Expression of H-2 class I genes in murine extra-embryonic tissues.

Murine major histocompatibility complex class I genes are transcribed at high levels in placental tissues, lower levels in yolk-sac tissues and at barely detectable levels in the embryo at Day 13.5 of gestation. Genes are expressed at equivalent levels whether inherited maternally or paternally, and the genetic background has no effect on class I gene transcription. These results show that potential alloantigens are expressed in extra-embryonic tissues intimately associated with maternal tissues and blood supply and yet fail to induce immunological rejection.