The contribution of specific type I collagen remodeling in angiogenesis was studied in vivo using a quantitative chick embryo assay that measures new blood vessel growth into well-defined fibrillar collagen implants. In response to a combination of basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF), a strong angiogenic response was observed, coincident with invasion into the collagen implants of activated fibroblasts, monocytes, heterophils, and endothelial cells. The angiogenic effect was highly dependent on matrix metalloproteinase (MMP) activity, because new vessel growth was inhibited by both a synthetic MMP inhibitor, BB3103, and a natural MMP inhibitor, TIMP-1. Multiple MMPs were detected in the angiogenic tissue including MMP-2, MMP-13, MMP-16, and a recently cloned MMP-9-like gelatinase. Using this assay system, wild-type collagen was compared to a unique collagenase-resistant collagen (r/r), with regard to the ability of the respective collagen implants to support cell invasion and angiogenesis. It was found that collagenase-resistant collagen constitutes a defective substratum for angiogenesis. In implants made with r/r collagen there was a substantial reduction in the number of endothelial cells and newly formed vessels. The presence of the r/r collagen, however, did not reduce the entry into the implants of other cell types, that is, activated fibroblasts and leukocytes. These results indicate that fibrillar collagen cleavage at collagenase-specific sites is a ratelimiting event in growth factor-stimulated angiogenesis in vivo. IntroductionAngiogenesis is the process by which the preexisting vascular tree gives rise to new blood vessels. This process is tightly regulated during development and occurs only under highly specialized circumstances in the normal adult animal. Unregulated angiogenesis may be a pivotal element of disease etiology, such as during tumor growth and atherosclerosis. 1,2 All forms of angiogenesis, however, are thought to share certain basic features, including migration and mitogenesis of endothelial cells, lumen formation, connection of new vascular segments with the preexisting circulation, and extensive remodeling of the extracellular matrix by proteases. The detailed mechanisms by which proteolytic enzymes, such as the matrix metalloproteinases (MMPs), mediate some of these events in vivo remain unclear. 3,4 The MMPs constitute a large family of zinc-dependent endopeptidases that have been strongly implicated in both normal angiogenesis and tumor vascularization. 2,4 These enzymes are characteristically regulated at multiple levels, including gene expression, spatial localization, zymogen activation, and inhibition by the tissue inhibitors of metalloproteinases (TIMPs). 5 In vitro remodeling of extracellular matrices by cultured endothelial cells has consistently been shown to rely on MMPs. 4 However, it is unclear which aspects of cell culture models for angiogenesis may be directly extrapolated to the animal. In vivo studies also have foun...
The growing body of empirical evidence for sympatric speciation has been complemented by recent theoretical treatments that have identi¢ed evolutionary conditions conducive to speciation in sympatry. The Neotropical Midas cichlid (Amphilophus citrinellum) ¢ts both of the key characteristics of these models, with strong assortative mating on the basis of a colour polymorphism coupled with trophic and ecological di¡erentiation derived from a polymorphism in their pharyngeal jaws. We used microsatellite markers and a 480 bp fragment of the mitochondrial DNA control region to study four polymorphic populations of the Midas cichlid from three crater lakes and one large lake in Nicaragua in an investigation of incipient sympatric speciation. All populations were strongly genetically di¡erentiated on the basis of geography. We identi¢ed strong genetic separation based on colour polymorphism for populations from Lake Nicaragua and one crater lake (Lake Apoyo), but failed to ¢nd signi¢cant genetic structuring based on trophic di¡erences and ecological niche separation in any of the four populations studied. These data support the idea that sexual selection through assortative mating contributes more strongly or earlier during speciation in sympatry than ecological separation in these cichlids. The long-term persistence of divergent cichlid ecotypes (as measured by the percentage sequence divergence between populations) in Central American crater lakes, despite a lack of ¢xed genetic di¡erentiation, di¡ers strikingly from the patterns of extremely rapid speciation in the cichlids in Africa, including its crater lakes. It is unclear whether extrinsic environmental factors or intrinsic biological di¡erences, e.g. in the degree of phenotypic plasticity, promote di¡erent mechanisms and thereby rates of speciation of cichlid ¢shes from the Old and New Worlds.
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