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Fig. 10. (A) Schematic representation of the rib ossification process. At embryonic
day 12, the entire rib consists of cartilage. Starting at embryonic day 14, a
growth plate is established and ossification begins, proceeding ventrally
towards the sternum. The ossification process continues until the mice reach
adulthood and an area of permanent costochondral cartilage (arrow) remains
throughout life. Red rectangle indicates area represented in B,C. Cartilage is
blue and bone is brown. (B) Schematic representation of rib growth plate and
chondrocyte differentiation. (a) Around E14, the rib cartilage template
becomes invaded by blood vessels, osteoblasts and osteoclasts, and a growth
plate is established. Chondrocytes, which are not part of the growth plate,
express Col2. (b) When mice reach the age of 2 weeks, chondrocytes in the
center of the rib cartilage become hypertrophic and express Col10 (red arrow).
(c) When mice reach skeletal maturity, this hypertrophic region expands
towards the periphery of the perichondrium, while the growth plate becomes
smaller and replacement of cartilage with bone stops. (C) Schematic
representation of rib chondrocyte differentiation in Ext2 heterozygotes. (a)
During early stages of development, no differences can be observed in between
rib cartilage of wild-type and Ext2+/– mice. (b) By
the age of 2 weeks, however, nodules of Col10-expressing chondrocytes are
being formed near the perichondrium as a result of premature hypertrophic
differentiation. (c) Formation of an exostosis could be the result of the
growth plate moving past a nodule, creating a disruption in bone collar
formation. Alternatively, the passage of the growth plate might provide
vascularization to the nodule and initiate the bony outgrowth of an exostosis.
D, dorsal; V, ventral.
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