Figure 3. High densities of MUC1 at the cell surface inhibit intercellular adhesion in model systems by steric hindrance of receptor-mediated interactions (a; Wesseling et al., 1996). In mouse, MUC1 density at the cell surface is decreased by maternal endocrine signals at the implantation phase (b; Braga and Gendler, 1993; Surveyor et al., 1995). In rabbit, maternal MUC1 appears to be downregulated locally as the result of a paracrine signal from the embryo (c; Hoffman et al., 1998). In human, MUC1 is abundant in the mid secretory phase (see Figure 1), but highly anionic keratan sulphate oligosaccharides disappear from the surface at this time, which might facilitate embryo apposition (d; Aplin et al., 1998). Other alternative mechanisms cannot be excluded: either direct recognition of mucin-associated glycan by a lectin on the blastocyst (e; Lindenberg et al., 1988; Zhu et al., 1995) or binding of underglycosylated MUC1 core protein by ICAM-1 (f; Regimbald et al., 1996).
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