Rapid specialization and stiffening of the primitive matrix in developing articular cartilage and meniscus.

Understanding early patterning events in the extracellular matrix (ECM) formation can provide a blueprint for regenerative strategies to better recapitulate the function of native tissues. Currently, there is little knowledge on the initial, incipient ECM of articular cartilage and meniscus, two load-bearing counterparts of the knee joint. This study elucidated distinctive traits of their developing ECMs by studying the composition and biomechanics of these two tissues in mice from mid-gestation (embryonic day 15.5) to neo-natal (post-natal day 7) stages. We show that articular cartilage initiates with the formation of a pericellular matrix (PCM)-like primitive matrix, followed by the separation into distinct PCM and territorial/interterritorial (T/IT)-ECM domains, and then, further expansion of the T/IT-ECM through maturity. In this process, the primitive matrix undergoes a rapid, exponential stiffening, with a daily modulus increase rate of 35.7% [31.9 39.6]% (mean [95% CI]). Meanwhile, the matrix becomes more heterogeneous in the spatial distribution of properties, with concurrent exponential increases in the standard deviation of micromodulus and the slope correlating local micromodulus with the distance from cell surface. In comparison to articular cartilage, the primitive matrix of meniscus also exhibits exponential stiffening and an increase in heterogeneity, albeit with a much slower daily stiffening rate of 19.8% [14.9 24.9]% and a delayed separation of PCM and T/IT-ECM. These contrasts underscore distinct development paths of hyaline versus fibrocartilage. Collectively, these findings provide new insights into how knee joint tissues form to better guide cell- and biomaterial-based repair of articular cartilage, meniscus and potentially other load-bearing cartilaginous tissues. STATEMENT OF SIGNIFICANCE: Successful regeneration of articular cartilage and meniscus is challenged by incomplete knowledge of early events that drive the initial formation of the tissues' extracellular matrix in vivo. This study shows that articular cartilage initiates with a pericellular matrix (PCM)-like primitive matrix during embryonic development. This primitive matrix then separates into distinct PCM and territorial/interterritorial domains, undergoes an exponential daily stiffening of ≈36% and an increase in micromechanical heterogeneity. At this early stage, the meniscus primitive matrix shows differential molecular traits and exhibits a slower daily stiffening of ≈20%, underscoring distinct matrix development between these two tissues. Our findings thus establish a new blueprint to guide the design of regenerative strategies to recapitulate the key developmental steps in vivo.