The human placental bed: electron microscopic study of trophoblastic invasion of spiral arteries.

During normal human pregnancy the spiral arteries of the placental bed became so greatly distended that they are capable of delivering a tenfold increase in the supply of blood required by the fetoplacental unit in the third trimester. Studies performed with the use of light and electron microscopes have shown remarkable structural alterations in the walls of these arteries at the end of normal human pregnancy. In order to evaluate the various hypotheses of the histogenesis of these vascular physiologic changes, the present study with light and electron microscopes was carried out on the spiral arteries during the second trimester of normal human pregnancy. Special attention was given to the intravascular migration of trophoblast, as well as to the consequent interaction between fetal and maternal tissues.