Extragonadal primordial germ cells or placental progenitor cells?

Primordial germ cells (PGCs), the precursors of the gametes, are now claimed to segregate within the extra-embryonic tissues of three species of placental mammals. In this brief Commentary, I raise the question of whether the so-called PGCs are not PGCs at all, but rather, progenitor cells that build the fetal–placental interface in Placentalia.