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Neural transplantation of the rat midgestation trophoblast.

The trophoblast may serve as an example of tissue that is endowed with invasive properties under physiological conditions. Invasiveness of the trophoblast is enabled by the secretion of various proteases capable of degrading extracellular matrix components. Trophoblast invasive behavior is strictly controlled by anti-invasive factors produced by uterine decidual cells. To assess invasive abilities of trophoblast cells we have transplanted E9 and E14 rat trophoblast (i.e. the trophoblast obtained on embryonic day 9 and 14) into the brain of adult rats. The brain parenchyma as an immunologically privileged site is suitable for acceptance of grafts of different tissues. Moreover, the trophoblast placed into the CNS lacks inhibitory influence of anti-invasive factors that normally regulate trophoblast invasivity in the course of intrauterine gestation. To visualize migration of grafted cells the nuclei of the trophoblast were labelled with bromodeoxyuridine prior to neural grafting. The transplant of E9 and E14 trophoblast cells obtained a blood nourishment from host vessels. A proper vascularization is necessary for a further transplant growth. The transplant contained labyrinthine trophoblast cells and giant cells that are typical for the rat placenta. Vital trophoblast cells were found in all grafts whose age did not exceed a lifespan of normal rat trophoblast cells i.e. 21-22 days. In the centre of the graft, no blood vessels were observed. Interstitial spaces of neighbouring trophoblast cells were filled with the host blood and morphology of these spaces mimicked lacunae of the placental trophoblast. E9 and E14 rat trophoblast continued to differentiate after transplantation into the CNS of adult rats. Histological structure of the grafts were compared with microscopical morphology of the normal rat placenta. E9 and E14 trophoblast is considerably differentiated and it does not invade a neighbouring tissue. Trophoblast cells located at t the graft periphery may migrate on free surfaces but they do not invade through the host parenchyma. Migration occurs at a limited distance from the transplant and the cells remain in a close contact with other trophoblast cells in the graft via their cytoplasmatic processes. The ability to lyse host blood vessels and form vascular lacunae is well preserved in E9 and E14 trophoblast after grafting into the CNS. This ability is necessary for a proper transport of nutrients from the host blood stream to fetal tissue that normally occurs in the placenta.