Gradual conversion of cellular stress patterns into pre-stressed matrix architecture during in vitro tissue growth

Abstract

The complex arrangement of the extracellular matrix (ECM) produced by cells during tissue growth, healing and remodelling is fundamental to tissue function. In connective tissues, it is still unclear how both cells and the ECM become and remain organized over length scales much larger than the distance between neighbouring cells. While cytoskeletal forces are essential for assembly and organization of the early ECM, how these processes lead to a highly organized ECM in tissues such as osteoid is not clear. To clarify the role of cellular tension for the development of these ordered fibril architectures, we used an in vitro model system, where pre-osteoblastic cells produced ECM-rich tissue inside channels with millimetre-sized triangular cross sections in ceramic scaffolds. Our results suggest a mechanical handshake between actively contracting cells and ECM fibrils: the build-up of a long-range organization of cells and the ECM enables a gradual conversion of cell-generated tension to pre-straining the ECM fibrils, which reduces the work cells have to generate to keep mature tissue under tension.

The influence of curvature on three-dimensional mineralized matrix formation under static and perfused conditions: an in vitro bioreactor model

“…The total amount of tissue formed was dependent on the channel perimeter and was proportional to the local curvature. Other studies have also shown that the tissue growth rate was highly dependent on curvature [6,7,15]. Curvature sign was also found to be important, showing decreasing tissue growth from concave (negative curvature) to convex ( positive curvature) to flat (zero curvature) surfaces [6,[15][16][17].…”

“…Another reason for less mineralized tissue deposition within channels could be an impairment of the mechanical stability of the formed tissue due to the fluid flow applied. Only recently, it has been shown that impaired mechanical stability of ECM leads to deteriorated tissue organization and decreased tissue growth and reduces the ability of cells to assemble collagen fibres [7]. As a consequence, tissue organization and cytoskeletal changes need to be investigated further for our scaffold model especially by evaluating cytoskeletal differences of cells within the scaffold volume or the channel volume.…”

“…Cells aim to reduce these forces by contracting their cytoskeleton, which subsequently leads to a flattening of the tissue surface. This has been attributed to the fact that cellular growth is not only controlled by biological signalling pathways but also by mechanical control mechanisms [6,7].…”

“…The above-mentioned in vitro studies focused on nonmineralized extracellular matrix (ECM) formation only [6,7,14,15], or the formation of mineralization was initiated by soaking the scaffold material in simulated body fluid, which does not reflect cell-mediated mineralization [18]. Despite the fact that three-dimensional scaffolds have been used to investigate the influence of curvature on tissue growth, analyses were performed using confocal microscopy limited to a penetration depth of about 500 mm [6,7,14,15,17,19].…”

“…Despite the fact that three-dimensional scaffolds have been used to investigate the influence of curvature on tissue growth, analyses were performed using confocal microscopy limited to a penetration depth of about 500 mm [6,7,14,15,17,19]. To evaluate tissue growth along the whole height of the channels, it would have been necessary to cut the samples into thinner slices by destroying the samples, making them unavailable for longitudinal monitoring of tissue growth.…”