Evidence from the studies of Stern and Canning (1990) suggests that the epiblast is not the homogeneous, undifferentiated tissue that we have long assumed it to be. Rather, there appears to be differentiation in the epiblast cells even before primitive streak formation begins. These studies show that certain cells that are randomly scattered throughout the epiblast can be distinguished by a particular molecule (HNK-1, a sulfated form of glucuronic acid) on their cell surfaces. The cells expressing HNK-1 ingress individually into the blastocoel and migrate to the posterior margin. It is likely that the posterior marginal tissue secretes a chemical that attracts the cells expressing HNK-1, while the anterior marginal tissue secretes a repellent molecule (Jephcott and Stern, quoted in Stern, 1991). The HNK-1-expressing cells that collect at the posterior margin will give rise to the endoderm and mesoderm, and no cell expressing HNK-1 forms ectodermal derivatives. If the HNK-1 cells are selectively destroyed (by antibodies) while they are still on the epiblast, the embryo will not form any mesoderm or endoderm. These HNK-1-positive cells interact with the epiblast cells above them to form the initial rudiment of the primitive streak. The streak rudiment then undergoes a convergent extension process that narrows and extends the streak. When the streak has reached nearly its full extension, the HNK-1-positive cells dissolve the basal lamina of the central epiblast to form a groove through the primitive streak. This allows epiblast cells (which never had expressed HNK-1) to get recruited into the streak as it extends anteriorly and to contribute (along with HNK-1-positive cells) to the embryonic mesoderm and endoderm.
Stern, C. D. 1991. Mesoderm formation in the chick embryo revisited. In R. Keller, W. H. Clark, Jr. and F. Griffin (eds.), Gastrulation: Movements, Patterns, and Molecules. Plenum, New York, pp. 29\–41.
Stern, C. D. and Canning, D. R. 1990. Origin of cells giving rise to mesoderm and endoderm in chick embryo. Nature 343: 273-275.