The physical microenvironment of hematopoietic stem cells and its emerging roles in engineering applications

Abstract

Stem cells are considered the fundamental underpinnings of tissue biology. The stem cell microenvironment provides factors and elements that play significant roles in controlling the cell fate direction. The bone marrow is an important environment for functional hematopoietic stem cells in adults. Remarkable progress has been achieved in the area of hematopoietic stem cell fate modulation based on the recognition of biochemical factors provided by bone marrow niches. In this review, we focus on emerging evidence that hematopoietic stem cell fate is altered in response to a variety of microenvironmental physical cues, such as geometric properties, matrix stiffness, and mechanical forces. Based on knowledge of these biophysical cues, recent developments in harnessing hematopoietic stem cell niches ex vivo are also discussed. A comprehensive understanding of cell microenvironments helps provide mechanistic insights into pathophysiological mechanisms and underlies biomaterial-based hematopoietic stem cell engineering.

Keywords: Biomaterial; Biophysical signal; Bone marrow niche; Engineering; Hematopoietic stem cell.

Fig. 1

The hierarchical system model of HSC self-renewal and differentiation. HSCs locate at the top of the hematopoietic hierarchy. Multipotent progenitors have the full-lineage differentiation potential

Fig. 2

The logistic model of endosteal and perivascular niches in the bone marrow. Multiple factors such as niche cells, cytokines, signals, ECM, and oxygen concentration gradient regulate HSC activities directly or indirectly. HSCs show overall different behaviors between the different subniches. Endosteal niches contribute to the maintenance of LT-HSCs, while vascular niches activate cell cycle and initiate cell proliferation and differentiation. NG2⁺ arteriolar pericytes in arterioles subniche keep HSCs closing to arterioles in a quiescent state. LepR-expressing perisinusoidal cells are the main source of SCF and CXCL12, which are essential to HSC maintenance or mobilization

Fig. 3

The mechanosensitivity of HSCs and MSCs in response to substrate stiffness. a The ECM stiffness can be mimicked by varying stiffness of the 2D substrate, which affects cell adhesion and morphology. On the stiffer substrate, cell spreads out more obviously. b Naive MNCs are growing on the gel surface with a range of stiffness (scale bars, 20 μm) [71]. c Phase-contrast images show the morphological changes of HSCs cultured on soft and hard gels. On the soft substrate, HSCs remain small and round. On the harder substrate, cell protrusions appear, accompanied by enhanced cell spreading and polarity (scale bars, 10 μm) [72]. d Confocal image of HSC cytoskeleton (F-actin) and nucleus (DAPI) on hard (196 kPa) and soft (0.71 kPa) matrix [73]

Fig. 4

Design strategies of hydrogel encapsulation systems for HSC culture. (a) Confocal microscopy images of HSPC on 2D/3D collagen hydrogel constructs via cytoskeleton (F-actin) and nucleus (DAPI) staining. Yellow arrows at the lower right indicate the thin filopodial protrusions from cells are extending into the surrounding hydrogel. Scale bar, 5 μm [73]. (b) GAG-rich 3D starPEG-heparin hydrogel system. Gel stiffness can be controlled by the molar ratio of starPEG to heparinmaleimide [123]. (c) Scanning electron micrograph image of cocultured MSC-BM (purple) and HSPCs within porous PEG hydrogels. Scale bar, 20 μm [124]. (d) Mechanical stretching induces the morphological changes in BM-derived progenitor cells within 3D fibrin hydrogels. Stress or strain induces the organization of the surrounding matrix. Cells (nuclei, blue) and F-actin filaments (green) are randomly organized in unconstrained control group (left), while cells in the static stress group (center) and the cyclic strain group (right) are aligned parallel to the direction of the stress or strain (magnification × 40, insets × 100; scale bar,10 μm) [125]