An in vitro assay for growth regulation of embryonal carcinoma by the blastocyst.

An in vitro assay has been developed in order to examine the mechanism whereby the mouse blastocyst regulates embryonal carcinoma (EC). The assay measures the suppression of EC cell colony formation caused by exposure to the cavity of the blastocyst (i.e., blastocele), and the results are comparable to the previous results with blastocyst-mediated suppression of tumor formation for this same cell line. The assay has been used to determine the time necessary for the blastocyst to regulate the EC cell. In these experiments, immunosurgery is done to disrupt the interaction between the EC cell and the blastocyst, and the EC cell is then recovered and identified by its ability to form a colony. As compared to control cells, 84% of the EC cells are recovered after 2 hr of exposure to the blastocyst, 57% are recovered after 14 hr of exposure, and 27% are recovered after 24 hr of exposure. This long time course is similar to the prolonged doubling time of labeled EC cells within the blastocyst suggesting that the response of the EC cell may be related to the cell cycle within the blastocyst. Data presented show that synchronization of the EC cell cycle (using a mitotic selection procedure) produces synchronization of the response to the blastocyst. The response appears to be closely linked to the G1 phase of the EC cell cycle.