The Smith County Veteran Services Office will be closed Monday through Monday, August 15-22, 2016. The Smith County Veterans Service Office will be closed Monday through Monday, August 15-22 for training, and will reopen on Tuesday, August 23 for normal business hours.
The first differentiation event in the formation of the placenta is the formation of trophoblasts, specialized epithelial cells, that ultimately form a physical connection between the embryo and the uterus. Allocation of cells to the trophoblast lineage is dependent on their position at the morula stage.
Just prior to formation of the placenta, there are a total of six layers of tissue separating maternal and fetal blood. There are also three layers on the maternal side, but the number of these layers which are retained - that is, not destroyed in the process of placentation - varies greatly among species.