Autoradiographic evidence for migration and fusion of cells in the sheep placenta: resolution of a problem in placental classification.

The location of the fetomaternal junction in the placenta is important with respect to structural classification and the identification of any possible barrier to maternal immunorejection. Structural classification of the sheep placenta remains controversial on account of the uncertain origin of the syncytial layer. In this study [3H]thymidine was injected into the fetus and placentomes were removed between 4h and 21 days afterwards. Autoradiography showed that the syncytium is derived predominantly from the migration of fetal binucleate cells and not from the maternal uterine epithelium as most recent reports have suggested. In this respect the origin of the syncytium in the sheep placenta is similar to that reported in certain other eutherian mammals. The finding that cells originating from the fetal allograft survive after migration through the microvillous junction poses questions as to the mechanism by which the syncytial layer resists maternal immune rejection throughout gestation.