The universal stem cell source: Does it exist?

Abstract

In 1978, Schofield hypothesized the existence of cells in the proximity of stem cells, termed the stem cell niche, that have the ability to extrinsically exert influence on stem cell behavior. Indeed, a large body of evidence from a number of stem cell systems validated this hypothesis by affirming the critical importance of stem cell niche interactions and localized extracellular signals in regulating stem cell self-renewal and differentiation. This concept is precisely illustrated in the bone marrow transplantation setting in which the success of the transplant is contingent on the ability of hematopoietic stem cells (HSCs) to home and seed appropriate secondary supportive niches of intravenous infection. Experiments using parabiosis of genetically marked strains of mice demonstrated that HSCs constitutively migrate through blood and are able to re-engraft unconditioned bone marrow to niche function HSCs have emerged as the model system to study tissue-specific niche stem cells and their potential to regenerating secondary niche. This chapter reviews observations, olds and recents, suggesting that this plasticity may perhaps outstrech the marrow boundaries, so that HSCs (mesodermal in origin) can give rise to cells that normally derive from germ layers other than the mesoderm. This review discusses the inextricable relationship between adult stem cells and bone marrow-derived hematopoietic cells, and their roles in replenish adult stem cell niches.