The development of articular cartilage: evidence for an appositional growth mechanism

It is well-established that cartilage grows by a combination of matrix secr etion, cell hypertrophy and cell proliferation. The extent to which this gr owth is by appositional, as opposed to interstitial mechanisms, however, re mains unclear. Using the knee joints of the marsupial Monodelphis domestica to study cartilage growth, we have combined an immunohistochemical study o f the TGF-beta family of cartilage growth and differentiation factors betwe en 30 days postpartum to 8 months, together with a stereological analysis o f cartilage morphology during growth. Furthermore, to gain an insight into the generation of the characteristic zones within cartilage, we have examin ed the effects of intra-articular administration of bromodeoxyuridine, an a gent that is incorporated into DNA during cell division and blocks further cell cycling. During early growth, TGF-beta2 and -beta3 were widely express ed but TGF-beta1 was less so. After the formation of the secondary centre o f ossification, all isoforms became more restricted to the upper half of th e tissue depth and their distribution was similar to that previously descri bed for IGFs, and PCNA-positive cells. Stereological analysis of tissue sec tions from the femoral condylar cartilage at 3 and 6 months showed that the re was a 17% increase in total cartilage volume but a 31% decrease in cell density on a unit volume basis. Finally, cell-cycle perturbation with BrDU, which was injected into the knee joints of 3-month-old animals and analyse d 1 and 4 months post-injection, revealed that the chondrocytes occupying t he transitional zone were depleted 1 month post-injection, resulting in thi nning of the articular cartilage. This effect was reversed 4 months post-in jection. Immunohistochemical analysis revealed that BrDU-treatment altered the expression patterns of all TGF-beta isoforms, with a marked reduction i n labelling of TGF-beta1 and -beta3 isoforms in the upper half of the carti lage depth. Overall, the data lends further support to the notion of articu lar cartilage growing by apposition from the articular surface rather than by interstitial mechanisms.