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 Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

layer, generating forces that can bend or fold the cell sheet.

Morphogenetic role

Apical constriction plays a central role in important

morphogenetic Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

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 chordat ...

and

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

. 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 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. ...

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 Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula. The process begins when the notochord induces the form ...

, placode formation, and

primitive streak The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the br ...

formation.

Mechanism

Apical constriction occurs primarily through the contraction of

cytoskeletal The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compo ...

elements. The specific mechanism depends on the species, the cell type, and the morphogenetic movement.

Model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...

s that have been studied include the

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

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-know ...

'', and the

fly Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...

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 speci ...

''.

''Xenopus''

During ''

, 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 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 (t ...

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. ...

of the membrane at the apical side further reduces surface area. Active trafficking of these endocytosed

vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...

along

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 1 ...

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 In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, ...

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 family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...

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 In typography, the mean line is the imaginary line at the top of the x-height. upright 2.0, alt=A diagram showing the line terms used in typography Round glyphs will tend to break ( overshoot) the mean line slightly in many typefaces, since t ...

cells initiates invagination to create the ventral furrow. 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, 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