Stem cell heterogeneity and division

All stem cells must be able to generate daughter cells with different fates (Wolpert 1988), so m e rem aining as stem cells and som e becom ing transit am plifying cells. T h e o re tic a lly this m ay be a ch ieved by two possible m echanism s, e ith e r invariant, predeterm ined asymm etrical outcomes of cell division such that the daughters of a single division are always one stem cell and one transit amplifying cell, or by variation of outcome. If

Chapter One__________________________________________

Introduction

stem cell division has a variable outcome, the possible results of the division could be two stem cells, two transit cells or one of each. If this is the case then stem cell fate would require a mechanism of regulation. There are three likely ways this could be achieved;

Stochastic

Environmental regulation

Heterogeneous stem cell population

C learly these possible m echanism s need not be mutually exclusive and there is evidence for each occurring, reviewed in Hall and Watt (1989) and Horvitz and Herskowitz (1992). Evidence for stochastic division is provided by experim ents which demonstrate that individual haematopoietic stem cells from mice can be cultured separately, but in exactly the s am e conditions, and give rise to pluripotential and com m itted progeny exhibiting differentiation along different lineages in culture (Johnson and Metcalf 1977; M ayani et al. 1993b).

Environm ental regulation of outcom e is seen in Caenorhabditis eiegans vulval developm ent for exam ple w here the fate of each precursor cell depends on signals from adjacen t cells and mutations in the signalling pathway, e.g. Lin-12 alters cell fate and therefore vulval developm ent (Greenwald and Rubin 1992; Horvitz and Herskowitz 1992).

In haematopoiesis, numerous growth factors are required for differentiation of stem cells (H S C 's ) into the various lineages (Allen et ai. 1990; Dexter 1991; Haig 1992; O gaw a 1993), different lineages expressing different and progressively more restricted types of growth factor receptor. The role of these factors is unclear and they m ay be permissive of a particular pathway rather than directive (Mayani e ta l. 1993b).

C ell division can be driven by numerous growth factors or cytokines (Haig 1992; Verfaillie 1 9 9 4 ), and different factors favour different pathw ays of differentiation (e.g. granulocyte/m acrophage, erythroid or lymphoid). H ow ever these factors do not seem to d eterm in e the result of stem cell division as was first thought, but are perm issive of a particular pathway, possibly by favouring expansion or survival of a subpopulation of lineage committed precursors (Mayani e ta l. 1993b). Stem cell division in HSC's m ay be symmetric or asym m etric but this is unaffected by combinations of cytokines/growth factors in culture.

Chapter One__________________________________________

Introduction

O ther factors m ay be important, such as contact with the marrow strom a or extracellular matrix, reviewed by Allen and Dexter (1990), although direct contact with the stromal cells is not essential in culture for long term maintenance of colony forming cells (Verfaillie 1994).

Similar control of cell fate is seen in other cell types. For exam ple growth factors may restrict neural progenitor cells to a glial fate in the neural crest (Shah e t al. 1994) and rat optic nerve (Raff and Lillien 1988).

T h e epidermis also proliferates under the control of growth factors (W att 1988) such as epiderm al growth factor (E G F ) and there is considerable evidence that interactions with the extracellular matrix (E C M ) can also regulate proliferation and term inal differentiation. During term inal differentiation, kératinocytes lose the ability to adh ere to the basem ent m em brane (W att 1987) and fibronectin can inhibit terminal differentiation of kératinocytes in suspension (Adams and W att 1989). As described above, the level and function of adhesion m o lecu les (in teg rin s) on the cell surface is related to the pro liferative capacity in kératinocytes. Similar interactions have now been shown in haem atopoietic tissue (Verfaillie e ta l. 1990; Gordon 1994; Rafii et al. 1994; Simmons et al. 1994) and in colonic carcinoma cell lines in which morphological differentiation is correlated with the ability to adhere to collagen (Richman and Bodmer 1988).

Evidence for stem cell heterogeneity supports the suggestion that there m ay be a continuum of stem cell renewal capacity rather than discrete populations of stem cells, with different proliferation potentials, and transit amplifiers. Som e cells would have a high probability of self renewal and low likelihood of differentiation and, at the other end of the continuum, cells would be very likely to differentiate and have low self renewal capacity. It is possible that such a situation exists in the bone marrow (Lemischka e t al. 1986) and in the epidermis w here integrin levels are directly related to self renewal capacity ((Jones and Watt 1993) and C hapter 4).

Stem cell ageing, regulated by loss of telom eres for exam ple, could be a cause of heterogeneity. For instance in the epidermis it m ay be that numbers of self renewing cells decrease with increasing donor age (Barrandon and Green 1987b). However this is disputed

Chapter One__________________________________________

Introduction

by other workers and there is similar uncertainty about stem cell ageing in the bone marrow and it is possible that there is sequential activation of HSC's (Lemischka e ta l. 1986).