Asymmetric cell division
An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to normal, symmetric, cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

s, which give rise to daughter cells of equivalent fates. Notably, stem cell
Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...

s divide asymmetrically to give rise to two distinct daughter cells: one copy of original stem cell as well as a second daughter programmed to differentiate into a nment, it must rely on intrinsic
Intrinsic and extrinsic properties
An intrinsic property is an essential or inherent property of a system or of a material itself or within. It is independent of how much of the material is present and is independent of the form the material, e.g., one large piece or a collection of smaller pieces...

 asymmetry. The term asymmetric cell division usually refers to such intrinsic asymmetric divisions

Intrinsic asymmetry

Intrinsic asymmetric divisions but they follow mechanism. At mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

 certain protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s, RNA transcripts, and other macromolecules are localized asymmetrically to one half of the cell. A cell can accomplish this through a variety of processes such as localized molecule tethering as well as molecule transport (see below). Following this, the cell performs cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...

 and divides in two. Thus, the asymmetrically localized proteins, RNA transcripts, and other macromolecules are inherited differentially to only one of the daughter cells, causing that cell to assume a separate fate from its sibling. Because these molecules ultimately determine the identity of the daughter cell they are called cell fate determinants.

This mechanism raises two requirements: first, the mother cell must be polarized
Cell polarity
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...

; second, the mitotic spindle
Mitotic spindle
In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...

 must be aligned with the axis of polarity. The cell biology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...

 of these events has been most traditionally studied in three animal models: the mouse, the nematode Caenorhabditis elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

and the fruitfly Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

. Recent work in spiralia
Spiralia is a grouping of animals."Lophotrochozoan" and "spiralian" are sometimes considered equivalent.It receives its name from the spiral cleavage found in most members....

n development has also discovered insightful mechanisms of asymmetric cell division

Asymmetric cell division in C. elegans

In C. elegans, a series of asymmetric cell divisions in the early embryo are critical in setting up the anterior/posterior, dorsal ventral, and left/right axes of the body plan. After fertilization, events are already occurring in the one cell stage embryo to allow for the first asymmetric cell division. This first division produces two distinctly different blastomeres, termed AB and P1. When the sperm fertilizes the egg, the sperm nucleus and centrosomes are deposited within the egg, which causes a cytoplasmic flux resulting in the movement of the sperm pronucleus and centrosomes towards one pole. The centrosomes deposited by the sperm seem to be responsible for the establishment of the posterior pole within the one cell embryo. Studies have shown that the pole in which the sperm-derived centrosomes reside always becomes the posterior pole. Furthermore, sperm with mutant or absent centrosomes fail to establish a posterior pole, while enucleated sperm with intact centrosomes successfully fertilize the egg and set up the posterior pole. The establishment of this polarity initiates the polarized distribution of a group of proteins present in the zygote called the PAR proteins (partitioning-defective), which are a conserved group of proteins that function in establishing cell polarity during development. These proteins are initially distributed uniformly throughout the zygote and then become polarized with the creation of the posterior pole. This series of events allows the single celled zygote to obtain polarity through an unequal distribution of multiple factors.

The single cell is now set up to undergo an asymmetric cell division, however the orientation in which the division occurs is also an important factor. The mitotic spindle must be oriented correctly to ensure that the proper cell fate determinants are distributed appropriately to the daughter cells. The alignment of the spindle is mediated by the PAR proteins, which regulate the positioning of the centrosomes along the A/P axis as well as the movement of the mitotic spindle along the A/P axis. Following this first asymmetric division, the AB daughter cell divides symmetrically, giving rise to ABa and ABp, while the P1 daughter cell undergoes another asymmetric cell division to produce P2 and EMS. This division is also dependent on the distribution of the PAR proteins.

Asymmetric cell division of Drosophila neuroblasts

In Drosophila melanogaster, asymmetric cell division plays an important role in neural development. Neuroblasts are the progenitor cells which divide asymmetrically to give rise to another neuroblast and a ganglion mother cell (GMC). The neuroblast repeatedly undergoes this asymmetric cell division while the GMC continues on to produce a pair of neurons. Two proteins play an important role in setting up this asymmetry in the neuroblast, Prospero and Numb. These proteins are both synthesized in the neuroblast and segregate into only the GMC during divisions. Numb is a suppressor of Notch, therefore the asymmetric segregation of Numb to the basal cortex biases the response of the daughter cells to Notch signaling, resulting in two distinct cell fates. Prospero is required for gene regulation in GMCs. It is equally distributed throughout the neuroblast cytoplasm, but becomes localized at the basal cortex when the neuroblast starts to undergo mitosis. Once the GMC buds off from the basal cortex, Prospero becomes translocated into the GMC nucleus to act as a transcription factor.

Other proteins present in the neuroblast mediate the asymmetric localization of Numb and Prospero. Miranda is an anchoring protein that binds to Prospero and keeps it in the basal cortex. Following the generation of the GMC, Miranda releases Prospero and then becomes degraded. The segregation of Numb is mediated by Pon (the partner of Numb protein). Pon binds to Numb and colocalizes with it during neuroblast cell division.

The mitotic spindle must also align parallel to the asymmetrically distributed cell fate determinants to allow them to become segregated into one daughter cell and not the other. The mitotic spindle orientation is mediated by Inscuteable, which is segregated to the apical cortex of the neuroblast. Without the presence of Inscuteable, the positioning of the mitotic spindle and the cell fate determinants in relationship to each other becomes randomized. Inscuteable mutants display a uniform distribution of Miranda and Numb at the cortex, and the resulting daughter cells display identical neuronal fates.

Asymmetric cell division in Spiralian development

Spiralia is a grouping of animals."Lophotrochozoan" and "spiralian" are sometimes considered equivalent.It receives its name from the spiral cleavage found in most members....

(commonly synonymous with lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...

) represent a diverse clade of metazoan organisms whose species comprise the bulk of the bilaterian animals present today. Examples include mollusks, annelid worms, and the entoprocta. Although much is known at the cellular and molecular level about the other bilateralian clades (ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...

 and deuterostomia), research into the processes that govern spiralian development is comparatively lacking. However, one unifying feature shared among spiralia is the pattern of cleavage in the early embryo known as spiral cleavage
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...

. This pattern, which contributed to the initial phylogenetic placement of this group of organisms, depends heavily on asymmetric cell division.

The general cleavage pattern of a spiralian embryo follows a classic and predictable set of cellular divisions. The first two divisions yield four progenitor macromeres which specify four geometric quadrants of the developing organism. Following this, the individual macromeres undergo a series of divisions which generate a string of micromeres at the animal pole of the embryo. Micromere production occurs in an alternating clockwise and counterclockwise fashion and is accomplished via asymmetric division. For more information on this pattern of cellular division see the cleavage (embryo) page.

Although the conserved cleavage of spiralian macromeres to generate micromere quartets is well established, more has been discovered about the first two cleavages at the molecular level. The initial cleavage can occur with several, sometimes overlapping, outcomes (See Figure, left panel). For example, in symmetrically cleaving embryos, the zygote can cleave twice to yield four cells of equal size and equipotent fate. In contrast, asymmetrically cleaving spiralians develop an embryonic polarity beginning with the first cellular division. The zygote cleaves once to generate a smaller AB cell and a larger CD cell. The second division usually occurs asynchronously, and generates three similarly sized macromeres (A, B, and C) as well as a larger D macromere. Known as D quadrant specification, this example of asymmetric cell division sets up the D quadrant of the embryo such that it has a specific and independent fate, often for mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...


Mechanisms of asymmetric division (See Figure, right panel):
  • Tubifex tubifex: The sludge worm Tubifex tubifex
    Tubifex tubifex
    Tubifex tubifex, also called the sludge worm, or sewage worm, is a species of tubificid segmented worm that inhabits the sediments of lakes and rivers on several continents...

    has been shown to demonstrate an interesting asymmetric cell division at the point of first embryonic cleavage. Unlike the classic idea of cortical differences at the zygotic membrane that determine spindle asymmetry in the C. elegans embryo, the first cleavage in tubifex relies on the number of centrosome
    In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...

    s. Embryos inherit a single centrosome which localizes in the prospective larger CD cell cytoplasm and emits radial microtubules during anaphase that contribute to both the mitotic spindle as well as cortical asters. However, the microtubule organizing center of the prospective smaller AB cell emits only microtubules that commit to the mitotic spindle and not cortical bound asters. When embryos are compressed or deformed, asymmetric spindles still form, and staining for gamma tubulin reveals that the second microtubule organizing center lacks the molecular signature of a centrosome. Furthermore, when centrosome number is doubled, tubifex embryos cleave symmetrically, suggesting this monoastral mechanism of asymmetric cell division is centrosome dependent.

  • Helobdella robusta: The leech Helobdella robusta
    Helobdella robusta
    Helobdella robusta is a leech of the family glossiphoniidae. Its genome has been sequences by the Joint Genome Institute, and its early development has been studied extensively.- References :...

    exhibits a similar asymmetry in the first embryonic division as C. elegans and tubifex, but relies on a modified mechanism. Compression experiments on the robusta embryo do not affect asymmetric division, suggesting the mechanism, like tubifex, uses a cortical independent molecular pathway. In robusta, antibody staining reveals that the mitotic spindle forms symmetrically until metaphase and stems from two biastral centrosomes. At the onset of metaphase, asymmetry becomes apparent as the centrosome of the prospective larger CD cell lengthens cortical asters while the asters of the prospective smaller AB cell become downregulated. Experiments using nocodazole
    Nocodazole is an anti-neoplastic agent which exerts its effect in cells by interfering with the polymerization of microtubules. Microtubules are one type of fibre which constitutes the cytoskeleton, and the dynamic microtubule network has several important roles in the cell, including vesicular...

     and taxol support this observation. Taxol, which stabilized microtubules, forced a significant number of embryos to cleave symmetrically when used at a moderate concentration. Moreover, embryos treated with nocodazole, which sequesters tubulin dimers and promotes microtubule depolymerization, similarly forced symmetric division in a significant number of embryos. Treatment with either drug at these concentrations fails to disrupt normal centrosome dynamics, suggesting that a balance of microtubule polymerization and depolymerization represents another mechanism for establishing asymmetric cell division in spilarian development.

  • Ilyanasa obsoleta: A third, less traditional mechanism contributing to asymmetric cell division in spiralian development has been discovered in the mollusk Ilyanasa obsoleta. In situ hybridization
    In situ hybridization
    In situ hybridization is a type of hybridization that uses a labeled complementary DNA or RNA strand to localize a specific DNA or RNA sequence in a portion or section of tissue , or, if the tissue is small enough , in the entire tissue...

     and immunofluorescence
    Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on biological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to specific biomolecule targets within a cell, and therefore allows...

    experiments show that mRNA transcripts colocolize with centrosomes during early cleavage. Consequently, these transcripts are inherited in a stereotypical fashion to distinct cells. All mRNA transcripts followed have been implicated in body axis patterning, and in situ hybridization for transcripts associated with other functions fail to exhibit such a localization. Moreover, disruption of microtubule polymerization with nocodazole, and of actin polymerization with cytochalisin B, shows the cytoskeleton is also important in this asymmetry. It appears that microtubules are required to recruit the mRNA to the centrosome, and that actin is required to attach the centrosome to the cortex. Finally, introducing multiple centrosomes into one cell by inhibiting cytokenesis shows that mRNA dependably localizes on the correct centrosome, suggesting intrinsic differences between each centrosomal composition. It is important to note that these results reflect experiments performed after the first two divisions, yet still demonstrate a different molecular means of establishing asymmetry in a dividing cell.

These examples of mechanisms that establish asymmetric cell division in early spiralian development showcase the plasticity of evolution. A variety of methods can contribute to differences in daughter cells in order to found independent cell fates.

The role of asymmetric divisions in development

Animals are made up of a vast number of distinct cell type
Cell type
A cell type is a distinct morphological or functional form of cell. When a cell switches state from one cell type to another, it undergoes cellular differentiation. A list of distinct cell types in the adult human body may include several hundred distinct types.-References:...

s. During development these are generated from a single cell, the zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

. Asymmetric divisions contribute to this expansion in cell type diversity by making two types of cells from one. For example, it is thought that many of the cells in the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 derive from asymmetric divisions.

Cells may divide asymmetrically to produce two novel cells at the expense of the mother cell. For example, in plants, an asymmetric division of an unspecialized epidermal
Epidermis (botany)
The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...

 cell can produce a guard cell mother cell that divides again to produce two guard cells, the cells that control the closing and opening of stomata. However, asymmetric divisions often give rise to only one novel cell type in addition to a new copy of the mother cell. Such divisions are called self-renewing. Self-renewal is a hallmark of stem cell
Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...

s, and there is growing evidence that stem cell
Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...

s self-renew through asymmetric division. In this way the production of new cell types (differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...

) is precisely balanced by renewal of the stem cell population.

Asymmetric division of somatic cells also creates a drift in cell function through the human life span contributing to aging of the organism. It is due to the asymmetric distribution of DNA between daughter cells.
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