The stem cell continuum: cell cycle, injury, and phenotype lability.

Area of Science:

• Stem cell biology
• Hematopoiesis
• Tissue regeneration

Background:

• Hematopoietic stem cell (HSC) phenotype is labile, affected by cell cycle and tissue injury.
• Published studies show reversible changes in HSC engraftment, gene expression, and differentiation potential.
• HSCs identified by Sca-1, c-kit, or Hoechst staining are cycling populations, with phenotype shifts linked to cell cycle phase.

Purpose of the Study:

• To investigate the continuum model of stem cell regulation.
• To explore the plasticity of marrow-derived cells in tissue repair.
• To identify factors influencing the conversion of marrow cells to non-hematopoietic lineages.

Main Methods:

• Analysis of cytokine-induced cell cycle transit in HSCs.
• Separation and characterization of cycling HSC populations using Sca-1, c-kit, and Hoechst staining.
• In vivo transplantation studies examining marrow cell conversion under various injury conditions.
• Investigation of microvesicle transfer from injured tissue to marrow cells.

Main Results:

• Cytokine-induced cell cycle transit causes reversible changes in HSC phenotype, contradicting unidirectional hierarchical models.
• Marrow cells can convert to non-hematopoietic lineages (e.g., skeletal muscle, lung) dependent on injury, cell cycle, and other variables.
• Microvesicles from injured lung tissue can induce lung-specific gene and protein expression in marrow cells, suggesting a mechanism for plasticity.

Conclusions:

• Stem cell regulation follows a continuum model, not a strict hierarchy.
• Marrow cells exhibit significant plasticity, capable of responding to tissue injury and repairing diverse tissues.
• Microvesicle-mediated transfer offers a potential mechanism for observed stem cell plasticity and tissue repair phenomena.