Origin and establishment of embryonic polar axes in amphibian development.

Publisher Summary In the growing oocyte or the mature egg, cephalocaudal and dorsoventral polarities cannot yet be distinguished. Usually only a single primary, animal-vegetative polarity is evident from the very beginning. The primary dorsoventral polarization of the egg often becomes manifest only shortly before cleavage. The relationships between these primary polarities of the unicellular egg and the ultimate polar axes of the developing embryo are described, and the origin and establishment of the various polarities are also discussed in this chapter. A distinction is made between the primary polarities of the unicellular egg and the definitive polarities of the developing embryo. Experimental evidence and theoretical considerations support the view that the primary polarities of the egg reside in the cortical layer that acts as a functional unit. In amphibians, the primary animal-vegetative polarity of the egg has the character partially of a discontinuous heterogeneity and partially of a continuous heterogeneity. The definitive polarities of the embryo become established only during gastrulation. The dorsoventral and mediolateral polarities arise as a result of the regional distribution of the mesoderm-inducing activity along the periphery of the vegetative yolk mass, the craniocaudal polarity, as a result of the decremental extension of the mesoderm-inducing action into the animal, ectodermal moiety. The position of the latter polarity becomes reversed during gastrulation.