Novel articular cartilage structure in the South American opossum, Monodelphis domestica.

Articular cartilage provides smooth surfaces for low-friction, unrestricted movement of opposing skeletal elements. The surface topography of articular cartilage has been the subject of numerous studies and, with few exceptions, is considered to be smooth (at least at the light microscopic level). Some studies have reported 'humps' on the articular surface which have been related to underlying chondrocytes residing very close to the surface. Here we report on a highly nodular form of articular cartilage in the distal limb joints of the South American opossum, Monodelphis domestica. Unlike previous reports, these articular 'humps' are visible under a dissecting microscope. Each 'hump' or 'nodule' represents the surrounding matrix of single or sometimes paired rounded chondrocytes. Flattened chondrocytes normally associated with mammalian articular cartilage were absent from these joints. Interestingly, the articular cartilage of the more proximal limb joints such as the knee showed more typical features of articular cartilage including flattened superficial chondrocytes.