Which cells form the dorsal lip of the blastopore?
During amphibian gastrulation, marginal zone cells at dorsal side undergo actin-myosin mediated apical constriction, accumulating F-actin and phosphorylated myosin apically and shrinking outer surface while expanding basally. These flask-shaped bottle cells invaginate inward, pulling adjacent epithelium and creating initial blastopore pit that becomes dorsal lip of blastopore. Bottle cell constriction initiates involution of chordamesoderm and marks Spemann organizer region secreting BMP antagonists like chordin and noggin, essential for axial patterning, archenteron formation, and internalization of dorsal mesoderm during early development.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Gastrulation bottle cells and dorsal blastopore lip.