欧宝娱乐

“ERV envelope genes possess unique properties that make them suitable for use in forming the placenta: they are fusogenic proteins and they have immunosuppresive properties. Eutherian (placental) mammals distinguish themselves from nonplacental animals in the ability of the female to nurture the fertilized ovum and growing embryo within the body. The placenta is a transient tissue of embryonic origin whose evolution made it unnecessary to partition the embryo into a protective egg, which matured outside the mother's body. It serves two purposes for the maturing embryo: it is a conduit for respiratory gasses and nourishment supplied by the mother, and it provides an environment of immune tolerance. The fetus is necessarily half-foreign tissue, an allograft within the mother. It draws half of its genetic, and hence antigenic, identity from maternal and half from paternal genes. If the fetus is to mature within the mother, it must be isolated from the maternal immune system such that a graft-versus-host response does not reject it. The placenta forms early after implantation of the embryo. Syncytins mediate the formation of a continuous fused layer of cells around the embryo, isolating it from the mother, yet allowing essential nutrients to traverse from the mother's system. Although the observations on human syncytin-1 and -2 were compelling, it was left to scientists to definitively link syncytins to placental formation by studying mice. Here two syncytins (dubbed A and B) from murine ERVs were implicated, and genetic experiments with mice defective in these genes confirmed that their dysfunction disrupted placental formation. Notably, however, syncytin-A and -B were not syntenic with the human syncytins. That is, the human and mouse genes are not descended fron common ancestral syncytins; they have arisen by separate ERV gene capture events from different families of ERV in human and mouse ancestors.”
― Michael G Cordingley, Viruses: Agents of Evolutionary Invention