Perspectives on intrinsic articular cartilage interfaces

According to the current concept the joint is an organ composed of multiple tissues forming manifold communicating interfaces. Moreover, cartilage itself presents typical intrinsic zonal interfaces. Since the healthy joint cartilage is not vascularized, soluble mediators are distributed by the synovia fluid flow in the cartilage extracellular matrix (ECM) of the joint compartments. This distribution is dependent on intermittent mechanical loading and water flow in the joint. Hence, soluble factors play a pivotal role in articular cartilage interfacial communication. These mediators must traverse the cartilage ECM, whose permeability and binding capacity for growth factors and signaling molecules vary according to its structural integrity (e.g. pathology) and homeostasis.

Cartilage lesions, e.g., due to osteoarthritis (OA), substantially alter the interfacial communication. Chondrocyte phenotype, subpopulations and cartilage ECM density changes in OA. Superficial zones, and during disease progression, deep zones, are lost over time. OA is closely linked to metabolic disorders like type 2 diabetes mellitus (T2DM), characterized by hyperglycemia. Elevated glucose levels promote aberrant glycosylation of cellular and ECM glycoproteins, formation of advanced glycation end products (AGEs), excessive ECM cross-linking, reduced elasticity, and chondrocyte aging associated with the senescence-associated secretory phenotype (SASP). Cartilage immunobiology comprises a dysregulation of complement split fragments and pro-/anti-inflammatory mediators: their release influences chondrocyte phenotype.

Intrinsic cartilage interface communication changes in joint cartilage disorders such as OA and associated systemic diseases. The role of immunobiological complement factors and pleiotrophic cytokines is still to be elucidated in articular cartilage in vivo and in OA patients.