The development of articular cartilage: evidence for an appositional growth mechanism
It is well-established that cartilage grows by a combination of matrix secretion, cell hypertrophy and cell proliferation. The extent to which this growth is by appositional, as opposed to interstitial mechanisms, however, remains unclear. Using the knee joints of the marsupial Monodelphis domestica to study cartilage growth, we have combined an immunohistochemical study of the TGF-β family of cartilage growth and differentiation factors between 30 days postpartum to 8 months, together with a stereological analysis of cartilage morphology during growth. Furthermore, to gain an insight into the generation of the characteristic zones within cartilage, we have examined the effects of intra-articular administration of bromodeoxyuridine, an agent that is incorporated into DNA during cell division and blocks further cell cycling. During early growth, TGF-β2 and -β3 were widely expressed but TGF-β1 was less so. After the formation of the secondary centre of ossification, all isoforms became more restricted to the upper half of the tissue depth and their distribution was similar to that previously described for IGFs, and PCNA-positive cells. Stereological analysis of tissue sections from the femoral condylar cartilage at 3 and 6 months showed that there 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 analysed 1 and 4 months post-injection, revealed that the chondrocytes occupying the transitional zone were depleted 1 month post-injection, resulting in thinning of the articular cartilage. This effect was reversed 4 months post-injection. Immunohistochemical analysis revealed that BrDU-treatment altered the expression patterns of all TGF-β isoforms, with a marked reduction in labelling of TGF-β1 and -β3 isoforms in the upper half of the cartilage depth. Overall, the data lends further support to the notion of articular cartilage growing by apposition from the articular surface rather than by interstitial mechanisms.
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