Apical constriction

Apical constriction is the process in which contraction of the apical side of a cell causes the cell to take on a wedged shape. Generally, this shape change is coordinated across many cells of an epithelial layer, generating forces that can bend or fold the cell sheet.

Apical constriction plays a central role in important morphogenetic events in both invertebrates and vertebrates. It is typically the first step in any invagination process and is also important in folding tissues at specified hingepoints.

During gastrulation in both invertebrates and vertebrates, apical constriction of a ring of cells leads to blastopore formation. These cells are known as bottle cells, for their eventual shape. Because all of the cells constrict on the apical side, the epithelial sheet bends convexly on the basal side.

In vertebrates, apical constriction plays a role in a range of other morphogenetic processes such neurulation, placode formation, and primitive streak formation.

Apical constriction occurs primarily through the contraction of cytoskeletal elements. The specific mechanism depends on the species, the cell type, and the morphogenetic movement. Here, we present a few well-studied examples in model organisms.

During Xenopus gastrulation, bottle cells are located in the dorsal marginal zone and apically constrict inwards to initiate involution of the blastopore. In these cells, apical constriction occurs when actomyosin contractility folds the cell membrane to reduce the apical surface area. Endocytosis of the membrane at the apical side further reduces surface area. Active trafficking of these endocytosed vesicles along microtubule tracks is also believed to be important, since the depolymerization (but not stabilization) of microtubules reduces the extent of apical constriction.

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