Controlling self-renewal and differentiation of stem cells via mechanical cues

Abstract

The control of stem cell response in vitro, including self-renewal and lineage commitment, has been proved to be directed by mechanical cues, even in the absence of biochemical stimuli. Through integrin-mediated focal adhesions, cells are able to anchor onto the underlying substrate, sense the surrounding microenvironment, and react to its properties. Substrate-cell and cell-cell interactions activate specific mechanotransduction pathways that regulate stem cell fate. Mechanical factors, including substrate stiffness, surface nanotopography, microgeometry, and extracellular forces can all have significant influence on regulating stem cell activities. In this paper, we review all the most recent literature on the effect of purely mechanical cues on stem cell response, and we introduce the concept of "force isotropy" relevant to cytoskeletal forces and relevant to extracellular loads acting on cells, to provide an interpretation of how the effects of insoluble biophysical signals can be used to direct stem cells fate in vitro.

Figure 1

The adult mammalian stem cell niche is defined as a microenvironment that facilitates (a) the survival and (b) the self-renewing capacity of the stem cells, as well as (c) the production of actively dividing precursors leading to the generation of a differentiated progeny. The stem cells retain their function as long as they remain anchored to the supporting cells and their divisions occur in such a way that one daughter cell keeps its contact with the supporting cell while the other one loses this contact, migrates from the niche, and (d) proceeds to generate terminally differentiated cells.

Figure 2

The concept of force isotropy applied to cells: (a) anisotropic and (b) isotropic cytoskeletal tension caused by cell contraction either in the niche, or during cell migration through the extracellular matrix; (c) anisotropic and (d) isotropic extracellular forces; σ is cyclic matrix stress, τ is cyclic fluid-induced shear, and p is cyclic pressurization.

Figure 3

Examples of engineering strategies used to study the effects of anisotropic versus isotropic cytoskeletal tension on cultured stem cells. Substrates that mimic (a) soft versus (d) stiff ECM induce differential fates in human embryonic stem cell-derived cardiomyocytes, due to differential patterns of focal adhesions and actin-myosin stress fibres (in green, nuclei in blue). Reprinted and adapted from [8] with permission from John Wiley and Sons. In rat neural progenitor stem cells, (b) nonpatterned substrates induce formation of cell protrusions oriented randomly, while (e) micropatterned surfaces that mimic the native presentation and orientation of ECM proteins to cells induce alignment of elaborated processes in the direction of the grooves. Reprinted and adapted from [9] with permission from Elsevier. In rat mesenchymal stem cells, ultraprecise microscaffolds (c) with pore dimension of 20 microns allow for cell migration and isotropic attachment to the internal 3D lattice, whereas (f) for pores below 10 microns in size, cell migration into the scaffold pores is limited, inducing cell spreading on the top surface of the micro-scaffold (dividing nuclei in pink, cytoskeletal actin in green). Reprinted and adapted from [10] with permission from Elsevier.

Figure 4

Examples of engineering strategies used to study the effects of anisotropic versus isotropic extracellular forces on cultured stem cells. Pig bone marrow-derived stem cells, subjected to cyclic hydrostatic pressure, (a) retain a spherical morphology when encapsulated in softer agarose, whereas (b) spread when encapsulated in stiffer fibrin hydrogels. Reprinted and adapted from [11] with permission from Elsevier. Rat bone marrow-derived stem cells (c) appear randomly distributed in unsolicited culture conditions, whereas (f) anisotropic strain induces cell elongation and upregulation of cardiomyocyte-related markers. Adapted with permission from [12]. In human bone marrow-derived mesenchymal stem cells and meniscal fibrochondrocytes cultured on a randomly organized nanofibrous scaffold, (d) biaxial strain induces a roundish shape of the cell nucleus, with small protrusion in all directions, while (e) uniaxial strain determines an elongated morphology of the cell nucleus. Reprinted and adapted from [13] with permission from Elsevier.

Figure 5

Synthesis of in vitro experimental evidence of the effects of mechanical cues and oxygen tension on embryonic stem cell fate.

Figure 6

Synthesis of in vitro experimental evidence of the effects of mechanical cues and oxygen tension on the fate of mesenchymal stem cells derived from the endosteal compartment.