PAST LETTERS FROM THE PRESIDENT
Melton JTK, Clarke NMP, Roach HI.
University Orthopaedic Department, Southampton University Hospital.
Chondro-epiphyseal cartilage is generally resistant to vascular invasion. At the time of formation of the secondary ossification centre in skeletal ‘long’ bones, the anti-angiogenic nature of cartilage is altered in favour of angiogenesis and vascular invasion takes place. We studied the control of this angiogenic ‘switch’ by experimentally investigating two factors which might influence vascular invasion. MMP 9 is a 92Kda gelatinase which degrades collagen types IV, V and X and gelatine (denatured collagen). It has been implicated in the control of endochondral ossification at the growth plate and has been shown to modulate endothelial cell morphogenesis. Basic Fibroblast Growth Factor (b-FGF) is a cytokine with well established angiogenic capability and has also been implicated in the development of the growth plate. We investigated whether MMP-9 caused an effect on the development of the vasculature of the chondro-epiphysis of neo-natal rabbits and compared this with the effects of b-FGF.
CAM Culture consists of placing a small tissue explant onto the the chorioallantoic membrane of 10 day old chick embryo and continuing culture for a further 10 days. CAM derived vessels will invade the tissue, unless anti-angiogenic factors are present. Hence CAM culture is used as an assay system for angiogenesis and factors that will influence it. We utilised the CAM culture model to investigate vascular in-growth into explants of femoral and humeral heads from 4 day postnatal rabbits to test the influence of MMP-9 and b-FGF. A small nylon membrane, pre-soaked in a solution containing the factor, was placed onto a tangential cut across the perichondrium. The explant was then cultured on the CAM for 3-10 days.
In control epiphyses, the in-growth of CAM derived blood vessels was rare and invasion of cartilage canals through the perichondrium seldom occurred, thus confirming the anti angiogenic nature of epiphyseal cartilage. The initial presence of MMP 9 caused a tremendous increase in the de novo vascular invasion. MMP 9 treated epiphyses contained numerous and large cartilage canals. In b-FGF treated epiphyses, a greater level of vascular in-growth was seen compared with controls, but this was not as marked as with MMP 9.
Our findings indicate that b-FGF and perhaps, more interestingly, MMP-9 are implicated in the activation of the angiogenic ‘switch’ at the chondro-epiphysis leading to a vascular invasion. The fact that MMP-9 can act as a stimulator on angiogenesis is a novel finding. The mechanism of action remains unclear although it is possible that it is involved in the deactivation of inhibitors of vasculogenesis or the activation of angiogenic factors or indeed both.