Frequency and localization of binucleate cells in the placentomes of ruminants.

Throughout most of normal pregnancy 15 to 20 per cent of the cells of the trophectodermal epithelium of the cow, deer, goat and sheep placentomes were found to be binucleate when a counting technique suitable for electron microscopy was used. Of this binucleate cell population about one-fifth were discovered to be migrating up to and across the microvillar junction at all stages of pregnancy. This migration appears to serve at least two functions: the transfer of the characteristic granules of the fetal binucleate cell to the maternal circulation; and the formation of the placentomal syncytium bounding the maternal connective tissue in sheep and goat.