2 Axial Relationships between Egg and Embryo in the Mouse

Publisher Summary This chapter discusses the axial relationship between egg and embryo in the mouse. Mammals are no exception to the rule that eggs are polarized cells because for the first and second meiotic divisions that occur, respectively, before and after sperm penetration the chromosomes are assembled on spindles that are localized cortically. Insect and cephalopod eggs are unusual in exhibiting an obvious bilaterally symmetrical morphology well before fertilization. More commonly, bilateral symmetry originates more subtly, thereafter, through sperm penetration or subsequent male pronuclear migration inducing the displacement of one or more cytoplasmic regions from a symmetrical distribution about the animal vegetal axis. More complex cytoplasmic rearrangements accompany migration of the pronucleus of the fertilizing sperm in ascidians, and these have also been implicated in the bilateral localization of several putative determinants. The very few relevant experimental studies that have been undertaken relate principally to the Em–Ab axis of the blastocyst.