Substrate elasticity regulates the biophysical properties of hematopoietic stem and progenitor cells

Physiological environments of the HSC niches exhibit a range of stiffness, ranging from soft marrow ( 34 kPa) (Patel, Smith et al. 2005; Engler, Sen et al. 2006; Discher, Mooney et al. 2009). In order to decouple the effects of substrate elasticity, ligand concentration, and dimensionality on hematopoietic stem cell (HSC) fate (quiescence, self-renewal, differentiation, mobilization, homing, and apoptosis), HSCs harvested from C57B6 mouse femurs and tibias were cultured in or on top of collagen hydrogels or on top of 2D polyacrylamide (PA) gels with varying mechanics and ligand densities. With collagen hydrogels and type I collagen-coated PA gels with varying stiffness, simple in vitro biomaterials system to probe the effects of substrate mechanics on the biophysical properties of HSCs were created. When cultured for 24 hours, HSCs exhibited varying degrees of changes in their biophysical properties (cell viability, spread area, cell morphology) with increasing substrate stiffness. In general, HSCs spread out more and showed more irregular morphology with increasing substrate elasticity and ligand density. The observed behaviors were different from those of 32D cells, which are further differentiated IL-3 dependent murine myeloid progenitor cells from a cell line, that showed overall much more spread out and amorphous morphology with optimal spreading at an intermediate ligand density