Overview I

The placenta is a specialized pregnancy-specific structure that develops concurrently with development of the embryo and fetus. From an evolutionary perspective, the placenta was the essential factor in permitting viviparity, a reproductive strategy in which fetal development proceeds within the female reproductive tract. Viviparous species are able to provide greater protection from environmental risks and can more precisely control the development of their progeny while they reside in utero . The placenta is comprised of numerous cell types. Among the cell types are specialized epithelioid cells, called trophoblast, that possess several important functions enabling viviparous development (1 ,2 ). Trophoblast cells play key roles in protecting the embryo/fetus from noxious substances, programming maternal support, and preventing maternal immune rejection while at the same time ensuring appropriate bidirectional nutrient/waste flow required for growth and maturation of the embryo. Although placenta functions are highly conserved, species-specific elements of placenta organization and activity are evident. Consequently, placental research has benefited and will continue to benefit from a comparative approach. Each species presents experimentally valuable attributes that can be exploited to better understand the biology of the placenta and viviparity.