Persistence of an immunoreactive MUC1 protein at the feto-maternal interface throughout pregnancy in the mare

A polyclonal human mucin-1 (MUC1) antibody was used to stain immunohistochemically for the presence of MUC1 on the endometrium and fetal membranes in mares between 20 and 309 days of gestation. Western blot analysis demonstrated the presence of a protein equivalent in size to a human MUC1 isoform, MUC1/Y, in equine endometrium, allantochorion and amnion. At all stages of gestation examined immunoreactivity to the MUC1 antibody was detected on the apical surface of the lumenal epithelium of the endometrium and the epithelium lining the mouths and apical regions of the endometrial glands. Furthermore, it persisted unchanged on the surface of the lumenal epithelium lying beneath the highly-invasive chorionic girdle component of the trophoblast before, during and after development of the endometrial cups. The MUC1 immunoreactive protein was also present on the trophoblast and other components of the fetal membranes during the post-fixation, pre-attachment period of gestation (20-40 days) and it persisted on the apical surface of the non-invasive trophoblast of the allantochorion before, during and after attachment, microvillous interdigitation and development of the microcotyledonary epitheliochorial placenta. Hence, the delayed placentation response in mares appears to occur independently of the persistence of an immunoreactive MUC1 protein at the feto-maternal interface.