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In their paper about the therapeutic potential of adult bone-marrow stem cells (October 2004 JRSM1) Professor Hassan and Dr El-Sheemy refer to ethical controversies over use of embryonic stem cells. Apart from the ethical objections, cell replacement therapy from embryos and fetal tissue is complicated by difficulties with standardizing the procedure, ensuring viability of tissue and obtaining tissue at optimal time points. By induction of fate-specific cells from an in-vitro-expanded population of isolated progenitor/stem cells, bone marrow stem cells may be more readily exploited. Differentiation of expanded cells can be induced by mitogen withdrawal or by exposure to cytokines, hormones or vitamins that cause lineage restriction.
Cytokines involved in the clonal expansion and lineage restriction of stem/progenitor cells in the haematopoietic system may play an important analogous role in the developing nervous system. For example, interleukin-1 can induce expression of the dopaminergic marker tyrosine hydroxylase. Furthermore, in a study by Carpenter et al.2 leukaemia inhibitory factor, a member of the interleukin-6 cytokine family, when combined with other mitogens was shown to enhance proliferation of human forebrain neural stem cells. Importantly, we need to consider the possibility that bone marrow stem cells are able to redifferentiate. Already there is evidence that neural stem cells can become bone-marrow-like: they were shown capable of reconstituting the haematopoietic systems of mice that had undergone marrow ablation.3 These studies suggest that the lineage path taken by multipotent cells is influenced by the environments to which they are exposed.