Stem cells: the generation and maintenance of cellular diversity (1989)

Cell divisions during embryonic development give rise to new differentiated cell types or increase the total number of cells in the embryo. In contrast, the major role of cell division in adult life is to maintain the number of differentiated cells at a constant level: to replace cells that have died or been lost through injury.The rate at which new cells are produced in the adult is a measure of how rapidly the cell population is turning over and, on this basis, tissues can be divided into three broad categories (Leblond, 1963). In tissues with static cell populations, such as nerve and skeletal muscle, there is no cell division and most of the cells formed during development persist throughout adult life. In tissues containing conditional renewal populations, such as liver, there is generally little cell division, but in response to an appropriate stimulus most cells can divide to produce daughters of the same differentiated phenotype. Finally, tissues with perma-nently renewing populations, including blood, testis and stratified squamous epithelia, are characterized by rapid and continuous cell turnover in the adult: the terminally differentiated cells have a short lifespan and are replaced through proliferation of a distinct sub-population of cells, known as stem cells.The simplest definition of a stem cell is that it is any cell with a high capacity for self-renewal, extending throughout adult life. In addition, stem cells are usually considered to have the potential to produce differen-tiated progeny and, as such, a stem cell may have a less 'mature' or less 'differentiated' phenotype than its daughters (Lajtha, 1979). Using these criteria, most of the progenitor cell populations that arise during embry-onic development are not stem cells, since they do not self-renew; however, as we shall describe, they have a number of properties in common with the stem cells of adult organisms, including differentiation potential and capacity for asymmetric cell division.Three adult mammalian tissues in which stem cells have been extensively studied are the haemopoietic system, the epidermis and intestinal epithelium. In our review, we shall begin by describing these stem cell populations: the evidence that they exist; their identifi-cation, location and differentiated progeny. Three com-mon features that emerge are illustrated in Fig. 1A. First, stem cells have the capacity for unlimited self-renewal, in the context of the fifespan of the organism. Second, they have the potential for asymmetric div-isions, such that one daughter is itself a stem cell while the other is committed to undergo terminal differen-tiation. Third, the differentiation process is irreversible, since the daughters of a cell that is committed to terminal differentiation are never stem cells.In haemopoiesis and in intestinal epithelium, the stem cells are pluripotential, giving rise to more than one type of differentiated cell. In the epidermis, in contrast, there appears to be only a single pathway of terminal differentiation (Fig. IB). In all three tissues, (here is evidence that differentiation of stem cell progeny occurs via transit amplifying, or progenitor, populations that have a more limited capacity for self-renewal (Fig. 1B).We shall discuss what is known of the mechanisms that regulate these aspects of stem cell behaviour, drawing on the properties of analogous cells in a range of other tissues and organisms.The terminally differentiated cell types of blood, shown in Fig. 2, are all derived from pluripotential stem cells that are located in bone marrow. The first evidence for the existence of such a stem cell population came from experiments in which bone marrow cells from healthy mice were injected into mice that had received lethal doses of radiation (Ford et al. 1956; Till & McCulloch, 1961). The donor marrow was able to reconstitute all the differentiated blood cell types in the host and, when low numbers of cells were injected, colonies derived from single marrow cells (CFU-S) were found to contain both myeloid and erythroid cells (Wu et al. 1968). The use of chromosome markers induced by radiation or retroviral integration has confirmed that cells of myeloid and lymphoid lineages can all be derived from a single cell with high self-renewal ca-pacity (Abramson et al. 1977; Joyner et al. 1983; Williams et al. 1984; Dick et al. 1985; Lemischka et al. 1986).The haemopoietic stem cells do not give rise to terminally differentiated cells directly, but via prolifer-ating progenitor populations, as illustrated in Fig. 2. In this way, a relatively small number of stem cells (estimated as 0·4% of the total population of haemo-poietic cells; Lord & Testa, 1988) can give rise to a large number of terminally differentiated cells (Lajtha, 1979). Identification of the progenitors and investigation of the mechanisms regulating their profiferation and differen-tiation have been made possible by the development of cell culture systems that support the proliferation of CFU-S and differentiation along multiple cell lineages (Pluznik & Sachs, 1965; Bradley & Metcalf, 1966; Dexter et al. 1977).Although the existence of haemopoietic stem cells is well established, their identification has proved diffi-cult. The strategy has been to separate subpopulations of marrow cells on the basis of buoyant density, sensitivity to antimitotic agents or expression of cell surface antigens, and to look for enrichment of stem cells on the basis of in vitro or marrow reconstitution assays. In mouse, the following profile of the bone marrow reconstituting cells is emerging: they do not express any markers of either granulocyte, macro-phage, B or T cell lineages (Müller-Sieburg et al. 1988), but bind wheat germ agglutinin (Visser et al. 1984; Lord & Spooncer, 1986), express low levels of Thy-1 (Müller-Sieburg et al. 1988) and express a surface antigen called SCA-1 (for Stem Cell Antigen 1) (Spangrude et al. 1988). In humans, 1-4% of the marrow cells bear the CD34 antigen; these cells include virtually all the stem cells detected by in vitro assays, and baboon cells bearing CD34 are able to reconstitute the marrow of irradiated animals (Berenson et al. 1988). However, although subpopulations of bone marrow cells that contain haemopoietic stem cells can be defined, no unique markers of stem cell populations have yet been identified.Cells within the bone marrow appear to lack the high degree of spatial organization characteristic of cells in epithelia. Nevertheless, studies of the distribution of different subpopulations of bone marrow cells suggest that it is not random (Western & Bainton, 1979; Lord & Testa, 1988). In mouse femur CFU-S with the highest self-renewal capacity, which are turning over very slowly, are found in the centre, while most CFU-S proliferation takes place in the vicinity of the bone. GM-CFCs are concentrated close to the bone surface, whereas maturing granulocytic cells accumulate towards the centre of the marrow space (Lord & Testa, 1988). A similar distribution has been reported in human rib (Testa, 1985).The epidermis comprises multiple layers of epithelial cells, called kératinocytes (Fig. 3). The deepest layer, which is adherent to a basement membrane, contains most of the cells that are capable of dividing. Kératino-cytes that leave the basal layer no longer divide and they undergo terminal differentiation as they move towards the tissue surface. The end-point of the differ-entiation pathway is an anucleate, keratin-filled squame that is specialized to protect the underlying living cell layers from desiccation and mechanical damage. Squames are continuously shed from the surface of the epidermis and are replaced through proliferation of cells in the basal layer. In order that the epidermis remains a constant thickness, the rates of cell pro-duction and cell loss must be equal.A number of observations suggest that not all of the kératinocytes that are capable of dividing are stem cells. For example, when mouse epidermis is severely damaged by radiation (Withers, 1967; Potten & Hendry, 1973) only about 10 % of the basal kératino-cytes have sufficient proliferative potential to form recognizable foci of new epidermis. These clonogenic cells have been interpreted as constituting the stem cell population, whereas the further 50 % of basal cells that are capable of dividing may constitute a transit amplify-ing (progenitor) population. The remaining 40% of cells in the basal layer are presumably postmitotic and committed to terminal differentiation (Iversen et al. 1968; Potten & Morris, 1988; Fig. 3).Further evidence for proliferative heterogeneity has come from studies of cultured human epidermal kéra-tinocytes. Sheets of cultured cells grafted onto suitable recipients form epidermis that persists for years, indi-cating that stem cells are not lost in culture (Gallico et al. 1984). However, only a small proportion of the cells in culture undergo extensive proliferation; the rest either fail to divide or form small abortive colonies in which all the cells terminally differentiate (Barrandon, & Green, 1987b). Experiments in which the clone-forming ability of cells isolated from parental clones that differed in growth potential were examined indi-cate that transitions from clones of high growth poten-tial to those with none occur in culture and are unidirectional (Barrandon & Green, 1987b; and see Fig. 1A).Although extensive proliferation in culture or during recovery from radiation is thought to be characteristic of epidermal stem cells, there is evidence that under normal steady-state conditions the stem cells divide more slowly than the transit amplifying population. Thus kinetic analysis of mouse epidermis and in vitro experiments with cultured human kératinocytes suggest that stem cells have a longer cell cycle time and shorter S phase than transit amplifying cells, and that they retain [3H]thymidine for much longer than the transit population (Bickenbach, 1981; Dover & Potten, 1983; Clausen et al. 1984; Jensen et al. 1985b; Morris et al. 1985; Albers et al. 1986). In addition, cells with stem-like characteristics (high capacity for self-renewal) are resistant to treatments that induce premature terminal differentiation in culture, such as exposure to TPA or cultivation in suspension (Parkinson & Emmerson, 1982; Parkinson et al. 1983; Hall & Watt, in prep-aration). However, although stem and transit amplify-ing kératinocytes can be distinguished by these func-tional criteria there are, to date, no molecular markers for each subpopulation. 'In some stratified epithelia, there is evidence that the stem cells occupy a specific location within the basal layer. In the cornea, the stem cells appear to be located at the limbus (Schermer et al. 1986; Cotsarelis et al. 1989) and, in tongue papilla, they are found in those regions of the basal layer that project deeply into the underlying connective tissue (Hume & Potten, 1976). During wound healing, interfollicular epidermis can be regenerated from the outer root sheath of the hair follicles, suggesting that this is a source of stem cells (Lenoir et al. 1988). The basal layer in human and primate epidermis is not flat, but undulates, and there is some evidence that the stem cells -identified as-slowly cycling cells -are located in the troughs (Lavker & Sun; 1983).In some regions, such as the dorsal skin of the mouse, the basal layer is flat and kératinocytes in the suprabasal layers are arranged in columns, like a stack of coins (Mackenzie, 1970; Christophers, 1971). It has been proposed that each stack corresponds to an epidermal proliferative unit (EPU), consisting, in the basal layer, of a stem cell surrounded by 5-6 transit amplifying cells and less than four postmitotic cells (i.e. committed to terminal differentiation) (Potten, 1974, 1981). The model is based on cell kinetic analysis and is supported by the findings that the basal cells that retain [3H]thymi-dine (putative stem cells -see earlier) fie at the centre of the stacks of suprabasal cells in mouse dorsal epidermis (Morris et al. 1985) and that gap junctional communication compartments are approximately the same size as EPUs (Pitts et al. 1988). However, a columnar arrangement of kératinocytes is lacking in most body sites, and the pattern of mosaicism revealed in the epidermis of chimaeric mice is not consistent with the EPU model (Schmidt et al. 1987).The lining of the small intestine is a. monolayer of epithelial cells organized into crypts and villi (Fig. 4). Cell proliferation is confined to the crypts; differen-tiated cells migrate to the surface and are shed from the tips of the villi. There are four differentiated cell types: Paneth cells at the base of the crypts and columnar, goblet and entero-endocrine cells in the rest of the crypts and on the villi; none of these mature cells divide, suggesting that they are renewed by prolifer-ation of a stem cell population. Evidence for the existence of a single stem cell compartment generating all four types of progeny came first from the observation that, after radiation-induced cell death, surviving pro-liferative cells phagocytose cell debris and the phago-somes can be used as markers of their progeny; within 30 h phagosomes are found in all the differentiated cell types (Cheng & Leblond, 1974). Further evidence that intestinal stem cells are pluripotential comes from the finding that lines derived from single human adenocar-cinoma cells can differentiate into columnar, goblet and entero-endocrine cells in culture (Huet et al. 1987; Kirkland, 1988).The stem cells are thought to be located at or near the base of each crypt, since proliferating cells higher in the crypt migrate upwards and are not therefore permanent residents (Potten et al. 1987). It has been proposed that stem cells give rise to committed progenitors for each differentiated cell type (Cheng & Leblond, 1974; see Fig. 4). Kinetic studies suggest that there are 4-16 stem cells per crypt (Potten & Loeffler, 1987; Potten et al. 1987) .However, studies with mouse aggregation chim-eras or mice heterozygous for markers that can be detected histochemically show that the epithelium of individual crypts in the small and large intestine of adult mice is always composed of cells of a single parental type, and hence that each crypt is probably derived from (and maintained by) a single stem cell during development (Ponder et al. 1985; Griffiths et al. 1988; Schmidt et al. 1988; Winton et al. 1988). In adult colonic epithelium there is evidence that each crypt is main-tained by proliferation of one stem cell (Griffiths et al. 1988; Winton et al. 1988; Fig. 5). No molecular markers for intestinal stem cells have so far been reported.In the previous sections, we described evidence for the existence of stem cells in bone marrow, epidermis and intestinal epithelium, and what progress has been made towards their identification. In all three tissues, a relatively small number of stem cells give rise to a large number of terminally differentiated cells via a transit amplifying (progenitor) population and the stem cells differ from their progeny in phenotype and/or location. In each tissue, the processes of stem cell renewal and production of differentiated progeny must be tightly coordinated, in order to meet the varying requirements of the body for differentiated cells. These common features enable us to ask a number of general questions about stem cells:-1. What mechanisms determine asymmetry of fate, such that stem cells produce both stem cells and daughters committed to terminal differentiation?2. What mechanism allows for unlimited self-renewal of stem cells, but a finite number of rounds of division within the transit/progenitor populations?3. Is differentiation from the stem cell compartment irreversible?Observations from a range of different tissues and organisms provide partial answers to these questions.One feature of all three stem cell systems is their capacity to produce both stem cells and cells that are committed to terminal differentiation, represented symbolically as A→ A + B. This can, theoretically, be achieved in two ways. In the first, the outcome of every division is predetermined and invariant: A→ A + B. In the second, individual stem cell divisions may have different outcomes, such that A→A+A or B + B or A + B, but, on a population basis, the end result is the production of equal numbers of A and B. This second situation could arise in three ways: if the outcome of each division is stochastic; if it is environmentally regulated; or if the stem cell population is hetero-geneous.Some of the best examples of invariant asymmetric divisions are found in yeast, Caenorhabditis elegans and plants, in which lineage maps of cell fate can be constructed. In cases where lineage is not the sole determinant of the outcome of divisions, such as in adult mammalian tissues, in Hydra and certain C. elegans tissues, some progress has been made in ident-ifying regulatory factors.Yeast cells can either be haploid or diploid, the diploid state resulting from fusion of two haploid cells that differ in mating type. Although the progeny of haploid cells normally inherit the mating type of the parental cell, switching of mating type can occur. It is the ability to switch that is inherited asymmetrically (Strathern & Herskowitz, 1979; Nasmyth, 1983; Klar, 1987a; Nas-myth & Shore, 1987).In the budding yeast, Saccharomyces cerevisiae, only the mother cells, and not their daughters (the buds), have the ability to switch mating type. Mating type is determined by gene conversion at a specific locus, which occurs through the action of a specific endonu-clease, encoded by the HO gene, such that the gene conferring one mating type is excised and replaced with a copy of the gene conferring the other type (Hicks & Herskowitz, 1976). HO expression and mating-type switching are restricted to a short period in late Gi, after the cell has become committed to another mitotic cycle (Nasmyth, 1983). HO transcription is controlled both positively and negatively by the products of several other genes, at least one of which interacts directly with the HO promoter (Nasmyth et al. 1987; Sternberg et al. 1987). The ability of mother, but not daughter, cells to transcribe HO appears to be due to asymmetric distribution of these gene products at div-ision (Klar, 1987b).In the fission yeast, S. pombe, mating type switching occurs such that of the four progeny of a single cell obtained after two generations, only one is switched; in other words, only one of the two daughters in the first generation is competent to produce a switched cell in the subsequent generation. As in 5. cerevisiae, mating type is determined by gene conversion but, in S. pombe, the ability to switch is inherited chromosomally, not through unequal distribution of cytoplasmic or nuclear factors (Egel, 1984; Klar, 1987a,b). Current models suggest that asymmetry from of parental (Klar, elegans development occurs via an of cell divisions and lineage maps of the of every cell in the adult can be 1988). There is some evidence that lineages normal For example, in the lineages of the each cell to one cell and one cell & the to produce one cell at each division that a single cell that is for the germ et al. major towards the basis of C. elegans cell lineage has been the identification and of in the in cells, or mother and to have different 1988). For example, in of the lineages by one of the two daughters of a division to express its normal fate and the fate of its mother this in a lineage et al. 1981). result in both cells the same fate & 1988), and, in certain cells express that be or in development & and S. provide examples of two possible mechanisms for daughters that in either that some or is inherited asymmetrically or that the parental are different In S. unequal of the factors that regulate HO transcription may occur at (Klar, 1987b). mechanisms resulting in of include asymmetric of specific 1988) or asymmetric of transcription factors et al. 1985; There is evidence for in & and the stem cells of such as However, in C. elegans development the parental & of is a feature of the of which that, during an asymmetric stem cell division, the new stem cell always the parental while the daughter that is to differentiate the This provide a for stem cell from during the [3H]thymi-dine in mouse intestinal crypts and tongue are consistent with the (Potten et al. but it remains to be mechanism that asymmetry of fate of stem cell progeny be for spatial location to determine The cell that the of the parental cell itself be a stem cell, while the other cell be committed to of cell fate and only constitute a predetermined in the that the of the two daughters are for example, in the of the et al. to the outcome of stem cell divisions is is to individual stem cells, place in culture under and the fate of their progeny. the stem cells are all yet found colonies different of and and different types of differentiated this that fate is Experiments that this in the haemopoietic have been by & and et al. The is a of haemopoiesis in mice and individual cells from this tissue are able to form colonies in to erythroid cells, or & cells isolated from blood were to clones of cells; individual cells were cultured and the differentiation of their clones after The experiments that within the first three divisions of a haemopoietic stem cell and its at least one cell with can be produced and at least two committed progeny can arise from the two or three divisions & Metcalf, 1979). et al. that the two daughters of a CFU-S can differentiate along different it must be that the was the same for each cell, the experiments show that self-renewal and can occur in the of a mechanism that that stem cell divisions produce both stem cells and differentiated cells be if the stem cell population is with some stem cells a high of self-renewal and a high of differen-tiation. There is evidence for heterogeneity of haemo-poietic stem cells in their ability to and irradiated bone marrow et al. 1977; et al. et al. This has to the that the stem and transit populations are not shown in Fig. but a from cells of high self-renewal capacity and low of differentiation to those of low self-renewal capacity and high to differentiation et al. 1977; et al. et al. source of heterogeneity could be stem cell This has not been for haemopoietic cells within the of individual since the capacity of or mouse marrow is the same & et al. 1984). However, there is a in self-renewal capacity with of haemo-poietic stem cells animals et al. et al. & 1982; & 1976). & have a in the of human kératinocytes capable of self-renewal in culture with donor model of that there is of stem cells during the lifespan of an organism. This is supported by the observations of Lemischka et al. and et al. Lemischka et al. used gene to haemopoietic stem cells and their fate after into irradiated In most or stem cell clones for the of the mature haemopoietic cells at any one and different clones were at different in the life of the et al. used a of haemopoietic stem cells from two normal mice to mouse from a with a in In some there was a and rise and in the proportion of haemopoietic cells of the two normal the However, et al. 1988), haemopoiesis in chimaeric have obtained evidence that stem cells are and the for the is development of C. elegans is there is evidence for the existence of small of cells, known as which arise from very similar lineages and the same In such cell result in cells following different and, if these are the differentiation fate of the cells is examples of this are development & 1986; 1988) and the of in the adult germ & 1987). In the the fate of each cell on a of two an from the cell and a from cells. in the et al. and this gene a with epidermal growth et al. 1988). In the with a cell, the cell, is for if the cell is all germ cells and form The of a gene is for germ cell In of the cell is but there is a in the response of cells to the the appears to a & 1987).In Hydra all the differentiated cell types and the and are derived from a population of stem cells which have an extensive capacity for renewal & 1987; et al. 1987). The number of the stem cells in different body regions most are found in the with in the or In than lineage the major role in the fate of stem cell spatial in the distribution of different differentiated cell types is a of stem cell A called has been isolated which is by nerve cells and stem cells to differentiate into nerve cells & 1981). is for as to growth and In addition, there is an in the and in the body is when gap junctional communication is et al. further of on stem cell in Hydra is the mechanism that the of stem cells to epithelial cells is the number of stem cells is there is a rapid increase in the size of the stem cell population the normal stem cell is This is achieved by an increase in the of self-renewal of the stem cells et al. 1987). mechanisms presumably maintain stem cell in other organisms, but they have to be