Microstructural and elastic properties of the extracellular matrices of the superficial zone of neonatal articular cartilage by atomic force microscopy.

The structural and mechanical properties of the superficial zone of articular cartilage are not well understood. Most previous studies have focused on the overall properties of articular cartilage in the adult. In the present work, the extracellular matrices of the superficial zone of the jaw-joint condyle in the 7-day-old rabbit were subjected to dynamic indentation with atomic force microscopy (AFM). The surface topography of four equally divided regions of the entire articular surface lacked substantial variations, with mean roughness from 95.4 nm (+/- 28.0) to 130.1 nm (+/- 13.8). Indentations of the articular surface and the microdissected, orthogonal transverse surface revealed a narrow distribution of Young's moduli ranging from 0.92 MPa (+/- 0.12) to 1.02 MPa (+/- 0.22). These rather uniform structural and mechanical properties of the superficial zone of the neonatal articular cartilage are in contrast to our previous finding of a gradient distribution of Young's moduli of the superficial zone of adult articular cartilage from 0.95 (+/- 0.06 MPa) to 2.34 (+/- 0.26 MPa) (Hu et al.: J Struct Biol 2001:136:46-52), indicating that the mechanical properties of the articular surface are modified during development. Thus, articular cartilage's anisotropic mechanical properties may be specific to the adult, rather than the neonatal. It is further postulated that the structural and mechanical properties of the superficial zone of articular cartilage are regulated by chondrocytes in addition to their unidirectional development pathway toward subchondral bone formation.