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We have histologically examined vascular invasion and calcification of the hypertrophic zone during endochondral ossification in matrix metalloproteinase (MMP)-9 deficient (MMP-9-/-) mice and in their littermates at 3 days, 3 weeks and 6 weeks after birth. Capillaries and osteoclasts at the chondro-osseous junction showed an intense MMP-9 immunopositivity, suggesting that they recognize chemical properties of cartilaginous matrices, and then release MMP-9 for cartilage degradation. CD31-positive capillaries and tartrate-resistant acid phosphatase-reactive osteoclasts could be found in the close proximity in the region of chondro-osseous junction in MMP-9-/- mice, while in wild-type mice, vascular invasion preceded osteoclastic migration into the epiphyseal cartilage. Although MMP-9-/- mice revealed larger hypertrophic zones, the index of calcified area was significantly smaller in MMP-9-/- mice. Interestingly, the lower layer of the MMP-9-/- hypertrophic zone showed intense MMP-13 staining, which could not be observed in wild-type mice. This indicates that MMP-13 may compensate for MMP-9 deficiency at that specific region, but not to a point at which the deficiency could be completely rescued. In conclusion, it seems that MMP-9 is the optimal enzyme for cartilage degradation during endochondral ossification by controlling vascular invasion and subsequent osteoclastic migration.

Histochemical aspects of the vascular invasion at the erosion zone of the epiphyseal cartilage in MMP-9-deficient mice