Light and electron microscope immunocytochemical studies on the role of binucleate cells in villus growth in goat placentomes.

The monoclonal antibody SBU-3 exclusively labels granules and Golgi cisternae in fetal trophectodermal binucleate cells but only similar granules in the adjacent syncytium in goat placentomes. This localisation supports previous morphological evidence that binucleate cells migrate and form the syncytium by fusion. SBU-3 reactivity is not demonstrable in binucleate cells or syncytium before the start of placentomal fetal chorionic villus formation (30-34 days post coitum (d.p.c.)) but then rises rapidly to a peak during the period of maximal villus growth into the caruncle (36-95 d.p.c.). During this period the highest reactivity and numbers of granules are concentrated in the most extensive syncytium and largest binucleate cells at the advancing tips of the villi. After the villi have reached maximum length (90-100 d.p.c.) the SBU-3 reactivity decreases and the binucleate cell distribution becomes more diffuse, although even at term (145-150 d.p.c.) SBU-3 positive granules are present in binucleate cells and the syncytium. In the interplacentomal regions, where no chorionic villi form, no binucleate cells or syncytium show any SBU-3 activity at any stage of pregnancy despite having equivalent ultrastructure (including granules) to their placentomal neighbours. These results support the concept of two populations of binucleate cells of equivalent ultrastructure but different granule content and function in the goat placenta. The distribution of the SBU-3 positive population suggests a role in placentomal villus formation.