Cell-surface enrichment of fucosylated glycoconjugates in the 8- to 16-cell mouse embryo : An autoradiographic study.

The destination of 3H-fucose was examined in preimplantation mouse embryos at the 8- to 16-cell stage by autoradiography of semi-thin sections. At the end of a 16-18 h incubation in medium containing 200 μCi/ml L-[6-3H] fucose the density of silver grains over the combined surface of cells where they faced their neighbours was 40% greater than that over the outer polar cell surface and 80% greater than that over the cytoplasm. These differences were statistically significant (p<0.01). The outer cell surface also had a grain density above that found over the cytoplasm (p<0.01). There was a rapid decrease of 18-24% in the incorporated radioactivity during the first 1.5 to 3 h in 'chase' medium containing 10 mM 'cold' fucose. Following this the grain density remained relatively stable until it decreased further between 24 and 48 h. By 48 h there was no significant difference between the density of grains over cytoplasm, apposed cell surfaces and outer cell surface: all were reduced compared to the initial values, to 29% in the case of the surfaces and 10% in the case of the cytoplasm. These results indicate that there is an enrichment in the incorporation of 3H-fucose into macromolecules at the cell surfaces where neighbouring cells confront one another relative to the outer polar cell surface, as well as the cytoplasm, at the 8- to 16-cell stage. Conclusions about the relative density of fucoconjugates per unit length of apposed and polar membrane cannot be reached from such a light microscopic study. Fucoconjugates appear to belong to two groups: the first are turned over relatively rapidly (within 3 h after removal from radioactive medium) while the second are more stable, decaying between 24 and 48 h after removal from 3H-fucose.