Background: An investigation of matrix metalloproteinase-9 (MMP-9)
and its influence on vascular invasion in the secondary ossification center at
the chondroepiphysis of developing long bones was undertaken. The effect of
MMP-9 was compared with that of basic fibroblast growth factor (b-FGF), a
potent angiogenic factor, and we assessed the chorioallantoic membrane (CAM)
culture as a model for angiogenesis in osteochondral tissue.
Methods: Seventy-two femoral and seventy-two humeral heads of
thirty-six four-day postnatal rabbits were dissected immediately after each
animal was killed. Solutions of MMP-9, b-FGF, and phosphate-buffered saline
solution were applied, and the femoral and humeral chondroepiphyseal explants
were incubated for ten days in CAM culture. This was used as an in vivo model
to investigate the growth of blood vessels into the femoral and humeral heads
of the neonatal rabbit. The explants were harvested from the CAM culture and
analyzed histologically. A three-day incubation was also performed to look for
early signs of vascular ingrowth into the cartilage matrix.
Results: One hundred and twenty epiphyses from thirty rabbits were
placed onto CAM culture successfully; of these, two were harvested at three
days to assess early changes and 118 were harvested at ten days. Forty of the
118 cultures were still viable when harvested after ten days, giving a 33%
yield. Both MMP-9 and b-FGF caused an increased vascular invasion into the
chondroepiphysis. New blood vessels derived from the chorioallantoic membrane
within cartilage canals were more numerous in MMP-9 treated epiphyses, and
larger canals were more commonly seen when compared with a control group.
Conclusions: These findings confirmed that b-FGF is angiogenic at
the chondroepiphysis. Matrix metalloproteinase-9 appears to be implicated in
vascular invasion and induces the formation of new cartilage canals at the
chondroepiphysis. The CAM culture model was a useful model for investigating
angiogenesis in osteochondral tissue.
Clinical Relevance: This study adds to the understanding of the
complex biochemical interaction that occurs in cartilage when the advancing
vasculature begins growing into the chondroepiphysis. A better knowledge of
this angiogenic process will enable a better understanding of the pathological
failure or disturbance of vasculogenesis, which results in dysplastic growth
disorders and osteonecrosis.