Perichondrial and Endochondral Components of Mandibular Condylar Growth: Morphometric and Autoradiographic Quantitation in Rats

In an attempt to determine the contribution of the perichondrial and endochondral components to rapid and slow overall condylar growth, mandibular condyles from rats aged 21 and 45 d as well as 3 and 6 months were examined by light microscopy. A morphometric analysis served to estimate the size of cells and the amount of extracellular matrix per cell in the various tissue layers beneath the articular surface. Quantitative evaluation of autoradiographic labelling due to the presence of [3H]-thymidine, [3H]-proline, and [35S]-sulphate 1.5 or 2 h and 5 d after injection of the tracer was used to determine the velocity of overall condylar growth. A combination of the two types of results allowed the calculation of daily rates in (1) the generation of new chondrocytes, constituting the perichondrial growth component, and the endochondral components comprising (2) net accumulation of extracellular matrix in the cartilage and (3) enlargement of the chondrocytes. It was observed that the 3 factors contributed, respectively, by about 10-25, 30 and 45-60% to rapid growth and by about 75, 15 and 10% to slow growth. Thus when growth slowed, the contribution of chondrocyte enlargement declined markedly and that of matrix accumulation slightly, while the contribution of new cell generation increased correspondingly. Declines in chondrocyte enlargement and in accumulation of cartilage matrix that was related mainly to decreasing proteoglycan formation were in good agreement with age-associated variation in weight gain, while peak rates in generation of new chondrocytes appeared to be delayed against peak somatic growth. It is concluded that (1) endochondral components contribute most to particularly rapid condylar growth, exceeding the perichondrial component, and (2) endochondral components of condylar growth are regulated by systemic factors that also control somatic growth.