DETERMINANTS AND EMBRYONIC POLARITY

Most Metazoa have a main somatic polarity. Diploblasts have only one p olarity. Many of them are radially organized around an axis of symmetr y defined by the position of the mouth. This axis is known as the oral -aboral axis. All triploblasts have an anterior-posterior and a dorsal -ventral polarity. The animal's primary polarity becomes established e arly in development. Although isotropic by shape, a typical metazoan e gg has a clear polarity, since its animal pole differs from its vegeta l pole. Various experiments demonstrate that the egg of many animals c ontains instructions required to specify the primary polarity of the e mbryo. These instructions consist of cortex-bound macromolecules, call ed determinants. In the simplest cases, the determinants are localized in the vegetal hemisphere and apparently distributed symmetrically wi th respect to the animal-vegetal axis of the egg. The egg axis determi nes the primary axis of the embryo, which is the oral-aboral axis in d iploblasts, and the anterior-posterior axis in most triploblasts. Thes e correlations strongly suggest that the oral-aboral axis of the diplo blasts corresponds to the anterior-posterior axis of the triploblasts. In evolutionary terms, this hypothesis implies that a common ancestor of all living Metazoa was already able to organize its own embryo alo ng a primary axis. This could be done by concentrating determinants ne ar the egg's vegetal pole. During early development the determinants a re thought to trigger a directional wave of morphogenesis, which progr esses towards the animal pole, thereby specifying the primary axis of the embryo.