Apical constriction
Apical constriction describes 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 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 a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...

 in both invertebrates and vertebrates, apical constriction of a ring of cells leads to blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the 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
Neurulation is the stage of organogenesis in vertebrate embryos, during which the neural tube is transformed into the primitive structures that will later develop into the central nervous system....

, placode
A neurogenic placode is an area of thickening in the embryonic epithelial layer where some organ or structure later develops. The term usually refers to cranial placodes, peripheral nervous system structures associated with the special senses and cranial ganglia...

 formation, and primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...



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.

Bottle cells

During Xenopus
Xenopus is a genus of highly aquatic frogs native to Sub-Saharan Africa. There are 19 species in the Xenopus genus...

gastrulation, bottle cells are located in the dorsal marginal zone and apically constrict inwards to initiate involution
In mathematics, an involution, or an involutary function, is a function f that is its own inverse:-General properties:Any involution is a bijection.The identity map is a trivial example of an involution...

 of the blastopore. In these cells, apical constriction occurs when actomyosin contractility folds the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 to reduce the apical surface area. Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...

 of the membrane at the apical side further reduces surface area. Active trafficking of these endocytosed vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...

 along microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

 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

Neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...

 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 globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 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 .


The molecular picture of apical constriction is most complete for Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

. During Drosophila gastrulation, apical constriction of midline 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. Folded Gastrulation (Fog), a secreted protein, and Concertina
A concertina is a free-reed musical instrument, like the various accordions and the harmonica. It has a bellows and buttons typically on both ends of it. When pressed, the buttons travel in the same direction as the bellows, unlike accordion buttons which travel perpendicularly to it...

, a G alpha
G alpha subunit
Guanine nucleotide binding proteins are membrane-associated, heterotrimeric proteins composed of three subunits: alpha , beta and gamma . G proteins and their receptors form one of the most prevalent signalling systems in mammalian cells, regulating systems as diverse as sensory perception, cell...

 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 .

External links

  • http://worms.zoology.wisc.edu/urchins/SUgast_primary4.html
  • http://www.sdbonline.org/fly/newgene/foldgs1.htm
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