Embryology Question:

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Under normal conditions, there is no passage of cells across the placenta during gestation. The placenta has a smooth mucosa separating the richly vascularized region in contact with the mother’s endometrium from the umbilical cord in contact with the fetal blood. This barrier is known as placental barrier. Although permeable to some substances (selective permeability), the placental barrier forbids the passage of cells.

Image Diversity: umbilical cord placental barrier