NOVEL ARTICULAR-CARTILAGE STRUCTURE IN THE SOUTH-AMERICAN OPOSSUM, MONODELPHIS-DOMESTICA

Articular cartilage provides smooth surfaces for low-friction, unrestr icted movement of opposing skeletal elements. The surface topography o f articular cartilage has been the subject of numerous studies and, wi th few exceptions, is considered to be smooth (at least at the light m icroscopic level). Some studies have reported 'humps' on the articular surface which have been related to underlying chondrocytes residing v ery close to the surface. Here we report on a highly nodular form of a rticular cartilage in the distal limb joints of the South American opo ssum, Monodelphis domestica. Unlike previous reports, these articular 'humps' are visible under a dissecting microscope. Each 'hump' or 'nod ule' represents the surrounding matrix of single or sometimes paired r ounded chondrocytes. Flattened chondrocytes normally associated with m ammalian articular cartilage were absent from these joints. Interestin gly, the articular cartilage of the more proximal limb joints such as the knee showed more typical features of articular cartilage including flattened superficial chondrocytes.