Cell polarity
Encyclopedia
Cell Polarity refers to spatial differences in the shape, structure, and function of cells. Almost all cell types exhibit some sort of polarity, which enables them to carry out specialized functions. Classical examples of polarized cells are described below, including epithelial cells with apical-basal polarity, neurons in which signals propagate in one direction from dendrites to axons, and migrating cells.
of internal cavities, and the basolateral membrane oriented away from the lumen. The basolateral membrane refers to both the lateral membrane where cell-cell junctions connect neighboring cells and to the basal membrane where cells are attached to the basement membrane
, a thin sheet of extracellular matrix
proteins that separates the epithelial sheet from underlying cells and connective tissue
. Epithelial cells also exhibit planar cell polarity,in which specialized structures are orientated within the plane of the epithelial sheet. Some examples of planar cell polarity include the scales of fish being oriented in the same direction and similarly the feathers of birds, the fur of mammals, and the cuticular projections (sensory hairs, etc.) on the bodies and appendages of flies and other insects.
extension to the synapse, where neurotransmitters are released to propagate the signal to another neuron or effector cell (e.g., muscle or gland). The polarity of the neuron thus facilitates the directional flow of information, which is required for communication between neurons and between neurons and effector cells.
. Here, actin
polymerization in the direction of migration allows cells to extend the leading edge of the cell and to attach to the surface. At the rear of the cell, adhesions are disassembled and bundles of actin microfilaments, called stress fibers
, contract and pull the trailing edge forward to keep up with the rest of the cell. Without this front-rear polarity, cells would be unable to coordinate directed migration.
and embryonic development
The bodies of vertebrate animals are asymmetric along three axes: anterior-posterior (head to tail), dorsal-ventral (spine to stomach), and left-right (for example, our heart is on the left side of our body). These polarities arise within the developing embryo through a combination of several processes: 1) asymmetric cell division
, in which two daughter cells receive different amounts of cellular material (e.g. mRNA, proteins), 2) asymmetric localization of specific proteins or RNAs within cells (which is often mediated by the cytoskeleton), 3) concentration gradients of secreted proteins across the embryo such as Wnt
, Nodal
, and Bone Morphogenic Proteins (BMPs), and 4) differential expression of membrane receptors and ligands that cause lateral inhibition, in which the receptor-expressing cell adopts one fate and its neighbors another.
In addition to defining asymmetric axes in the adult organism, cell polarity also regulates both individual and collective cell movements during embryonic development such as apical constriction
, invagination
, and epiboly
. These movements are critical for shaping the embryo and creating the complex structures of the adult body.
. Many of the molecules responsible for regulating cell polarity are conserved across cell types and throughout metazoan species. Examples include the PAR complex (Cdc42
, PAR3, PAR6, atypical protein kinase C
), Crumbs complex (Crb, PALS, PATJ, Lin7), and Scribble complex (Scrib, Dlg, Lgl). These polarity complexes are localized at the cytoplasmic side of the cell membrane, asymmetrically within cells. For example, in epithelial cells the PAR and Crumbs complexes are localized along the apical membrane and the Scribble complex along the lateral membrane. Together with a group of signaling molecules called Rho GTPases
, these polarity complexes can regulate vesicle transport and also control the localization of cytoplasmic proteins primarily by regulating the phosphorylation of phospholipids called phosphoinositides. Phosphoinositides serve as docking sites for proteins at the cell membrane, and their state of phosphorylation determines which proteins can bind.
Epithelial cells
Epithelial cells adhere to one another through tight junctions and adherens junctions, forming sheets of cells that line the surface of the animal body and internal cavities (e.g., digestive tract and circulatory system). These cells have an apical-basal polarity defined by the apical membrane facing the outside surface of the body, or the lumenLumen
Lumen can mean:* Lumen , the SI unit of luminous flux* Lumen , the cavity or channel within a tubular structure* Thylakoid lumen, the inner membrane space of the chloroplast* Phenobarbital...
of internal cavities, and the basolateral membrane oriented away from the lumen. The basolateral membrane refers to both the lateral membrane where cell-cell junctions connect neighboring cells and to the basal membrane where cells are attached to the basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
, a thin sheet of extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
proteins that separates the epithelial sheet from underlying cells and connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
. Epithelial cells also exhibit planar cell polarity,in which specialized structures are orientated within the plane of the epithelial sheet. Some examples of planar cell polarity include the scales of fish being oriented in the same direction and similarly the feathers of birds, the fur of mammals, and the cuticular projections (sensory hairs, etc.) on the bodies and appendages of flies and other insects.
Neurons
A neuron receives signals from neighboring cells through branched, cellular extensions called dendrites. The neuron then propagates an electrical signal down a specialized axonAxon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
extension to the synapse, where neurotransmitters are released to propagate the signal to another neuron or effector cell (e.g., muscle or gland). The polarity of the neuron thus facilitates the directional flow of information, which is required for communication between neurons and between neurons and effector cells.
Migratory cells
Many cell types are capable of migration, such as leukocytes and fibroblasts, and in order for these cells to move in one direction, they must have a defined front and rear. At the front of the cell is the leading edge, which is often defined by a flat ruffling of the cell membrane called the lamellipodium or thin protrusions called filopodiaFilopodia
Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...
. Here, actin
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...
polymerization in the direction of migration allows cells to extend the leading edge of the cell and to attach to the surface. At the rear of the cell, adhesions are disassembled and bundles of actin microfilaments, called stress fibers
Stress fiber
Stress fibers are high order structures in cells consisting of actin filaments , crosslinking proteins , and myosin II motors.Actin is a globular protein...
, contract and pull the trailing edge forward to keep up with the rest of the cell. Without this front-rear polarity, cells would be unable to coordinate directed migration.
Vertebrate development
See also embryogenesisEmbryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
and embryonic development
The bodies of vertebrate animals are asymmetric along three axes: anterior-posterior (head to tail), dorsal-ventral (spine to stomach), and left-right (for example, our heart is on the left side of our body). These polarities arise within the developing embryo through a combination of several processes: 1) asymmetric cell division
Asymmetric cell division
An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to normal, symmetric, cell divisions, which give rise to daughter cells of equivalent fates...
, in which two daughter cells receive different amounts of cellular material (e.g. mRNA, proteins), 2) asymmetric localization of specific proteins or RNAs within cells (which is often mediated by the cytoskeleton), 3) concentration gradients of secreted proteins across the embryo such as Wnt
WNT
WNT is a three-letter abbreviation with multiple meanings, as described below:*Windows NT*WNT *Wnt signaling pathway, a complex protein network...
, Nodal
Nodal
Nodal can refer to* Nodal, a protein encoded by the gene NODAL and responsible for left-right asymmetry* Nodal , a novel music composition program...
, and Bone Morphogenic Proteins (BMPs), and 4) differential expression of membrane receptors and ligands that cause lateral inhibition, in which the receptor-expressing cell adopts one fate and its neighbors another.
In addition to defining asymmetric axes in the adult organism, cell polarity also regulates both individual and collective cell movements during embryonic development such as apical constriction
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...
, invagination
Invagination
Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.* Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...
, and epiboly
Epiboly
Epiboly is a cell movement that occurs in the early embryo, at the same time as gastrulation. It is one of many movements in the early embryo that allow for dramatic physical restructuring . The movement is generally characterized as being a thinning and spreading of cell layers...
. These movements are critical for shaping the embryo and creating the complex structures of the adult body.
Molecular basis
Cell polarity arises primarily through the localization of specific proteins to specific areas of the cell membrane. This localization requires both the recruitment of cytoplasmic proteins to the cell membrane and polarized vesicle transport along cytoskeletal filaments to deliver transmembrane proteins from the golgi apparatusGolgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
. Many of the molecules responsible for regulating cell polarity are conserved across cell types and throughout metazoan species. Examples include the PAR complex (Cdc42
CDC42
Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :...
, PAR3, PAR6, atypical protein kinase C
Protein kinase C
Protein kinase C also known as PKC is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins. PKC enzymes in turn are activated by signals such as increases in...
), Crumbs complex (Crb, PALS, PATJ, Lin7), and Scribble complex (Scrib, Dlg, Lgl). These polarity complexes are localized at the cytoplasmic side of the cell membrane, asymmetrically within cells. For example, in epithelial cells the PAR and Crumbs complexes are localized along the apical membrane and the Scribble complex along the lateral membrane. Together with a group of signaling molecules called Rho GTPases
Rho family of GTPases
The Rho family of GTPases is a family of small signaling G protein , and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found in all eukaryotic organisms including yeasts and some plants...
, these polarity complexes can regulate vesicle transport and also control the localization of cytoplasmic proteins primarily by regulating the phosphorylation of phospholipids called phosphoinositides. Phosphoinositides serve as docking sites for proteins at the cell membrane, and their state of phosphorylation determines which proteins can bind.
See also
- Epithelial polarityEpithelial polarityCell polarity is a fundamental feature of many types of cells. Epithelial cells are one example of a polarized cell type, featuring distinct 'apical', 'lateral' and 'basal' plasma membrane domains. Epithelial cells connect to one another via their lateral membranes to form epithelial sheets that...
- Cell migrationCell migrationCell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations...
- EmbryogenesisEmbryogenesisEmbryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
- Embryonic development
- Asymmetric cell divisionAsymmetric cell divisionAn asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to normal, symmetric, cell divisions, which give rise to daughter cells of equivalent fates...