Practice question

Bottle cells arise at dorsal marginal zone through active apical constriction driven by actomyosin contraction, mediated by Shroom3 and RhoA pathways regulating cytoskeleton. Microtubules and actin accumulate apically, shrinking outer surface while expanding basal cytoplasm, producing flask shape with narrow neck and bulbous body. This mechanical deformation pulls neighboring cells inward, initiating blastopore invagination and archenteron formation. No mitosis occurs during this transformation; shape change depends on cytoskeletal remodeling and membrane trafficking. Resulting invagination creates opening for archenteron and continuous involuting mesoderm ingress that establishes body plan.