First trimester development of human chorionic villous vascularization studied with CD34 immunohistochemistry.

Normal chorionic villous vascularization is essential for the undisturbed development of pregnancy. Defective vasculogenesis may play a role in pathological pregnancy. To assess pathological chorionic villous vascularization, normal vascularization has to be defined first. Few data are available on this topic. The aim of this study was therefore to investigate normal chorionic villous vascularization in ultrasound-dated first trimester pregnancies from week 5 menstrual age to week 12 (n = 41), using quantitative CD34 immunohistochemistry. Two important processes in chorionic villous vascularization were quantitatively illustrated: (i) maturation, reflected by an increase of the total number of luminized vessels as opposed to non-luminized haemangioblastic cords and (ii) margination, due to a decrease of villous stromal area and an increase of total villous vascular area. The percentage of villous stromal area occupied by vascular elements (area difference %) increased from 0.7% in week 5-2.5% in week 10. Therefore, the area of the villous stroma occupied by vascular elements increases and the vessels are situated closer to the trophoblastic layer suitable for fetal-maternal exchange. There was also a trend in increased number of peripheral vessels (2.0 in week 5 to 4.6 in week 10), supporting both developmental mechanisms. In conclusion, in exactly dated normal human first trimester pregnancies, development of the chorionic villous vascular system seems to be mostly characterized by maturation of luminized vessels from primitive haemangioblastic cords, and margination to a situation of peripherally located vessels.