Establishment Of Chorioallantoic Placentation

There are several different types of chorioallantoic placentae that differ greatly in appearance. However, all chorioallantoic placentae develop from the same types of embryonic and maternal tissues and are built around a basic framework of vascular mesodermal allantoic villi.

The trophoblast, although varying greatly in structure and number of layers, is present, as are fetal mesenchyme and connective tissue, fetal blood vessels, and both fetal and maternal blood. All vascular placentae are supplied with either vitelline or allantoic vessels. Choriovitelline placental areas are usually temporary and often remain smooth (without villi). Smooth, or avillous, chorio-allantoic placental areas occur temporarily in early gestation in some species, but in later gestation are supplemental to villous or labyrinthine areas. Vascular mesodermal allantoic villi are the basic framework of all definitive chorioallantoic placentae of Eutheria, whether villous, trabecular, or labyrinthine in pattern. These meso-dermal villi are covered by trophoblast.

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