Invagination, involution, ingression and convergent extension are the types of cell movement during gastrulation. Invagination and involution maintain epithelium. In amphibians, the dorsal lip of the blastopore invaginates, involutes and then spreads out between the ectoderm and endoderm to form the mesoderm. Sea urchin embryos invaginate to form mesoderm. Birds and mammals' blastodisc ingresses at the primitive groove to give rise to mesoderm.
In invagination, the epithelium buckles inward like a finger poking into a soft ballon, thus forming an invagination. More technically, groups of contiguous epithelial cells actively constrict at their apical pole by contraction of the band of actin microfilaments located there. Thus, the epithelial sheet folds in forming a tubular (or vesicular) endoderm with its apical surface facing a lumen.
If invagination happens passively, for example in the wake of a neighboring population of cells that actively invaginates, it is called involution. This movement can also be seen in sea urchin embryos. Most morphogenetic movements in which cells withdraw from the surface to form inner tubular structures are achieved by a combination of invagination and involution.
In ingression, tight and adherens junctions are lost, epithelial cells become mesenchymal and this mesenchyme reforms a polarized epithelium. This occurs in higher vertebrates, including birds and mammals. Birds form a disc-shaped blastula whose cells migrate toward the midline when gastrulation begins. These cells pile up to form a visible structure called the primitive streak. The primitive streak ingresses inward; ingressed cells spread out laterally and reorganize into an epithelium (endoderm) that gradually spreads around the yolk.
Epithelia change in length and width by convergent extension and individual cells slide past each other. Cells that start out positioned beside each other in one row intercalate. Thereby, the shape of an epithelium that was wide and short before the convergent extension changes to narrow and long afterwards. More technically, a cell layer elongates in one axis via intercalation (interdigitation) of cells along a perpendicular axis. Two types of convergent extension are meso lateral intercalation and radial intercalation.
Convergent Extension: Meso-Lateral Intercalation
Cellular intercalation during convergent extension involves degradation and reformation of tight and adherens junctions. Many organisms, including amphibians like Xenopus, use convergent extension to convert the embryo from a sphere to an elongated rod more like the final shape of the mature organism.
Convergent Extension: Radial Intercalation (Epiboly)
Epiboly is the process in which the layer of cells spread out and expand to cover the yolk or yolk-filled cells (in fish and amphibians) at the vegetal pole. This spreading of the cells occurs through an increase in area due to a flattening of individual cells and an intercalation of cells.