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**Mammals Placentation**:
– Placenta forms after embryo implants into uterine wall.
– Fetus connected to placenta via umbilical cord.
– Mammalian placentas classified based on tissues separating maternal from fetal blood.
– Types include endotheliochorial, epitheliochorial, hemochorial placentation.
Placentation involves remodeling of blood vessels to supply needed blood.

**Reptiles Placentation**:
– Over 100 origins of live birth in squamata.
– Squamates have frequent placentation occurrences.
– Most reptiles exhibit strict epitheliochorial placentation.
– Placentae form following implantation into uterine tissue.
– Regional specialization for gas, amino acid, and lipid transport in some species.

**Research on Placenta**:
– Placenta evolved multiple times independently.
– Genetic mechanisms studied in reptiles, seahorses, mammals.
– Study of evolution of new structures and functions in placenta.
– Intermediate forms of placenta exist in living species.
– Placenta serves as a model to study complex organ evolution.

**Evolution of Placental Structures**:
– Placentas evolve through utilization of existing tissues.
– New structures can evolve within pre-existing tissues.
– Uterus becomes regionally specialized in some viviparous reptiles.
– Male seahorses have modified skin to form pouch for embryos.
– Southern grass skink shows regional specialization in placental functions.

**Evolution of Placental Functions**:
– Functions include nutrient transport, gas exchange, waste removal.
– Processes involve re-purposing ancestral tissues and genes.
– Maternal-fetal communication via signaling molecules.
– Utilization of existing signaling molecules in placental tissues.
– Evolution of new molecular processes for placental functions.

Placentation (Wikipedia)

Placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo. Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.

Placentation in the human resulting from cleavage at various gestational ages
Anatomical terminology

The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange. This definition is modified from the original Mossman (1937) definition, which constrained placentation in animals to only those instances where it occurred in the uterus.

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