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.

Morphogenetic role

Apical constriction plays a central role in important morphogenetic events in both

invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

and vertebrates. It is typically the first step in any invagination process and is also important in folding tissues at specified hingepoints. During

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. ...

in both invertebrates and vertebrates, apical constriction of a ring of cells leads to blastopore formation. These cells are known as

bottle cells A bottle is a narrow-necked container made of an impermeable material (such as glass, plastic or aluminium) in various shapes and sizes that stores and transports liquids. Its mouth, at the bottling line, can be sealed with an internal ...

, for their eventual shape. Because all of the cells constrict on the apical side, the epithelial sheet bends convexly on the

basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is nec ...

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

Mechanism

Apical constriction occurs primarily through the contraction of cytoskeletal elements. The specific mechanism depends on the species, the cell type, and the morphogenetic movement. Model organisms that have been studied include the

frog A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...

'' Xenopus'', and the fly ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many s ...

''.

''Xenopus''

During '' Xenopus''

, bottle cells are located in the

dorsal marginal zone Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsa ...

and apically constrict inwards to initiate involution of the blastopore. In these cells, apical constriction occurs when actomyosin contractility folds the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...

to reduce the apical surface area.

Endocytosis Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...

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. Although apical constriction is always observed, it is not necessary for gastrulation, indicating that there are other morphogenetic forces working in parallel. Researchers have shown that the removal of bottle cells does not inhibit gastrulation, but simply makes it less efficient. Bottle cell removal does, however, result in deformed embryos. Neural tube cells in ''Xenopus'' apically constrict during the initial invagination as well as during hingepoint folding. Here, the mechanism depends upon the protein Shroom3, which is sufficient to drive apical constriction. Because Shroom3 is an actin-binding protein and accumulates on the apical side, the most likely mechanism is that Shroom3 aggregates the

actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...

meshwork, generating a squeezing force. Ectopic Shroom3 has been shown to be sufficient to induce apical constriction, but only in cells with apico-basal polarity.

''Drosophila''

The molecular picture of apical constriction is most complete for ''

''. During ''Drosophila'' gastrulation, apical constriction of midline cells initiates invagination to create the

ventral furrow Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position ...

. Like in ''Xenopus'', actomyosin contractility plays a major role in constricting the apical side of the cell. The constricting cells have an actin meshwork directly beneath the apical membrane as well as circumferential actin belts lining the adherens junctions between cells. Pulsed contractions of the actin meshwork are believed to be primarily responsible for reducing the apical surface area. In Drosophila, researchers have also pinpointed the molecules responsible for ''coordinating'' apical constriction in time. ''Protein folded gastrulation'' (Fog), a secreted protein and ''Concertina'', a G alpha protein, are members of the same pathway that ensure that apical constriction is initiated in the right cells at the right time. The transmembrane protein T48 is part of a redundant pathway that is also needed for coordination of apical constriction. Both pathways must be disrupted in order to completely block ventral furrow formation. Both pathways also regulate the localization of

RhoGEF2 RhoGEF domain describes two distinct structural domains with guanine nucleotide exchange factor (GEF) activity to regulate small GTPases in the Rho family. Rho small GTPases are inactive when bound to GDP but active when bound to GTP; RhoG ...

, a member of the Rho family GTPases, which are known to regulate actin dynamics.

References

External links

* http://worms.zoology.wisc.edu/urchins/SUgast_primary4.html * http://www.sdbonline.org/fly/newgene/foldgs1.htm {{DEFAULTSORT:Apical Constriction Cell biology