactions in preimplantation embryos: evidence for involvement of saccharides of the poly-N-acetyllactosamine series.

Roles of cell surface carbohydrates containing the 3-fucosyl-N-acetyllactosamine and poly-N-acetyllactosamine sequences (SSEA-1 and I antigens, respectively) in the compaction of mouse embryos have been investigated using the endo-beta-galactosidase of Bacteroides fragilis to modify the surface of cleavage-stage embryos. Treatment with this enzyme abolished SSEA-1 activity and diminished I antigen activity on the embryonic cell surface. Embryos cultured in the presence of endo-beta-galactosidase from the 2- to 4-cell stage onwards, or treated with the enzyme at the compacting 8-cell stage, continued to compact and proceeded to form blastocysts at the normal rate. However, when compacted 8- to 16-cell embryos were experimentally decompacted in calcium-free medium, treated for 1 h with endo-beta-galactosidase and returned to normal culture medium, the time taken for 50% of the embryos to recompact was prolonged five-fold. There was an even greater delay if these embryos were maintained in culture medium containing the enzyme. Blastocysts were eventually formed under both conditions. Thus, endo-beta-galactosidase did not affect compaction unless the embryos were first decompacted. On the assumption that recompaction and de novo compaction occur by similar mechanisms, we propose that carbohydrate-binding molecules are involved which have high affinities for poly-N-acetyllactosamine structures and protect them from digestion by endo-beta-galactosidase.