Invasiveness of mouse trophoblastic cells in connective tissue

Ectoplacental cones from 8-day-old mouse embryos were grafted into the dorsal subcutaneous tissue of host mice. The grafts were collected between the 3rd and 8th days after transfer and processed for morphological analysis by light and electron microscopy. Approximately 60% of the grafts formed hemorrhagic nodules in which only invasive, giant trophoblastic cells developed. These cells shared many morphological features with the trophoblastic cells involved in normal implantation. At the periphery of the nodules, trophoblastic giant cells were frequently seen growing toward the host connective tissue. The association between the trophoblast and fibrilar components of the extracellular matrix was examined. No direct association with collagen fibrils was noted; however, many areas of the surface of invasive cells were in close proximity with microfibrils of the extracellular matrix. Since only the invasive trophoblast cells exhibited such an association, a direct comparison was made with the process of trophoblast migration within the connective tissue.