Cytokines in mammalian reproduction and speculation about their possible involvement in nonmammalian viviparity.

In recent years, the secretion of immunoregulatory factors (cytokines) at the maternofetal interface in mammals has been widely documented. Although cytokine production seems to be a specific phenomenon in mammalian reproduction, the specific roles of these substances in different species are still not clear. However, a balance of different cytokine activities appears to be crucial for regulation of the establishment and survival of the semiallogeneic embryo in maternal tissues. The apparent immunological role of placental cytokines in the mechanisms of implantation and embryonic development in mammals has raised the question of whether cytokines are also involved in the reproduction of nonmammalian vertebrates. Our studies have shown that the production of cytokines by the maternofetal unit is not limited to mammalian species, but that interleukin-1 (IL-1)alpha, IL-1beta, and transforming growth factor-beta (TGF-beta) are secreted by the placenta of a viviparous squamate reptile, Chalcides chalcides. Our finding of this parallelism between reptilian and mammalian reproduction suggests that immunological mechanisms, possibly mediated by the secretion of cytokines, played an important role in the evolution of viviparity.