Glucosephosphate isomerase subunit-reassociation tests for maternal-fetal and fetal-fetal cell fusion in the mouse placenta

The trophoblast of the mouse placenta is known to include multinucleated "syncytia" and also phagocytic giant cells with high nuclear DNA content. In order to learn whether such cells originate by cell fusion and whether there is appreciable mixing and/or conservation of fusion components, electrophoretic allelic strain differences in the dimeric enzyme glucosephosphate isomerase (GPI) were experimentally used. The heterodimeric GPI enzyme variant is ordinarily found only in cells of F₁ hybrids between pure strains of the two different allelic types, and is experimentally formed in vivo in multinucleated allophenic skeletal muscle heterokaryons. With our methods for starch gel assay of GPI, hybrid enzyme can be detected in amounts below 1% of the total GPI. To test for cell fusion between maternal and fetal cells in the placenta, blastocysts of one allelic pure strain ( Gpi-1^b Gpi-1^b) were surgically transferred to uteri of "incubator" mothers of the other allelic pure strain ( Gpi-1^a Gpi-1^a). To test for cell fusion between fetal cells in the placenta, allophenic blastocysts with cells of both pure strains ( Gpi-1^a Gpi-1^a ↔ Gpi-^b Gpi-1^b combination) were transferred to uteri of pure-strain ( Gpi-1^a Gpi-1^a) mothers. In both tests, the relevant component(s) of the placentas were found to contain the two pure-strain electrophoretic variants of GPI but lacked the heterodimeric variant. These results, taken in conjunction with other available evidence, support the conclusion that, although syncytial trophoblast may in fact arise by cell fusion, there is no appreciable mixing of the macromolecular contents of fused cells. Giant trophoblast cell origin by endoreduplication rather than cell fusion is supported by the present results. Nuclei or cytoplasmic constituents such as GPI monomers of cells phagocytized by giant trophoblast cells are not conserved.