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In pipefish placentas form following the interaction with the egg and the skin. Despite the placenta forming from pre-existing tissues, in many instances new structures can evolve within these pre-existing tissues. For example, in male seahorses the underbelly skin has become highly modified to form a pouch in which embryos can develop.
[7] [8] This protects the fetus and placenta from oxidative stress during early development in the histiotrophic (glandular nutrition) stage. As fetal nutrition switches to the hemotrophic (blood-derived nutrition) stage, EVT plugs dissolve and perfusion of maternal blood begins, allowing further development of both the fetus and placenta. [9] [5]
The placenta derives from the embryonic chorion (the portion of the chorion that develops villi) and the underlying uterine tissue of the mother. [4] [5] The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called the blastula.
Trophoblasts are specialized cells of the placenta that play an important role in embryo implantation and interaction with the decidualized maternal uterus. [5] The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetus’ circulation via the umbilical cord.
Placenta shown with attached fetal membranes. The fetal membranes surround the developing embryo and form the fetal-maternal interface. [3] The fetal membranes are derived from the trophoblast layer (outer layer of cells) of the implanting blastocyst. [3]
In the placenta, the intervillous space is the space between chorionic villi, and contains maternal blood.. The trophoblast, which is a collection of cells that invades the maternal endometrium to gain access to nutrition for the fetus, proliferates rapidly and forms a network of branching processes which cover the entire embryo and invade and destroy the maternal tissues.
The chorion and the amnion together form the amniotic sac. In humans it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes; [ 1 ] the chorionic villi emerge from the chorion, invade the endometrium , and allow the transfer of nutrients from maternal blood to fetal blood.
There is an extensive variation in the type of trophoblast cells, and structures of the placenta across the different species of mammals. [8] Of the five recognised stages of implantation including two pre-implantation stages that precede placentation , the first four are similar across the species.