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Skeletons grow through three phases of chondrocyte enlargement
DOI:10.1038/bonekey.2013.113
The differences in skeletal size between different vertebrate species are partly due to variation in bone length. Differential increases in length occur as the volume of hypertrophic chondrocytes within the growth plate increases dramatically during development, around the point of terminal differentiation. In this study, Cooper et al. used quantitative phase microscopy to study volume increase in mammalian chondrocytes.
Three definite phases of volume increase were observed. The first is characterized by a 3 × increase in cell volume due to hypertrophic cell enlargement. In the second, cells swell to four times their original size. In the final phase, increases in fluid volume and dry mass cause the cells to double in size again.
It is this final phase that varies most significantly between growth plates that are elongating either slowly or rapidly. Mice with a knockout for the gene that codes for insulin-like growth factor 1 (Igf1) completed the first two phases but failed to progress into phase 3, which resulted in much lower bone elongation, demonstrating the mechanism through which Igf1 controls bone growth.
Editor’s comment: The volume increase in rodent chondrocytes is rapid, lasting only about 12 h; this rate of cell height increase in growth plates can reflect a circadian secretion of growth hormone that is pulsating, whereas increments of bone lengths are irregular (saltatory).
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