Transdifferentiation means conversion of one differentiated cell type into another. It is a subset of a wider class of cell type transformations called metaplasias.

This website explains about how one cell type of a human or animal can be converted into another. It deals with naturally occurring transformations, pathological transformations and transformations induced by molecular genetic methods. It explains why transdifferentiation has been the source of fierce controversy and the practical significance of its occurrence or non-occurrence.

It is written by Jonathan Slack (University of Minnesota and University of Bath) and David Tosh, (University of Bath). Webpage design by James Corbett.

Cells, tissues and organs

Cells

Cells are the ultimate structural unit of an animal or plant body. Each has a nucleus containing the genetic material (DNA), and a cytoplasm containing a complex mixture of proteins and other sorts of molecule that perform particular biochemical or mechanical tasks. There are about 210 kinds of cell in a human body (Alberts et al., 1994). Most of them are what we call differentiated cells, each type of which has a specific function and a particular appearance when viewed down the microscope. For example cells of the liver (hepatocytes), or of the heart muscle (cardiomyocytes), or of the brain (neurons), are well known types of differentiated cell. The differentiated type that a cell belongs to depends on which particular genes are active in its nucleus. Each gene encodes one specific protein and the repertoire of genes that are active, and thus of proteins that are produced, defines the type of the cell. The complete set of genes present in the cell nucleus is called the genome, and to a first approximation the genome is the same for every cell in the body. An undifferentiated cell is one that does not have any obvious specialisation of gene expression and has a bland generic appearance down the microscope. But just because you cannot see specialisation this does not mean that it does not exist. Most undifferentiated cells are specialised in some way, especially in terms of restrictions into what other types of cell they can become. Undifferentiated cells are found in the embryo, where they develop into various types of differentiated cells in the course of time. They are also found in some cancers, where lack of differentiation often indicates a poor grade of tumour, liable to respond poorly to treatment. Undifferentiated cells are sometimes, but by no means always, stem cells.

What is a cell type?

Differentiated cell types are, by definition, discrete and persistent. The “state” of a cell is defined as the steady state concentration of all the substances in the cell, comprising both macromolecules and small metabolites. This is essentially equivalent to the pattern of gene expression: the list of which genes are active, and to what extent they are active. It can be experimentally determined by RNA analysis: for example employing the RNA seq technique, which effectively counts the number of each type of mRNA molecule in a sample. The steady state level of one specific mRNA will reflect both the rate of synthesis by transcription and the rate of degradation. A cell state may not be persistent, it can be very short lived, and evolve spontaneously into a different cell state. This happens all the time during embryonic development, and during postnatal development of differentiated cells from stem cells.

There are two traditions in theoretical biology that try to capture the nature and evolution of cell states (
Karlebach and Shamir, 2008). One uses continuous mathematics and represents the cell by a set of differential  equations, one for each substance, describing how its concentration changes in time. The other uses Boolean logic, representing each gene as a binary variable: 1 for “on” and 0 for “off”. Both types of formalism can be used to build complex models which capture certain aspects of how cells behave. But both have their limitations. Large arrays of nonlinear differential equations are mathematically intractable and require huge amounts of computing power to make simulations. Boolean logic cannot easily deal with those aspects of cellular biochemistry that really are continuous rather than discrete.

The classification of human or animal cell types is based on the appearance of cells under the light microscope, following staining with appropriate dyes. In reality there are more than the quoted figure of 210 cell types because some cells that are known to differ in their properties, especially different sorts of lymphocyte or neuron,  may nonetheless look the same. Visibly distinguishable cell types normally contain relatively large quantities of a few proteins associated with their specific function, for example the contractile proteins found in muscle fibres, or the neurofilament proteins found in nerve axons. Such cells are called “differentiated”, contrasting with undifferentiated cells found in the early embryo or as stem cells in adult tissues. Differentiated cell types usually do not divide, so they are called “postmitotic”. The differentiated state is considered to be stable, in contrast to the determined state of progenitor cells, which is transient. Some types of differentiated cell persist for the life of the animal, for example many neurons or muscle fibres. Others have a short lifetime, and are replaced by stem cells, for example the keratinocytes of the epidermis.

Tissues

The structure of tissues is the subject of the science of histology. A tissue is an assemblage of cells, normally including more than one type of differentiated cell. From a developmental biology standpoint, a tissue is the set of cell types arising from one sort of progenitor or stem cell. For example, the intestinal epithelium is a tissue. It contains four cell types: absorptive, goblet, Paneth and enteroendocrine cells together with undifferentiated cells within the crypts of Lieberkuhn. The stem cells are located at the crypt base and produce progenitor cells, called transit amplifying cells, that divide a few times before differentiating into one of the four mature cell types (
van der Flier and Clevers, 2009). The tissue comprises the stem cells, the transit amplifying cells, and the differentiated cells. This is an example of a renewal tissue, in which there is continuous production and replacement of differentiated cells. Not all tissues are renewal tissues although most display a small degree of renewal or the ability to replace the constituent cell types following damage. For example the central nervous system arises from the neuroepithelium of the early embryo and contains a variety of types of neuron together with glial cells, especially astrocytes and oligodendrocytes. Because it arises from a clearly identified progenitor cell population, the central nervous system is one tissue. Most of the differentiated cells of the central nervous system are postmitotic and persist for the lifetime of the animal. However, there is some multiplication of astrocytes, especially following damage. There is some continued production of oligodendrocytes throughout the CNS from progenitor cells (oligodendrocyte precursor, or NG2 cells). There are two regions of the mammalian CNS where populations of neural stem cells persist, the subventricular zone, and the dentate gyrus of the hippocampus (Zhao et al., 2008). Neural stem cells produce neurons, astrocytes and oligodendrocytes continuously throughout life. In some cases the precise status of tissues within the animal remains unclear. For instance there are cells called “mesenchymal stem cells” that can be isolated from bone marrow, adipose tissue or other connective tissues (Nombela-Arrieta et al., 2011). In vitro these cells produce smooth muscle, adipose cells and osteocytes, although it is unclear whether they actually serve as progenitors for all these cell types in vivo.

In cases where differentiated cells are derived from different stem or progenitor cell populations, they cannot be considered as a single tissue. For example, the thyroid gland contains many follicles of thyroglobulin-producing epithelial cells. It also contains endocrine cells producing the hormone calcitonin. However, the latter do not arise from the same endodermal epithelial bud as the rest of the thyroid. Instead they arise from the neural crest of the embryo. Likewise, many tissues contain macrophages (histiocytes) derived from the haematopoietic stem cells of the bone marrow. These cells are properly considered to belong to the blood/immune tissue rather than the epithelia or connective tissues in which they reside.

Organs

Organs are the familiar structures in the body which are each associated with a particular function. They are typically composed of several tissue layers and arise from multiple developmental origins. For example, the stomach has the function of preliminary food digestion. The lining of the stomach is one tissue, the gastric epithelium, derived from gastric stem cells. The outer layers consist of smooth muscle and connective tissues, blood vessels, nerve fibres, and cells of the blood and immune system. Together these make up a discrete and integrated body part. A single named muscle contains many muscle fibres, which are postmitotic multinucleate cells. It also contains muscle satellite cells, which are stem cells that can regenerate fibres. The fibres and the satellite cells are one tissue, arising from the myotome of the somites of the embryo. Surrounding the bundles of fibres are connective tissue sheaths composed of fibroblasts and extracellular matrix derived from lateral plate mesoderm. Again, there are always blood vessels, nerve fibres and cells of the blood and immune system. A muscle is one organ, composed of several tissues, each consisting of more than one cell type.

Transdifferentiation in normal development 

Normal embryonic development follows a hierarchical process. Starting from the blastula (or blastoderm or epiblast depending on the species), any particular tissue rudiment or cell type is formed by a sequence of developmental decisions. This process is now understood in great detail (
Slack, 2012). As an example, the progenitors of the beta cells of the pancreas undergo a series of steps of developmental commitment to become definitive endoderm, foregut endoderm, dorsal or ventral pancreatic bud, endocrine precursor cell, and finally beta cell (Zaret and Grompe, 2008) (Fig.1). At each step the production of a particular combination of transcription factors is activated or repressed in response to a particular extracellular signal, which may be composed of one or more substances (called “inducing factors” in an embryonic context). Each step leads to multiple pathways, a developmental “choice”.

Developmental Hierarchy
Fig.1. The developmental hierarchy. This shows the normally accepted mode of formation of pancreatic beta cells, involving six developmental steps, each controlled by one or more inducing factors. The inducing factors are shown in color. The final step distinguishes the insulin-producing beta cells from other types of endocrine cell also present in pancreatic islets (α, δ, ɛ, PP). Reproduced from Fig.1 in (Slack, 2008) .

Different concentrations of the inducing factor will result in the activation or repression of genes encoding different transcription factors, and therefore the adoption of different developmental pathways. Repeated occurrence of this process enables an early embryo, consisting initially of a simple mass of similar cells, to develop autonomously into an organism with a very complex pattern of structures.

In the hierarchical process of development, cell populations in a particular state of commitment often change into populations with a new state of commitment, for example the change from foregut endoderm to pancreatic bud. This is not transdifferentiation because the cell states in question are not discrete and persistent, they are transient states of commitment that will spontaneously progess along a default pathway even in the absence of any inducing factors. However there are a few examples where visible cell type does change. One is the development of the epithelial lining of the oesophagus. This is formed as a columnar epithelium, like the rest of the gut tube, and later becomes transformed to a squamous epithelium (Fig.2) (Yu, 2005)

Image of tissue differentiating from columinar to squamous epitheliumFig.2. Transdifferentiation from columnar to squamous epithelium in the normal development of the mouse oesophagus. A shows an embryonic (E15) oesophagus, stained with antibody to cytokeratin 8, B shows an adult oesophagus stained for cytokeratin 14. Reproduced from Fig.2 in (Yu, 2005).

Another is the occasional occurrence of pancreatic endocrine cells, including beta cells, in the ducts of the extrahepatic biliary system (Fig.3) (Dutton, 2007).

Image of ectopic beta cells

Fig.3. Ectopic beta cells, immunostained for insulin (green), located near an extrahepatic bile duct in a normal mouse.

Transdifferentiation in Drosophila 

Transdetermination is the name given to the transformations between different imaginal discs in Drosophila. It is associated with regeneration but because the discs are not yet differentiated, it also has aspects of an embryonic heterotopia.

 Like other insects with a complete metamorphosis, the Drosophila embryo hatches as a larva.  This crawls around, feeds and undergoes two moults.  After about a week it becomes a pupa.  Within the pupal case, the body of the larva undergoes a drastic metamophosis in which most of the larval tissues autolyse and the adult tissues and structures arise from the imaginal discs and other reserve nests of cells (Fig.4) (Cohen, 1993). The discs are established during embryonic life and proliferate during larval life.  But they do not differentiate visibly until metamorphosis. There are different discs to form the head; the legs, wings and halteres of the thorax; and the genitalia.
 

Imaginal discs in Drosophila

Fig.4. Imaginal discs in the larva of Drosophila form adult structures at metamorphosis.


Imaginal discs can survive in the undifferentiated state if they are transplanted into the abdomen of an adult fly. Here they are nourished by the haemolymph of the host and can grow indefinitely without differentiating. The fragmentation of discs necessary for repeated transplantation means that they are in a state of continuous regeneration.  If they are returned to a late larva in which the hormonal conditions signify the start of metamorphosis, then the implants differentiate in concert with the metamorphosis of the host.  In most cases the structures formed are appropriate to the original character of the disc, but sometimes they are appropriate to a different disc (Maves and Schubiger, 2003).  For example, tissue from a leg disc may produce wing structures. In a classic but rather intricate experiment involving combined clonal labelling and transdetermination, it was shown that transdetermination could sometimes occur in groups of cells as well as in single cells (Gehring, 1967).  This indicated that somatic mutation could not be the mechanism as the same mutation would not be expected to occur in a group of cells simultaneously.

Although transdetermination of Drosophila imaginal discs was traditionally considered a rare event, it can be caused to occur at high frequency by cutting the discs at the right places, the so-called “weak points”, and one such place is the dorsal part of the prothoracic leg disc, where a cut will provoke transdetermination to wing with 95% frequency. Such high frequency enables detailed investigation to take place. It turns out that wounding activates the JNK signalling pathway and this represses expression of Polycomb group genes. Polycomb proteins normally repress expression of the wingless gene, hence Wingless protein starts to be produced and its signalling activity upregulates expression of the vestigial gene, which encodes a transcription factor (Maves and Schubiger, 1995; Lee et al., 2005).

This example indicates the central role of a transcription factor, Vestigial.  This is normally expressed in the dorsal discs (wing, haltere) but not ventral ones (legs), and serves to distinguish wing from leg character.  It also indicates a role for intercellular signalling (Wingless is the Drosophila homologue of the Wnt factors), which explains why transdetermination may occur in many cells simultaneously.  It also introduces a role for chromatin organisation.  The Polycomb group proteins tend to shut down gene activity in regions of the chromosome to which they bind, by a variety of mechanisms including the promotion of inhibitory histone methylases and the inhibition of chromatin remodelling proteins.

Transdifferentiation in regeneration

Comparable phenomena to transdetermination are found in other groups of arthropod.  For example, the so called serial heteromorphosis is found in the regeneration of appendages in both crustacea and insects. It has long been known that if a claw is amputated from the shrimp Palinurus, it may regenerate not as a claw but as an antenna (Fig.5). In several types of insect, an amputated antenna may be replaced by parts of a leg (Villee, 1942). Serial heteromorphosis has remained uninvestigated for a long time, but has all the hallmarks of a typical metaplasia.

An individual of Palinurus with one eye replaced by an antenna.Fig.5. An individual of Palinurus with one eye replaced by an antenna.

Wolffian lens regeneration

The only example of transdifferentiation that is conceded to exist by sceptics is the regeneration of the lens found in various species of urodele amphibia (newts and salamanders) (Okada, 1991; Tsonis et al., 2004).  Ironically this is a case where careful cell lineage studies have not been done, perhaps because the morphological appearance of transdifferentiation is so compelling.

In Wolffian regeneration, after removal of the lens of the eye, the cells of the dorsal iris start to proliferate, undergo depigmentation, and eventually redifferentiate to form a new lens. The two cell types are very different: iris cells are pigmented epithelial cells similar to those of the pigmented retina while the lens is composed of modified keratinocytes containing high concentrations of crystallin proteins that impart the characteristic transparency. This system has been studied extensively in vitro and it was shown that it was possible to obtain development of lentoids from a clonal culture initiated by just a single pigmented iris epithelial cell of a chick embryo (Eguchi and Okada, 1973). Remarkably the in vitro culture system works for all vertebrates including human foetal iris and retina (Yasuda et al., 1978), and not just for the urodeles that will regenerate following lens ablation in vivo. Like the Drosophila discs, this system offers a connection between wounding and the initiation of a regenerative process.  However the mechanism seems to be rather different, consisting of enhanced activity of the proteolytic enzyme thrombin, a component of the blood clotting cascade.  Removal of the lens causes damage to vessels of the ciliary body, which release prothrombin into the aqueous humour. This becomes sequestered on the dorsal but not ventral iris because of the local presence of a substance called Tissue Factor, and is then processed to thrombin (Imokawa and Brockes, 2003).  The thrombin is necessary for the entry of the iris cells into S phase, and it probably works by releasing another factor present in a latent form in serum. 


Transdifferentiation in human pathology

Some metaplasias have a clinical significance because they predispose to development of cancer. In human histopathology it is not unusual to find foci of particular tissues in the wrong place. Examples are the occurrence of bone in the soft connective tissue, or squamous patches in an epithelium that is normally glandular in histology (Willis, 1962). Metaplasias virtually always arise in tissues that have been subjected to chronic trauma, infection or abnormal hormonal stimulation, hence undergoing continuous regeneration.  This association with regeneration is a point of resemblance to the arthropod examples. In some cases it is not clear from static preserved pathological specimens whether the ectopic tissue developed in situ, or migrated from elsewhere.  Obviously the latter situation is not relevant to our present concerns. But highly unlikely to arise by migration are the glandular metaplasias where patches of one tissue are found embedded within the epithelium of another. These can be found particularly in the gut and in the female reproductive system, perhaps because these two systems consist of a series of organs arranged as a tube, each organ being lined with a histologically different epithelium (Slack, 1985; Slack, 1986).  When a patch of metaplasia occurs it is often composed of the tissue type normally derived from a neighbouring region in the embryo.  For example intestinal metaplasia of the stomach means the occurrence of patches of intestinal tissue within the gastric mucosa; and intestine and stomach develop from adjacent territories of the endoderm in the early embryo (Fig.6,7).  Less obvious is the condition known as cystitis glandularis, where colonic-type tissue arises in the urinary bladder.  This is a quite separate organ to the intestine in the adult, but is derived from neighbouring endoderm in the embryo, as the urinary bladder forms from the proximal part of the allantoic evagination of the hindgut.

Intestinal metaplasiaFig.6. Intestinal metaplasia in a human stomach. The stomach is opened out flat, with the mucosa exposed. The red alkaline phosphatase stain indicates intestinal tissue. From (Matsukura et al., 1980).

Histology of intestinal metaplasia

Fig.7. Histology of intestinal metaplasia in the stomach.

Some metaplasias have a clinical significance because they predispose to development of  cancer. For example the bronchi are lined with columnar epithelium, but smokers often have patches of squamous metaplasia and it is from within these patches that lung cancer usually arises.  Adenocarcinoma of the oesophagus usually arises in areas of Barrett’s metaplasia, a condition in which the normally squamous epithelium of the lower oesophagus becomes converted to columnar type, with gastric and intestinal differentiation patterns. In such cases the metaplasia can be regarded as the first step in a multistep progression to cancer.

The transdifferentiation controversy

A number of studies around the year 2000 suggested that bone marrow stem cells are capable of colonising a wide variety of other tissue types when transplanted into irradiated hosts. Some of these were performed with unfractionated marrow, some with enriched or purified haematopoietic or mesenchymal stem cells.  The tissues colonized included virtually everything including numerous epithelia, muscle and neurons.  This work generated considerable controversy because it suggested a very different model of development from the conventional one. Instead of cell populations undergoing a series of decisions during embryonic development, in each of which their competence is restricted, the idea was that the whole body is continuously being renewed by highly pluripotent cells from the bone marrow (Blau et al., 2001).

The phenomenon became known as “transdifferentiation” although this is unfortunate as the term was previously used, and is used here, to refer to cases of direct transformation between differentiated cell types.

It now appears that some of the results were due to lodgement of cells in these tissue but without actual differentiation, and others were due to cell fusion, whereby the genetic markers from donor cells became incorporated into host cells (Wagers and Weissman, 2004). There may be some genuine reprogramming of marrow-derived cells to various other tissue types, but this certainly only occurs at very low frequency. Because the hosts are nearly always irradiated, and therefore have considerable tissue damage and widespread tissue regeneration all over the body, it is thought that this situation allows favorable circumstances for the occasional reprogramming event. It is not, however, at all likely that reprogramming occurs on a large scale, or that the bone marrow is a repository for cells that can regenerate the rest of the body.

 iPS cells

Induced pluripotent stem cells (iPS cells), which are extremely similar to embryonic stem cells (ESC), are made by introducing certain genes into normal cells, and then applying selective conditions such that those few cells that become reprogrammed to an ES-like state will grow into colonies. They are discussed here because the cell type transformation is
discrete and permanent, although the final outcome is not another differentiated cell type but an ES-like cell.

Generation of iPS cells

Fig.8. Generation of iPS cells

The original gene set used was Oct4, Sox2, Klf4 and Myc, called “Yamanaka factors” after the originator (Takahashi and Yamanaka, 2006). OCT4 and SOX2 are key transcription factors regulating the pluripotency of ES cells. The function of Klf4 and Myc are less well defined but they are both expressed in ES cells and known as oncogenes that can cause cancer if activated inappropriately. Their effect may be exerted by promoting cell division and/or by making sites in the chromatin more accessible to the OCT4 and SOX2. Delivery of the Yamanaka factors to mouse fibroblasts using retroviral vectors generates about one ES cell colony per 10,000 input cells. This frequency is typically lower for human cells, but it can be increased by including extra genes or by treating with substances, such as the histone deacetylase inhibitor valproic acid, that open chromatin and increase accessibility of transcription factors to the genome. Of the four genes in the Yamanaka set, only Oct4 is really obligatory, the others can be substituted by other genes, usually members of the ES cell pluripotency factor group, such as Nanog, or genes that stimulate cell division. The critical requirement is that overexpression of the genes can upregulate the autocatalytic network of pluripotency factors characteristic of ES cells.

Colonies of mouse iPS cells

Fig.9. Colonies of mouse iPS cells. Left: phase contrast. Right immunofluorescence for Nanog protein.

iPS cells can be made from a variety of mammalian species. Mouse iPS cells will integrate into the mouse embryo following injection into blastocysts and are capable of contributing to all tissues including the germ line. This is the “gold standard” test for pluripotency. It is also found possible to make iPS cells from other cell types than fibroblasts. In particular lymphocytes isolated from blood and stimulated to divide by treatment with appropriate growth factors or lectins can serve as the starting material. This is potentially important because blood samples are so easily obtained from individual people. iPS cells have been made from lymphocytes in which gene rearrangement of antibody or T cell receptor genes has already occurred, which shows that it is possible to reprogram even well differentiated cells by the introduction of suitable transcription factors (Hanna et al., 2008).

The genes are delivered using retroviral or lentiviral vectors that insert their DNA into the chromosomes of the cells and enable high level expression of the products. Eventually the viral-encoded genes usually become “silenced”, that is they cease to be active, probably because the chromatin around them becomes impermeable to the transcription machinery. Continued activity of the introduced genes is detrimental, because, being pluripotency factors, they tend to inhibit differentiation of the cells.

There is a set of standard tests that are used to characterize iPS cells and to establish whether a particular new line has a genuinely ES cell-like phenotype or not. These include: an appropriate gene expression pattern, presence of the correct cell surface markers and alkaline phosphatase activity, colony morphology, the removal of DNA methylation from promoters of endogenous pluripotency genes, and formation of embryoid bodies capable of differentiating into derivatives of all three germ layers. Also the cells should have a normal karyotype and viral genes should be fully silenced. One key property of mouse ES cells is the ability to form chimeras when injected into early mouse embryos.

Human iPS cells can be made using similar methods (Takahashi et al., 2007; Yu et al., 2007). For human iPS cells it is not possible for ethical reasons to inject the cells into embryos, so the standard approach is the teratoma assay where the cells are injected into an immunodeficient host animal. Good quality iPS cells should grow to form a teratoma, and this should contain tissues characteristically derived from all three embryonic germ layers: ectoderm, mesoderm and endoderm.

Although good quality of iPS cells closely resemble ES cells, the random elements in their formation mean that it is easy to isolate imperfect cell lines. It is commonly found that iPS cell lines carry some “memory”, probably encoded in DNA methylation, of the cell type that they used to be. This also leads to a subsequent bias in the ease of differentiation in favor of this cell type.


Direct reprogramming”

Differentiated cell types tend to be relatively stable and to persist long term, with or without cell division. However it is sometimes possible to reprogram one differentiated cell type to another by overexpression of specific transcription factors which are responsible for the relevant commitment processes in normal development (Zhou and Melton, 2008). The first example of this was the ability of the myogenic factor MyoD to reprogram a variety of tissue culture cell lines to a myogenic phenotype (Weintraub et al., 1989). More recently, several other examples have been described, including the conversion of pancreatic exocrine cells to hepatocytes (Shen et al., 2000), B lymphocytes into macrophages (Xie et al., 2004), pancreatic exocrine cells to endocrine cells (Zhou et al., 2008), fibroblasts to neurons (Vierbuchen et al., 2010) or fibroblasts to cardiomyocytes (Ieda et al., 2010). Between one and three transcription factors need to be overexpressed to achieve these transformations, typically ones that are involved in the normal embryonic development of the cell type in question. Their function is not simply to activate direct target genes, but to shift the cell into a new stable state of gene expression.

Barrett’s metaplasia

Barrett’s metaplasia is a condition in which the lining of the lower end of the oesophagus changes from the normal stratified squamous epithelium into a columnar epithelium with some features of stomach and intestinal linings (Falk, 2002). Barrett’s metaplasia predisposes to oesophageal cancer, whose incidence has risen rapidly in the last three decades and carries a poor prognosis. In a typical individual the zone of transformed tissue contains several different populations of cells with different mutations contributing to the abnormal appearance (Leedham et al., 2008).  The disease is associated with the reflux of acid and bile from the stomach and duodenum into the oesophagus, but the mechanism of the initiating event is poorly understood at present. There is also controversy about the cell of origin: it may be oesophageal basal cells, or gland cells, or cells from the adjacent region of the stomach (Wang et al., 2011)

In order to achieve a better understanding of what drives the initial changes in Barrett’s metaplasia we are studying the role of certain genes which are important in intestinal development. Recent evidence suggests that Cdx2, a gene normally important for the development of the intestine but not expressed in the oesophagus, may be involved in Barrett’s metaplasia. We are currently investigating the conditions under which expression of Cdx2 in the oesophagus can prompt transdifferentiation from stratified squamous epithelium into intestinal type epithelium. This should increase our understanding of the disease process and may ultimately enable a form of gene therapy to reverse the metaplastic process.

Intestine expresses cdx2 Fig.10. Normal intestine expresses cdx2 (stained green).

Key definitions

Chimera (also chimaera):  Chimeras are made by injecting cells of one genotype into an embryo of a different genotype. The embryo and resulting adult consist of a mixture of cells of the two genotypes.

Direct reprogramming: Transdifferentiation induced by the overexpression of selected transcription factors.

Embryonic stem cells (ES cells): Cells that can be cultured indefinitely in vitro, or with suitable treatments can be caused to differentiate into any of the cell types found in the normal body. They are derived from the inner cell mass of mammalian blastocyst stage embryos. However their stem cell property is manifested only in vitro. In vivo the ICM cells are a type of progenitor cell which soon becomes other cell types.

Germ layers: All animal embryos have an early stage of development at which the body is subdivided into three layers: the ectoderm on the outside, the mesoderm in the middle and the endoderm on the inside. In vertebrate embryos the ectoderm becomes the nervous system and epidermis; the mesoderm becomes the notochord, somites, kidney, gonads, limbs, lateral plate, and blood islands; the endoderm becomes the epithelial lining of the gut and respiratory tract.

Heterotopia: Formation of cells, tissue or organs in the wrong place during embryonic development. This can include misplacement due to abnormal cell or tissue movements as well as due to inappropriate differentiation.

Heteromorphosis: Usually applied to cases of regeneration where the type of tissue or organ regenerated differs from the original.

Induced pluripotent stem cells (iPS cells): Cells closely resembling ES cells, made by introducing selected transcription factors into other cell types.

Metaplasia: Transformation of one stable cell state to another, including transformations of stem cell types.

Pluripotency: The ability to develop into any of the cell types normally found in the body.

Progenitor cells: Undifferentiated cells that develop into another cell type after a few divisions.

Serial heteromorphosis: Heteromorphosis where the transformation is between different segments of the body, for example leg to antenna in arthropods.

Stem cells: Undifferentiated cells that persist for the lifetime of the animal and divide to replenish their own population as well as generating differentiated cells of a particular tissue type.

Teratoma: A tumour arising from pluripotent stem cells following implantation into an animal. Typically the tumours contain tissues derived from all three embryonic germ layers.

Transdifferentiation: Transformation of one differentiated cell type to another.

Transdetermination: Specifically refers to transformation of one type of imaginal disc into another, in Drosophila. This occurs in the course of disc regeneration.


Slack and Tosh bibliography dealing with transdifferentiation and related topics

 Background materials

1.    Slack, J.M.W. (2000). Stem cells in epithelial tissues. Science 287, 1431-1433.
2.    Slack, J.M.W. (2008). Origin of stem cells in organogenesis. Science 322, 1498-1501.
3.    Slack, J.M.W. (2012). Stem Cells – A Very Short Introduction. Oxford University Press. 
4.    Slack, J.M.W. (2012). Essential Developmental Biology, third edition.  Wiley-Blackwell.

General reviews

1.    Slack, J.M.W. (1985).  Homoeotic transformations in Man.  Implications for the mechanism of embryonic development and for the organization of epithelia.  J. Theor. Biol. 114, 463-490. 
2.    Slack, J.M.W. (1986).  Epithelial metaplasia and the second anatomy. The Lancet, August 2nd, 267-271.
3.    Tosh D. and Slack J.M.W. (2002). How cells change their phenotype. Nature Reviews Molecular Cell Biology. 3, 187-194.
4.    Slack, J.M.W. (2007). Transdifferentiation and metaplasia : from pure biology to the clinic. Nature Reviews Molecular Cell Biology 8, 369-378
5.    Slack, J.M.W. (2009). Metaplasia and somatic cell reprogramming. J.Pathol. 217, 161–168.

Transformation from pancreas to liver


1.    Shen, C.N., Slack, J.M.W. and Tosh, D. (2000). Molecular basis of transdifferentiation of pancreas to liver. Nature Cell Biology, 2, 879-887.

2.    Tosh, D., Shen, C-N. and Slack, J.M.W. (2002). Differentiated properties of hepatocytes induced from pancreatic cells. Hepatology, 36, 534-543.

3.    Tosh, D., Shen, C-N., Horb, M. and Slack, J.M.W. (2003). Transdifferentiation of pancreas to liver. Mech. Dev. 120, 107-116

Transformation from liver to pancreas

1.    Horb, M.E., Shen, C.N., Tosh, D. and Slack, J.M.W. (2003). Experimental conversion of liver to pancreas. Current Biology 13, 105-115.
2.   Li, W.C., Horb, M. E. , Tosh, D. and Slack, J.M.W.. (2005). In vitro transdifferentiation of hepatoma cells into functional pancreatic cells. Mech.Dev. 122, 835-847
3.  Coad, R.A. Dutton J.R., Tosh D., Slack J. M. W. (2009) Inhibition of Hes1 activity in gall bladder epithelial cells promotes insulin expression and glucose responsiveness. Biochemistry and Cell Biology 87, 975-987.
4.   Akinci, E., Banga, A., Greder, L.V., Dutton, J.R. and Slack, J.M.W. (2011). Reprogramming of pancreatic exocrine cells towards a beta cell character using Pdx1, Ngn3 and MafA. Biochem.J. 442, 539-550.
5.   Banga, A., Akinci, E., Greder, L.V., Dutton, J.R. and Slack, J.M.W. (2012) In vivo reprogramming of Sox9 positive cells in the liver to insulin-secreting ducts. Proc. Natl. Acad. Sci. USA 109, 15336–15341.

Transformation from oesophagus to intestine (Barrett’s oesophagus)

1.   Colleypriest, B.J., Burke, Z.D., Yu, W.Y., Jover, R., Quinlan, J.M., Farrant, J.M., Slack, J.M.W. and Tosh, D. (2012). Hnf4α is a master gene that can generate columnar metaplasia in esophageal epithelium. Submitted.
2. Colleypriest, B. J., Farrant, J. M., Slack, J. M. W. and Tosh, D. (2010). The role of Cdx2 in Barrett's metaplasia. Biochemical Society Transactions 38, 364-369.
3. Slack, J. M. W., Colleypriest, B. J., Quinlan, J. M., Yu, W. Y., Farrant, J. M. and Tosh, D.
(2010). Barrett's metaplasia: molecular mechanisms and nutritional influences. Biochemical Society Transactions 38, 313-319.

Lack of transdifferentiation in regeneration

1.   Gargioli C.and Slack J.M.W. (2004). Cell lineage tracing during Xenopus tail regeneration. Development 131, 2669-2679.
2.  Lin, G. Chen, Y. and Slack, J.M.W. (2007). Regeneration of melanophores and other neural crest derivatives in the Xenopus tadpole tail. BMC Developmental Biology, 7:56

Transdifferentiation during normal development

1.   Yu, W.Y., Slack, J.M.W. and Tosh, D. (2005). Conversion of columnar to stratified squamous epithelium in the developing mouse oesophagus. Dev. Biol. 284, 157-170.
2.  Dutton, J. R., Chillingworth, N.L., Eberhard, D., Brannon, C.R., Hornsey, M.A., Tosh, D and Slack, J.M.W. . (2007). Beta cells occur naturally in extrahepatic bile ducts of mice.  J. Cell Science 120, 239-245.

3. Eberhard, D., Tosh, D., and Slack J.M.W. (2008). Origin of pancreatic endocrine cells from biliary duct epithelium. Cellular and Molecular Life Sciences 65, 3467 – 3480.

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Cells, tissues and organs

Transdifferentiation in normal development

Transdifferentiation in Drosophila

Transdifferentiation in regeneration

Transdifferentiation in human pathology

The transdifferentiation controversy

iPS cells

Direct Reprogramming

Barrett's Metaplasia

Key definitions

Slack and Tosh Bibliography