The role of superficial layer cells in maintenance of articular cartilage surface integrity

The superficial layers (surface and sub-surface) of human articular cartilage represent specialized joint tissues with cellular and matrix compositions different to the deeper layers. The distinct properties of the superficial layers confer specialized functions and also provide a frontline barrier to degenerative insult (cellular, soluble, biomechanical) from the joint space. The design of interventions to protect/regenerate the superficial layers in the earliest stages of joint pathology will require that the details of cellular phenotype and matrix turnover in this location are more fully understood. In normal human joints (Grade 0), single cells in the surface layer, and the next 2–3 cell layers (down to about 20 μm) appear fibroblast-like and flattened parallel to the surface, unlike deeper cells (20–200 μm from surface), which are more oval and can be present in small groups.