ANTERIOR PATTERNING IN MOUSE

The anteroposterior axis of the vertebrate embryo becomes explicit dur ing gastrulation, the process that converts a relatively featureless e mbryonic precursor population into new tissues assembled into a recogn isable body pattern Vertebrate embryos arrive at gastrulation in very different states in terms of their size, cell number and reliance on f actors inherited from the unfertilized egg. However, they emerge from gastrulation looking very similar and there Is now extensive molecular genetic evidence to indicate that the bare essentials of the gastrula tion process have been well conserved during evolution Here, we review recent findings in the mouse that suggest that anterior identity is, in fact, established before gastrulation starts. They suggest that it is first manifest in extraembryonic tissue and that this tissue is ess ential for the embryo to develop normal anterior structures, such as t he forebrain. We also argue that this precocious anterior pattern coul d have a counterpart in other non-mammalian vertebrates.