Blastocoel formation in half and double mouse embryos.

We have tested the alleged causal relationship between the peripheral sealing of the embryo and the formation of the blastocoel by observing with the electron microscope the diffusion of a colloidal lanthanum tracer into advanced morulae and early blastocysts grown from half and double mouse embryos. In these experimental conditions and in the controls, the peripheral sealing of the embryos always precedes the appearance of the blastocoel, supporting a causal relationship between both phenomena. By counting the number of cells present at the morula-blastocyst transition in half and double embryos, as well as in control embryos, we have observed that neither the total number of cells nor the number of cell cycles correlate well with the appearance of the blastocoel. Results suggest that blastocoel formation depends on time elapsed since fertilization and that the experimental manipulation diminishes the cleavage rate.