In vivo studies on the second half stage of rabbit blastocyst implantation

Tateyama, I.; Hashii, K.; Kamitani, N.; Tominaga, T.; Mori, T.

We observed throught time from the morphological viewpoint the process of implantation of blastocysts in rabbits. In this report, we dealt with the point in time which must be called the second half stage of implantation, and we were able to clearly grasp the circumstances in which the syncytiotrophoblasts including mainly those of 7.75 d. p. c. penetrate into the endometrial folds. However, the process of penetration by trophoblasts is not simply an erosive activity from the blastocyte side, but complex interactions of the blastocyst and endometrium were anticipated. Moreover, it was throught that a complicated immunological mechanism intervenes throughout this entire process of interactions. Especially during this period, macrophages and large lymphocytes thought to be released from within the endometrium are present, and they are intimately mutually involved in implantation. During the implantation process in normal pregnancies, the penetration of trophoblasts extends to the capillaries of the endometrial stromal components, and in the case of rabbits, the hemochorial placenta forms in the capillary region of the placental folds. Endometrial stromal cells changing into decidual cells were previously reported as possessing necessarily an immunological barrier action, so that an intense immune action was surmised from the deep layers of the basement membrane. In contrast to this, in the upper layer regions of the uterine cavity or from the basement membrane, many macrophages and lymphocytes were present, so that a condition of immune tolerance due to hyperimmunity was anticipated. With the basement membrane as the border, the immunological imbalance was considered as a clue for explaining the mechanism of implantation.