Maturation State and Matrix Microstructure Regulate Interstitial Cell Migration in Dense Connective Tissues

Abstract

Few regenerative approaches exist for the treatment of injuries to adult dense connective tissues. Compared to fetal tissues, adult connective tissues are hypocellular and show limited healing after injury. We hypothesized that robust repair can occur in fetal tissues with an immature extracellular matrix (ECM) that is conducive to cell migration, and that this process fails in adults due to the biophysical barriers imposed by the mature ECM. Using the knee meniscus as a platform, we evaluated the evolving micromechanics and microstructure of fetal and adult tissues, and interrogated the interstitial migratory capacity of adult meniscal cells through fetal and adult tissue microenvironments with or without partial enzymatic digestion. To integrate our findings, a computational model was implemented to determine how changing biophysical parameters impact cell migration through these dense networks. Our results show that the micromechanics and microstructure of the adult meniscus ECM sterically hinder cell mobility, and that modulation of these ECM attributes via an exogenous matrix-degrading enzyme permits migration through this otherwise impenetrable network. By addressing the inherent limitations to repair imposed by the mature ECM, these studies may define new clinical strategies to promote repair of damaged dense connective tissues in adults.

Nuclear softening expedites interstitial cell migration in fibrous networks and dense connective tissues

“…Consistent with this, our recent in vitro models exploring cell invasion into devitalized dense connective tissue (knee meniscus sections) showed reduced cellular invasion in adult tissues compared to less dense fetal tissues (3). The density of collagen in most adult dense connective tissues is 30 to 40 times higher than that used within in vitro collagen gel migration assay systems (2,3), emphasizing the substantial barrier to migration that the dense ECM plays in these tissues.…”

“…While many tissues provide a permissive environment for such interstitial [threedimensional (3D)] cell migration (i.e., skin), adult dense connective tissues (such as the knee meniscus, articular cartilage, and tendons) do not support this migratory behavior. Rather, the extracellular matrix (ECM) density and micromechanics increase markedly with tissue maturation (2,3) and, as a consequence, act as a barrier for cells to reach the wound interface. It follows then that healing of these tissues in adults is poor (4,5) and that wound interfaces remain susceptible to refailure over the long term due to insufficient repair tissue formation.…”

Cell migration: implications for repair and regeneration in joint disease

“…The mechanical properties (such as the tensile modulus, compressive modulus and shear modulus) of these tissues vary because of the heterogeneity and hierarchical nature of the tissue building blocks. In general, increasing the concentration, density and/or degree of alignment of collagen increases the load-bearing capacity of a tissue and results in higher ECM mechanical properties (that is, the Young's modulus) 47,48 . Because of the heterogeneity in tissue mechanical properties, more homogeneous collagen-based hydrogels or synthetic hydrogels have most often been used to assess cell migration in different 3D microenvironments.…”

“…Nonetheless, decoupling the effect of ECM stiffness on cell migration from that of pore size and/or adhesivity is difficult given that hydrogel mechanics are often directly related to matrix density (and in the case of ECM-derived materials, adhesive ligand concentration). Moreover, as the collagen concentration, stiffness and fibril organization of most gels used in 3D assays are extremely low (~100 times lower concentration than that of dense connective tissues 47 ), to what extent these findings apply to migration in native dense connective tissues requires further investigation.…”

Influence of Fiber Stiffness on Meniscal Cell Migration into Dense Fibrous Networks

“…For instance, the nuclei of adjacent host cells are often too large and too stiff to migrate through the small pores . Attenuated cell migration might also favor new tissue formation at the scaffold interface, which could further act as a barrier to hinder cell migration into scaffolds …”