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The placenta (pl.: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation.It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal ...
The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception". Rapid growth occurs and the embryo's main features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells).
The chromosomes of the sperm are passed into the egg to form a unique genome. The egg becomes a zygote and the germinal stage of embryonic development begins. The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation. [2]
These buds become tubular, and form the glandular substance of the two lobes, which ultimately meet and fuse behind the urethra and also extend on to its ventral aspect. The median lobe of the prostate is formed as an extension of the lateral lobes between the common ejaculatory ducts and the bladder.
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.
This extraembryonic mesodermal ground substance forms the future Wharton's jelly. [2] The amniotic membrane and its contents form the umbilical cord that connects the embryo and the placenta. [3] [4] The root of the connecting stalk contains the allantois as a diverticulum of hindgut endoderm along with umbilical vessels. [5] [2]
There are two vessel-rich features in the amniote, the yolk sac and the allantois. When the chorion fuses with the former, the result is a choriovitelline placenta. When it fuses with the latter, the result is a chorioallantoic placenta. Most mammals first form a temporaty choriovitelline placenta, then the chorioallantoic placenta takes over.
In the late blastocyst, the trophectoderm is known as the trophoblast. [2] The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion (the portion of the chorion that develops villi) and the underlying uterine tissue of the mother.