Trophoblast cell invasiveness and capability for the cell and genome reproduction in rat placenta

Using 3H-thymidine labeling and cytophotometric DNA content measurement in nuclei and mitotic figures, proliferative activity and genome reproduction peculiarities have been studied in highly invasive, primary and secondary giant trophoblast cells (pGTCs and sGTCs, respectively) as well as in the low invasive junctional zone and labyrinth trophoblast cells (JTCs and LTCs) of rat placenta. It has been shown that different extent and patterns of phagocytic activity of pGTCs and sGTCs correspond to different developmental stages. An inverse relationship has been observed between capability for mitoses and invasive and phagocytic activities. The pGTCs and sGTCs lose their mitotic activity from the start of their differentiation. Nevertheless, they continue reproduction of their genome and undergo a series of endoreduplication cycles to reach the ploidy degree of 256-1024c. In contrast, the JTCs and LTCs have, as a rule, no invasive and phagocytic activities, while preserving proliferative properties up to 15 day of gestation. They undergo initial polyploidization via uncompleted polyploidizing mitoses up to octaploid level and then pass to the endoreduplication cycle that excludes mitoses. Such a way of cell reproduction has been suggested to play a protective role, as it rules out contacts of the trophoblast cell genome with chromosomes of the phagocyted allogenic maternal tissue.