Is the plane of first cleavage related to the point of sperm entry in the mouse?

Abstract

The notion that pattern formation cannot depend on cues that are already present in the zygote has dominated thinking about early development in mammals for more than 3 decades. However, recent studies in the mouse have rendered this notion untenable. The first pertinent study provided evidence that the axis of bilateral symmetry of the early blastocyst was aligned with the animal–vegetal axis of the zygote (Gardner, 1997). Subsequently, it was found that the embryonic–abembryonic axis and equatorial plane of the blastocyst were normally related to the plane of first cleavage, being approximately orthogonal and parallel to this plane respectively (Gardner, 2001; Piotrowska and Zernicka-Goetz, 2001; Piotrowska et al., 2001). Collectively, these findings argue that both axes of the blastocyst are already specified before cleavage in the mouse, and therefore raise the question whether the asymmetries necessary for their specification are intrinsic to the egg or imposed on it by the fertilizing spermatozoon. Using phytohaemagglutinin (PHA)-treated fluorescent microspheres (beads) as markers, Piotrowska and Zernicka-Goetz (2001) claim to have established a patterning role for the spermatozoa by demonstrating that the sperm entry point (SEP) consistently maps to the plane of first cleavage.