Fig. 5.

The major events of gastulation occur between E13 and E20. a. The onset of gastrulation is marked by the formation of the primitive streak and the primitive node. The primitive streak provides an opening to deeper embryonic layers. The primitive node is a critical molecular signaling center. On E13, cells from the epiblast layer begin to migrate toward the primitive node and streak (blue arrows). The dotted line indicates the cross-sectional view shown in panel B. b. The migrating cells first move to the primitive streak and then change direction and move down and under the upper layer (blue arrows). As the cells pass the node they receive molecular signals that induce gene expression in the migrating cells. By the end of gastrulation, the hypoblast layer is replaced by the newly formed endodermal layer and the epiblast layer by the ectodermal layer. Between these layers the mesodermal layer forms. c. Once under the upper layer, the cells change direction and begin migrating rostrally under the upper layer (blue arrows). The first cells to migrate form the most rostral regions of the newly forming endodermal and mesodermal layers. Later migrating cells form progressively more caudal regions of the layers. d. Cells that migrate along the axial midline send molecular signals that induce cells in the overlying epiblast layer to differentiate into neuroectodermal cells (red band) which are the neural progenitor cells. Migrating cells also receive a second set of signals from the node that induce anterior or posterior fate in different subpopulations of the neurectodermal cells. Early migrating cells signal anterior fate in the progenitor cells, while late migrating cells signal posterior fate. Illustrations by Matthew Stiles Davis reprinted by permission of the publisher from THE FUNDAMENTALS OF BRAIN DEVELOPMENT: INTEGRATING NATURE AND NURTURE by Joan Stiles, Cambridge, Mass.: Harvard University Press, Copyright © 2008 by the President and Fellows of Harvard College