A SMALL POPULATION OF ANTERIOR CELLS PATTERNS FOR FOREBRAIN DURING ZEBRAFISH GASTRULATION

During gastrulation in vertebrates, dorsal ectoderm is induced to form neural tissue that later gives rise to the brain and spinal card. Thi s induction depends on signals arising from a group of cells on the do rsal side of the gastrula, This group of cells constitutes the organiz er(1,2). It is thought that the organizer initially induces neural tis sue with anterior, or forebrain, character, and that other signals sub sequently posteriorize neural tissue in the trunk(2,3). Here we show t hat development of the anterior central nervous system of the zebrafis h embryo also depends on a small group of ectodermal cells located in the prospective head region. Removal of these ectodermal cells during gastrulation perturbs subsequent neural patterning and results in wide spread cell death, Transplantation of these cells shows that they can induce forebrain-specific gene expression in more posterior regions of the neural plate. Our results indicate that an early step in neural p atterning is the establishment of a small population of signalling cel ls within the most anterior region of the embryo. These cells are requ ired for patterning and survival of the anterior brain.