The Ultrastructure of the Arterial Supply of the Human Placenta in Early and Late Pregnancy

Summary Ultrastructural changes in the uterine arteries supplying the human placenta in early and late pregnancy have been studied. The arteries of the myometrium contained no elastic lamina between the endothelium and smooth muscle cells of the media. Intraluminal trophoblast cells are present within the spiral arteries of the decidua and in areas also form an intimal lining of the vessels. Within the media of the decidual spiral arteries in early and late pregnancy trophoblast cells are also present and together with amorphous material largely replace the smooth muscle cells of the media. The electron dense material surrounding the trophoblast in the media was identified as fibrin. In late pregnancy mural thrombi were found in both myometrial and decidual spiral arteries. The removal of the elastic lamina will reduce the compliance and variation of pressure in the arterial blood flow to the placenta. The type of structural modification of the media of the arteries supplying the placenta will facilitate the expansion of these vessels to provide increased blood flow as pregnancy advances and their occlusion and collapse after parturition.