Histological Classification of Placenta || Hemo-endothelial placenta

 Hemo-endothelial placenta

        In Hemo-endothelial relationship, there is approach to actual mixing of maternal blood and foetal blood as in case of higher rodents, rats, guinea pigs and rabbits. Here, the lining of chorionic villi are lost altogether along with the erosion of uterine lining. Thus endothelial blood lining of their vessels only separates the foetal blood from maternal blood in sinuses.    

        Now it may be clearly understood that the chorion is of main importance although the allantois also brought into close contact with uterus. It is more from the case of rodents and anthropoids where the chorion acquires all the placenta functions and the allantois is the form of vestigial structure or even lacking. But it must be recalled that allantoic vessels are important constituents of allantois and these allantoic vessels supply the chorionic placenta. Therefore, the placenta must be called the chorio-allantoic-placenta even if the allantois is lacking or vestigial.

        Many evidences have been recorded where the rate of transfer of substances from maternal to foetal increases with the decreases in the number of layers to be passed. The ascending order of efficient transfer is similar to the order in which the structural types have been described, i.e., Hemo-endothelial is the most efficient while the epithelio-chorial placenta is with lowest rate of transfer of material from maternal blood to foetal blood. The placenta of human beings is 250 times more efficient in the rate of transfer of sodium than that of cow's placenta. It can also be noted that there is a progressive thinning of placental tissue (barrier) as we go from epithelio-chorial to Hemo-endothelial placenta in the series. It must also be remembered that permeability (transfer of material from maternal blood to foetal blood) is only one kind of efficiency which is thought to be related with the rapidity of the growth and differentiation of foetus.