Cellular Choreography: Gastrulation Movements Quiz

Invagination is the process by which a sheet of cells folds inward to form a pocket or tube-like structure. During gastrulation, invagination is critical for creating the internal layers of the embryo. It is driven by changes in cell shape, particularly apical constriction, where cells narrow at their top surface and cause the sheet to buckle inward.

Convergent extension is a coordinated cell movement in which cells intercalate mediolaterally, narrowing the tissue perpendicular to the body axis while simultaneously elongating it along the axis. This movement is essential for shaping the embryo during gastrulation and is regulated by the planar cell polarity pathway.

The actomyosin network, consisting of actin filaments and myosin II motor proteins, drives apical constriction during invagination. Myosin II contracts the actin cortex at the apical surface of cells, causing them to narrow at the top. This generates a wedge shape that causes the epithelial sheet to curve inward and form a tube or pouch.

In sea urchin embryos, gastrulation begins with the primary invagination of the vegetal plate, a thickened region of cells at the vegetal pole. These cells buckle inward to initiate the archenteron, or primitive gut. This is followed by secondary invagination, which extends the archenteron further into the blastocoel.

Epiboly is a morphogenetic movement in which a sheet of cells spreads and thins as it moves over the surface of the embryo or yolk. It is particularly well-studied in zebrafish, where the blastoderm spreads to envelop the yolk cell. Epiboly involves cell intercalation, active migration, and changes in cell shape to achieve tissue spreading.

Invagination, ingression, and convergent extension are all morphogenetic movements that reshape the embryo during gastrulation. Invagination involves inward folding, ingression involves individual cells detaching and migrating inward, and convergent extension narrows and elongates tissues. Mitosis is cell division and is not itself a morphogenetic movement.

The blastopore is the opening that forms on the surface of the amphibian embryo through which cells move inward by involution during gastrulation. It begins as a small slit on the dorsal side and expands to encircle the yolk plug. The dorsal lip of the blastopore is the organizer, equivalent to Hensen's node in amniotes.

Epiboly describes the spreading of a cell sheet over the surface of another tissue, much like pulling a hood over an underlying structure. In zebrafish development, the blastoderm spreads over the yolk cell during gastrulation through a combination of cell intercalation and cytoskeletal-driven movement of the yolk syncytial layer.

Although both involve similar cellular processes, the ventral furrow in Drosophila forms by coordinated apical constriction of a large sheet of cells leading to invagination, rather than by individual cell ingression as seen at the vertebrate primitive streak. These are parallel solutions to the challenge of internalizing the mesoderm but differ mechanistically.

Fibronectin is an extracellular matrix protein that provides a substrate for directed cell migration during gastrulation. Migratory cells use integrin receptors to adhere to fibronectin fibers and move through the interior of the embryo. Disruption of fibronectin or integrin signaling leads to defects in cell migration and abnormal gastrulation.

Convergent extension is primarily regulated by the planar cell polarity pathway and the non-canonical Wnt signaling pathway. These pathways orient cells within the plane of the tissue and coordinate their polarized intercalation. The canonical Wnt/beta-catenin pathway is more involved in cell fate decisions, and Hedgehog is not a direct regulator of convergent extension.

The dorsal lip of the blastopore in Xenopus acts as the Spemann-Mangold organizer, a region capable of inducing a complete secondary body axis when transplanted to another embryo. It secretes BMP antagonists and other signals that pattern the embryo, making it functionally equivalent to Hensen's node in birds and mammals.

During convergent extension, cells intercalate by inserting themselves between neighboring cells. This requires dynamic changes in cell-cell adhesion, including the regulated remodeling of cadherin-based junctions. Cells must weaken adhesion in one direction to allow intercalation and strengthen it in another to produce net tissue elongation.

The archenteron is the primitive gut cavity that forms during gastrulation by the inward movement of cells through invagination or involution. It opens to the outside through the blastopore and is lined by cells of the endodermal and mesodermal germ layers. The archenteron is the precursor to the entire digestive and respiratory tracts.

Invagination of epithelial sheets is driven by apical constriction (narrowing of the apical surface), changes in cell-cell adhesion that allow rearrangements, and oriented cell division that can contribute to folding forces. Basal expansion is not a recognized driver of invagination and would counteract the inward buckling needed to fold the tissue.