VERTEBRATE HEAD INDUCTION BY ANTERIOR PRIMITIVE ENDODERM

In vertebrates the antero-posterior organization of the embryonic body axis is thought to result from the activity of two separate centers, the head organizer and the trunk organizer, as operationally defined b y Spemann in the 1920s. Current molecular studies have supported the e xistence of a trunk organizer activity while the presence of a distinc t head inducing center has remained elusive. Mainly based on analyses of headless mutants in mice, it has been proposed that the anterior ax ial mesoderm plays a determining role in head induction. Recent gain-a nd loss-of-function studies in various organisms, however, provide com pelling evidence that a largely ignored region, the anterior primitive endoderm, specifies rostral identity. In this review we discuss the e merging concept that the anterior primitive endoderm, rather than the prechordal plate mesoderm, induces head development in the vertebrate embryo.