New insights into critical events of avian gastrulation

The formation and progression of the primitive streak are key events of avi an gastrulation. We examine these processes in detail, using various morpho logical approaches. We show that formation of the primitive streak occurs l ocally at the caudal midline of the area pellucida, as cells in the caudal midline undergo an epithelial-to-mesenchymal transformation, and that exten sive migration of delaminated cells arising from more rostral or peripheral areas of the blastoderm is not involved in streak formation. Instead, such delamination occurs earlier and is restricted to the process of hypoblast formation. Moreover, we provide evidence that progression of the primitive streak involves two processes: convergent-extension movements within the st reak per se, and progressive delamination of midline epiblast cells in a ca udal-to-rostral sequence. We have identified a subpopulation of primitive-s treak cells located at its dorsal midline surface that undergoes extensive rostral displacement concomitant with streak progression. The fact that the se cells are located only dorsally and do not elongate ventrally as do adja cent ingressing cells, suggests that these cells retain their residency wit hin the primitive streak, at least until regression of the primitive streak occurs. Finally, by following labeled cells over time we establish the tim ing of movement of epiblast cells toward and into the primitive streak, pro viding direct evidence that cell-cell intercalation occurs within the primi tive streak during its progression. Collectively, our results provide new i nsight into complex and central events of avian gastrulation. Anat Rec 262: 238-252, 2001. (C) 2001 Wiley-Liss, Inc.