Live Tissue Imaging to Elucidate Mechanical Modulation of Stem Cell Niche Quiescence

Abstract

The periosteum, a composite cellular connective tissue, bounds all nonarticular bone surfaces. Like Velcro, collagenous Sharpey's fibers anchor the periosteum in a prestressed state to the underlying bone. The periosteum provides a niche for mesenchymal stem cells. Periosteal lifting, as well as injury, causes cells residing in the periosteum (PDCs) to change from an immobile, quiescent state to a mobile, active state. The physical cues that activate PDCs to home to and heal injured areas remain a conundrum. An understanding of these cues is key to unlocking periosteum's remarkable regenerative power. We hypothesized that changes in periosteum's baseline stress state modulate the quiescence of its stem cell niche. We report, for the first time, a three‐dimensional, high‐resolution live tissue imaging protocol to observe and characterize ovine PDCs and their niche before and after release of the tissue's endogenous prestress. Loss of prestress results in abrupt shrinkage of the periosteal tissue. At the microscopic scale, loss of prestress results in significantly increased crimping of collagen of periosteum's fibrous layer and a threefold increase in the number of rounded nuclei in the cambium layer. Given the body of published data describing the relationships between stem cell and nucleus shape, structure and function, these observations are consistent with a role for mechanics in the modulation of periosteal niche quiescence. The quantitative characterization of periosteum as a stem cell niche represents a critical step for clinical translation of the periosteum and periosteum substitute‐based implants for tissue defect healing. Stem Cells Translational Medicine 2017;6:285–292

Translating Periosteum's Regenerative Power: Insights From Quantitative Analysis of Tissue Genesis With a Periosteum Substitute Implant

“…The lack of increased tissue formation with inclusion of collagen sheets alone in implant pockets presented an unexpected result. We expected collagen membranes alone to promote bone generation within the implant because collagen is the predominant structural protein in the periosteum’s extracellular matrix [41, 49, 64] and promotes adhesion of osteoprogenitor cells [65]. Interestingly, ectopic bone formation within the muscle compartment (i.e., external to the outer boundary of the implant) was observed in defects treated with membrane implant and collagen (group 2).…”

“…Separation of the Sharpey's fibers changes the stress state of periosteum [40]. Such changes in periosteum's intrinsic stress change the conformation of collagen fibrils within the tissue concomitant with changes in the shape of periosteal cell nuclei; these changes are hypothesized to regulate quiescence of the progenitor cells inhabiting periosteal tissue, also called periosteum‐derived cells (PDCs) [41]. PDCs isolated from humans express mesenchymal stem cell (MSC) surface markers, including CD73, CD90, and CD105, more consistently and at significantly higher levels than seen with bone marrow stromal cells (BMSCs), which are MSCs from the bone marrow niche.…”

Origin of Reparative Stem Cells in Fracture Healing

“…51 Importantly, differentiation of these stem cells is mechanically sensitive and the relative stability of the microenvironment drives fate decisions. 52–55 Along the endosteal and periosteal surfaces, where the bone itself provides a high degree of stability, the progenitor cells differentiate directly into osteoblasts and contribute to fracture repair through intramembranous bone healing. In the fracture gap, where there is more mobility, periosteal progenitor cells differentiate into chondrocytes to form a soft cartilage callus to provide a temporary bridge between the broken bone ends.…”

Emergence of form from function—Mechanical engineering approaches to probe the role of stem cell mechanoadaptation in sealing cell fate

“…[63][64][65] Similarly, mechanical activation of a previously quiescent niche appears to drive specialization of the cells through mechanoadaptation and eventually differentiation. 2 The length and time scales of cellular processes play an integral role encompassing cellular memory, enabling the cell to adapt to best survive follow on experiences whose probability depends on previous experiences.…”