MUC-1 glycosylation in endometrium: possible roles of the apical glycocalyx at implantation.

MUC-1 is a major epithelial apical surface glycoprotein in human endometrium. It has a large, extended and highly glycosylated ectodomain that contains keratan sulphate chains. MUC-1 is abundant at the luminal epithelial surface in the receptive phase, but keratan sulphate disappears at this time. MUC-1 has been shown experimentally to inhibit cell-cell interactions by steric hindrance of binding interactions mediated by receptors, including integrins and cadherins, so its high abundance at the time of implantation is unexpected. Here, various models for MUC-1 function in implantation are considered and its expression in different species compared. The possible evolutionary advantages of a maternal 'barrier' to implantation are discussed.