The development and proliferation of trophoblast from ectopic mouse embryo allografts and isografts of increasing gestational age in nonimmune and specifically preimmunized hosts was studied. Specific preimmunization of recipients significantly inhibited haemorrhagic nodule development from 3-3½ and 4-4½ day post coitum homozygous blastocysts and from 5-5½ day homozygous egg cylinders. preimmunization had no effect on the percentage of haemorrhagic nodules which developed from 6-6½ day homozygous egg cylinders. Haemorrhagic nodule development was significantly increased for F1 hybrid allografts compared to homozygous allografts to preimmunized recipients. Coincident with the immune inhibition of haemorrhagic nodule formation was the microscopic finding that in some cases trophoblast cells developed without subsequent proliferation in preimmunized hosts.
The results are discussed in relation to the expression of transplantation antigens by the developing conceptus, the continued differentiation of the embryo after transplantation, and the inductive influences of the inner cell mass on trophoblast ontogeny.