Placenta

The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother s blood supply. True placentas are a defining characteristic of eutherian or placental mammals , but are also found in some snakes and lizards with varying levels of development up to mammalian levels. Note, however, that the homology of such structures in various viviparous organisms is debatable at best and, in invertebrates such as Arthropoda , is definitely analogous at best. However, a recent publication describes what amounts to a phylogenetically analogous, but physiologically and functionally almost identical structure in a skink . In some senses it is not particularly surprising, because many species are ovoviviparous and some are known as examples of various degrees of viviparous matrotrophy . However, the latest example is the most extreme to date, of a purely reptilian placenta directly comparable to a eutherian placenta. The word placenta comes from the Latin word for cake , from Greek plakóenta/plakoúnta , accusative of plakóeis/plakoús – πλακόεις, πλακούς , flat, slab-like , in reference to its round, flat appearance in humans. The classical plural is placentae , but the form placentas is common in modern English and probably has the wider currency at present. Prototherial (egg-laying) and metatherial (marsupial) mammals produce a choriovitelline placenta that, while connected to the uterine wall, provides nutrients mainly derived from the egg sac. The placenta functions as a fetomaternal organ with two components: the fetal placenta , or ( Chorion frondosum ), which develops from the same sperm and egg cells that form the fetus; and the maternal placenta , or ( Decidua basalis ), which develops from the maternal uterine tissue.

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