2 – Asymmetry and Prepattern in Mammalian Development

This chapter discusses the asymmetry and prepattern in mammalian development. The possibility that developmentally significant prepatterning of the oocyte or zygote occurs in eutherian mammals, as in most other animals, is generally felt to be incompatible with the marked capacity of early conceptuses to regulate their development in response to various perturbations. There are a number of concerns regarding the interpretation of the relevant experiments undertaken so far that somewhat undermine the force of this view. There is, furthermore, a growing body of data that is not readily accommodated within conceptual frameworks that envisage emergence of patterning as being wholly dependent on postzygotic events. Collectively, these data point to regularities that suggest that patterning is indeed rooted in egg organization in eutherian mammals, at least in unperturbed development. That the early mammalian conceptus seems to have evolved efficient ways of normalizing its development following experimental perturbation is likely to complicate this task.