Furthermore, the proliferative zones of the grow th plate became disorgan ized af ter age wk in null mice relative to thewildtype mice. Major defects were also seen in the development and shape of the secondaryNintedanib ossification centers. The secondaryossification centers that form af ter birth developed poorly in the null mice.For example, at age wk, the secondaryossification nuclei in the distal femur and prox imal tibia were absent in the null mice. Instead, there was a mass of hypertrophic chondrocy tes surrounded by chondroid matrixwith streaks of osteoid deposition.In contrast towildtype mice, no blood vessel formation wasvisible in the null mice, as shown by lack of stain ingwith antibodies against PECAM, except at the insertion of the cruciate ligaments where some vascular invasion of the cartilage occurred.At the chondroosseous junction, blood vessels and osteoclasts werevisible in bothwildtype and null mice.Immunostain ingwith antibodies against PECAM, an endothelialspecific marker, did not show sign ificant dif ferences in the ex isting blood vessels in terms of density, orientation, or dilation. The structure and composition of the bone marrow appeared to be histologically normal. Immunostain ingwith a polyclonal antibody to tartrateresistant acid phosphatase identified no dif ferences in the number of osteoclasts between null andwildtype mice. In both types of mice, osteoclasts were seen adjacent to the last intact transverse septum of the hypertrophic zones.The primarymetaphyseal trabeculae appeared normal in size and composition.The enlarged hypertrophic zone in the grow th plates in mutant mice and the fact that MTMMP is highly expressed in perichodrium led us to assess whether the absence of the enzyme disturbs the regulation of chondrocy te dif ferentiation and maturation.This indicates that activation of proMMP is impaired in the knockout mice, and that MTMMP is a major activator of proMMP in vivo.The delayed vascularization in the secondaryossification nuclei of long bones in null mice prompted us to explore whether the absence of MTMMP af fects angiogenesis.To examine this in vivo, a corneal micropocket assay was carried out in wkold mice.Micropellets of aluminum sulfate coatedwith the slow release polymerhydron contain ing human FGF were surgically implanted into the cornea of dayold null mice and theirwildtype littermates, and the angiogen ic response was examined days later. Complete absence of blood vessel grow th was obser ved in all five mice tested until their natural death at around wk of age.Here, we also describe additional studies to define a defect in vascular invasion of cartilage, both during endochondral ossification of grow th plates and in secondaryossification centers of Amifostine chondroepiphyses.Furthermore, our studies using an in vivo corneal assay demonstrated that this abnormality may be part of a more generalized defect in postnatal angiogenes in MTMMP deficiency.The present work also demonstrated that activation of proMMP in vivo is impaired in the absence of MTMMP.The specific enlargement of the hypertrophic zones obser ved in our null mice is attributable to delayed resorption of cartilage during endochondral ossification.A lthough both MMP and MTMMP appear to be independently required for resorption of calcified cartilage at the advancing ossification front, MTMMP, but not MMP, regulates cell proliferation in the grow th plate through an as yet unknown mechan ism.