Cellular differentiation
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[[Image:Cell differentiation.gif|400px|thumbnail|In the center of the diagram are three of the early steps in the development of a mammal. On the top and bottom are some of the fully-differentiated cell types that will eventually form in the adult.]]
In [[developmental biology]], '''cellular differentiation''' is the process by which a less specialized [[cell (biology)|cell]] becomes a more specialized [[cell type]]. Differentiation occurs numerous times during the development of a [[multicellular organism ]] as the organism changes from a single [[zygote]] to a complex system of [[Tissue (biology)|tissues]] and cell types. Differentiation is a common process in adults as well: [[adult stem cell]]s divide and create fully-differentiated [[Cell division|daughter cells]] during tissue repair and during normal cell turnover. Cell differentiation causes its size, shape, polarity, [[metabolism|metabolic activity]], and responsiveness to signals to change dramatically. These changes are largely due to highly-controlled modifications in [[gene expression]]. With a few exceptions, cellular differentiation almost never involves a change in the [[DNA]] sequence itself. Thus, different cells can have very different physical characteristics despite having the same [[genome]].
A cell that is able to differentiate into many cell types is known as '''''[[pluripotent]]'''''. These cells are called '''[[stem cell]]s''' in animals and '''[[meristem|meristematic cells]]''' in higher plants. A cell that is able to differentiate into all [[cell type]]s is known as '''''[[totipotent]]'''''. In mammals, only the zygote and early [[embryo]]nic cells are ''[[totipotent]]'', while in plants, many differentiated cells can become totipotent with simple laboratory techniques. In [[cytopathology]] the level of cellular differentiation is used as a measure of [[cancer]] progression. "[[Grading (tumors)|Grade]]" is a marker of how differentiated a cell in a tumor is.
==Mammalian cell types==
Three basic categories of cells make up the mammalian body: [[germ cell]]s, [[somatic cell]]s, and [[stem cell]]s. Each of the approximately 100,000,000,000,000 (10<sup>14</sup>) cells in an adult human has its own copy or copies of the [[genome]] except certain cell types, such as [[red blood cell]]s, that lack nuclei in their fully differentiated state. Most cells are [[diploid]]; they have two copies of each [[chromosome]]. Such cells, called somatic cells, make up most of the human body, such as skin and muscle cells.
Germ line cells are any line of cells that give rise to [[gametes]]—eggs and sperm—and thus are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development.
Development begins when a [[sperm]] fertilizes an [[egg (biology)|egg]] and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a [[blastocyst]]. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the [[inner cell mass]]. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. These cells are referred to as [[pluripotent]].
Pluripotent stem cells undergo further specialization into [[multipotent]] [[progenitor cell]]s that then give rise to functional cells. Examples of stem and progenitor cells include:
*''[[Hematopoietic stem cells]]'' (adult stem cells) from the [[bone marrow]] that give rise to [[red blood cell]]s, [[white blood cell]]s, and [[platelet]]s
*''[[Mesenchymal stem cells]]'' (adult stem cells) from the [[bone marrow]] that give rise to stromal cells, fat cells, and types of bone cells
*''[[epithelia|Epithelial]] stem cells'' (progenitor cells) that give rise to the various types of skin cells
*''Muscle [[satellite cell]]s'' (progenitor cells) that contribute to differentiated [[muscle]] tissue
==Dedifferentiation==
Dedifferentiation is a cellular process often seen in lower life forms such as [[worm]]s and [[amphibian]]s in which a partially or terminally differentiated cell reverts to an earlier developmental stage, usually as part of a [[Regeneration (biology)|regenerative]] process.<ref name="dediff1">Stocum DL; [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=14594207&cmd=showdetailview&indexed=google Amphibian regeneration and stem cells]; ''Curr Top Microbiol Immunol.'' 2004;280:1-70. PMID: 14594207</ref> <ref name="dediff2">CM Casimir, PB Gates, RK Patient and JP Brockes; [http://dev.biologists.org/cgi/content/abstract/104/4/657 Evidence for dedifferentiation and metaplasia in amphibian limb regeneration from inheritance of DNA methylation]; Development, Vol 104, Issue 4 657-668 </ref> Dedifferentiation also occurs in plants<ref>[http://www.rsnz.org/publish/nzjb/1971/47.php Dedifferentiation and Regeneration in Bryophytes: A Selective Review], K.L. Giles, New Zealand Journal of Botany 9: 689-94</ref>. Cells in [[cell culture]] can lose properties they originally had, such as protein expression, or change shape. This process is also termed dedifferentiation<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11795984&dopt=Citation Dedifferentiation-associated changes in morphology and gene expression in primary human articular chondrocytes in cell culture], M. Schnabel et al, Osteoarthritis and Cartilage, Volume 10, Issue 1 , January 2002, Pages 62-70.</ref>.
Some believe dedifferentiation is an aberration of the normal development cycle that results in [[cancer]],<ref>Stewart Sell; [http://www.jstor.org/view/00916765/ap060112/06a00040/0 Cellular Origin of Cancer - Dedifferentiation or Stem Cell Maturation Arrest?]; Environmental Health Perspectives, 1993</ref> whereas others believe it to be a natural part of the immune response lost by humans at some point as a result of evolution.
A small molecule dubbed [[reversine]], a [[purine]] analog, has been discovered that has proven to induce dedifferentiation in myotubes. These dedifferentiated cells were then able to redifferentiate into osteoblasts and adipocytes.<ref>Panagiotis A. Tsonis; [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15087480 Stem Cells from Differentiated Cells]; ''Molecular Interventions'' 4:81-83, (2004)</ref>
==Mechanisms==
Each specialized [[cell type]] in an organism [[Gene expression|expresses]] a [[subset]] of all the [[gene]]s that constitute the [[genome]] of that [[species]]. Each cell type is defined by its particular pattern of [[regulation of gene expression|regulated gene expression]]. Cell differentiation is thus a transition of a cell from one cell type to another and it involves a switch from one pattern of gene expression to another. Cellular differentiation during development can be understood as the result of a [[gene regulatory network]]. A regulatory gene and its cis-regulatory modules are nodes in a gene regulatory network; they receive input and create output elsewhere in the network <ref name=DeLeon>DeLeon SBT, EH Davidson; Gene regulation: Gene control network in development. '' Annual Review of Biophysics and Biomolecular Structure '' 36:191-212, 2007</ref>. The [[systems biology]] approach to developmental biology emphasizes the importance of investigating how developmental mechanisms interact to produce predictable patterns ([[morphogenesis]]).
A few [[evolution]]arily conserved types of molecular processes are often involved in the cellular mechanisms that control these switches. The major types of molecular processes that control cellular differentiation involve [[cell signaling]]. Many of the signal molecules that convey information from cell to cell during the control of cellular differentiation are called [[growth factor]]s. Although the details of specific signal transduction pathways vary, these pathways often share the following general steps. A ligand produced by one cell binds to a receptor in the extracellular region of another cell, inducing a conformational change in the receptor. The shape of the cytoplasmic domain of the receptor changes, and the receptor acquires enzymatic activity. The receptor then catalyzes reactions that phosphorylate other proteins, activating them. A cascade of phosphorylation reactions eventually activates a dormant transcription factor or cytoskeletal protein, thus contributing to the differentiation process in the target cell <ref name=Gilbert>Gilbert; ''Developmental Biology, eighth edition''. Sinaur Associates, Inc., p. 147, 2006</ref>. Cells and tissues can vary in competence, their ability to respond to external signals <ref name=Rudel>Rudel and Sommer; The evolution of developmental mechanisms. ''Developmental Biology'' 264, 15-37, 2003</ref>.
[[Induction]] refers to cascades of signaling events, during which a cell or tissue signals to another cell or tissue to influence its developmental fate <ref name=Rudel/>. Yamamoto and Jeffery<ref name=Yamamoto> Yamamoto Y and WR Jeffery; Central role for the lens in cave fish eye degeneration. '' Science '' 289 (5479), 631-633, 2000</ref> investigated the role of the lens in eye formation in cave- and surface-dwelling fish, a striking example of induction<ref name=Rudel/>. Through reciprocal transplants, Yamamoto and Jeffery<ref name=Yamamoto/> found that the lens vesicle of surface fish can induce other parts of the eye to develop in cave- and surface-dwelling fish, while the lens vesicle of the cave-dwelling fish cannot<ref name=Rudel/>.
Other important mechanisms fall under the category of [[asymmetric cell division]]s, divisions which give rise to daughter cells with distinct developmental fates. Asymmetric cell divisions can occur because of segregation of cytoplasmic determinants or because of signaling <ref name=Rudel/>. In the former mechanism, distinct daughter cells are created during [[cytokinesis]] because of an uneven distribution of regulatory molecules in the parent cell; the distinct cytoplasm that each daughter cell inherits results in a distinct pattern of differentiation for each daughter cell. A well-studied example of pattern formation by asymmetric divisions is [[Drosophila embryogenesis#Anterior-posterior axis patterning in Drosophila|body axis patterning in Drosophila]]. [[RNA]] molecules are an important type of intracellular differentiation control signal. The molecular and genetic basis of asymmetric cell divisions has also been studied in green algae of the genus ''[[Volvox]]'', a model system for studying how unicellular organisms can evolve into multicellular organisms <ref name=Rudel/>. In ''Volvox carteri'', the 16 cells in the anterior hemisphere of a 32-celled embryo divide asymmetrically, each producing one large and one small daughter cell. The size of the cell at the end of all cell divisions determines whether it will become a specialized germ or somatic cell <ref name=Rudel/> <ref name=Kirk>Kirk MM, A Ransick, SE Mcrae, DL Kirk; The relationship between cell size and cell fate in ''Volvox carteri''. ''Journal of Cell Biology'' 123, 191-208, 1993</ref>.
==See also==
*[[Morphogenesis]]
*[[Multipotent]]
*[[Germ layer]]
*[[Cell fate determination]]
{{cell potency}}
==References==
{{reflist}}
* [http://www.ncbi.nih.gov/About/primer/genetics_cell.html What is a cell?] 2004. A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI.
{{Stem cells}}
[[Category:Cellular processes]]
[[Category:Developmental biology]]
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