Apical constriction is the process in which contraction of the

apical Apical means "pertaining to an apex". It may refer to: * Apical ancestor, refers to the last common ancestor of an entire group, such as a species (biology) or a clan (anthropology) *Apical (anatomy), an anatomical term of location for features l ...

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 devel ...

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 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, wi ...

. 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 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 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 forma ...

placode A neurogenic placode is an area of thickening of the epithelium in the embryonic head ectoderm layer that gives rise to neurons and other structures of the sensory nervous system. Placodes are embryonic structures that give rise to structures suc ...

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 ...

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 comp ...

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 ''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-kno ...

'', 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 ''

, 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 Involution may refer to: * Involute, a construction in the differential geometry of curves * ''Agricultural Involution: The Processes of Ecological Change in Indonesia'', a 1963 study of intensification of production through increased labour input ...

of the blastopore. In these cells, apical constriction occurs when

actomyosin Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act toge ...

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 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 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 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 thi ...

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 Adherens junctions (or zonula adherens, intermediate junction, or "belt desmosome") are protein complexes that occur at cell–cell junctions, cell–matrix junctions in epithelial and endothelial tissues, usually more basal than tight 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