[Studies of implantation traces in rats. III. Histological examination]

We carried out a histological examination of the implantation traces in delivered rats. The implantation traces could be identified more than 500 days after delivery on the mesometrial side as black and brown spots. The implantation traces were recognizable as a cicatrix remaining in the parametrium, mesometrial triangle, which was formed by repair of injury caused by placental desquamation. In this area, metrial gland cells which were laid down through pregnancy were recognized for about two months after delivery. The implantation traces consisted of cicatrix tissue associated with collagen production and hemosiderin. It was possible to distinguish old and new traces by the size of the siderophile cells and by the degree of hemosiderin present. It was also possible to discriminate new traces as yellowish-brown areas, covered with a yellowish-white mass of degenerated metrial gland, in cleared uteri stained with 2% NaOH solution. Siderophile cells on the implantation traces were derived from giant cells which persisted around the peripheral region of the placental desquamation site, and these giant cells were considered to be identifiable with metrial gland cells. It was considered that formation of the cicatrix is essentially the same in abortion, stillbirth and normal delivery. However, it was found that the implantation traces had different histological appearances depending on the degree of injury to the endometrium and myometrium and time of placental desquamation. The iron content of the implantation traces corresponded quantitatively with the hemosiderin observed in the histological investigations. The iron content decreased rapidly up to 21 days after delivery, decreasing gradually thereafter. The iron in the implantation traces could, however, be analyzed quantitatively by atomic absorption spectroscopy until day 365 after delivery.