Mitotic spindles and cleavage planes are oriented randomly in the two-cell mouse embryo.

Most experimental embryological studies performed on the early mouse embryo have led to the conclusion that there are no mosaically distributed developmental determinants in the zygote and early embryo (for example see [1-6]). It has been suggested recently that "the cleavage pattern of the early mouse embryo is not random and that the three-dimensional body plan is pre-patterned in the egg" (in [7] for review see [8-10]). Two major spatial cues influencing the pattern of cleavage divisions have been proposed: the site of the second meiotic division [11, 12] and the sperm entry point [13-14], although the latter is controversial [15-17]. An implication of this hypothesis is that the orientations of the first few cleavage divisions are stereotyped. Such a define cleavage pattern, leading to the segregation of developmental determinants, is observed in many species [18]. Recently, it was shown that the first cleavage plane is not predetermined but defined by the topology of the two apposing pronuclei [19]. Because the position of the female pronucleus is dependent upon the site of polar body extrusion and the position of the male pronuclei is dependent upon the sperm entry point [19-20], this observation leaves open the possibility that the sperm may provide some kind of directionality [7]. But, even if asymmetries were set up only after fertilization, a stereotyped cleavage pattern should take place during the following cleavage divisions. Thus, we studied the cleavage pattern of two-cell embryos by videomicroscopy to distinguish between the two hypotheses. After the mitotic spindle formed, its orientation did not change until cleavage. During late metaphase and anaphase, the spindle poles appear to be anchored to the cortex through astral microtubules and PARD6a. Only at the time of cleavage, during late anaphase, do the forming daughter cells change their relative positions. These studies show that cleavage planes are oriented randomly in two-cell embryos. This argues against a prepatterning of the mouse embryo before compaction.