Immunohistochemical Identification of an Extracellular Matrix Scaffold that Microguides Capillary Sprouting In Vivo

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Abstract
To gain insight into how a naturally occurring scaffold composed of extracellular matrix (ECM) proteins provides directional guidance for capillary sprouting, we examined angiogenesis in whole-mount specimens of rat mesentery. Angiogenesis was studied in response to normal maturation, the injection of a mast cell degranulating substance (compound 48/80), and mild wounding. Confocal microscopy of specimens immunolabeled for elastin revealed a network of crosslinked elastic fibers with a density of 140.8 ± 37 mm of fiber/mm2 tissue. Fiber diameters ranged from 180 to 1400 nm, with a mean value of 710 ± 330 nm. Capillary sprouts contained CD31- and OX-43-positive endothelial cells as well as desmin-positive pericytes. During normal maturation, leading endothelial cells and pericytes were in contact and aligned with an elastic fiber in ≃80–90% of all sprouts. In wounding and compound 48/80-treated specimens, in which angiogenesis was markedly increased, leading endothelial cells remained in contact and aligned with elastic fibers in ≃60–80% of all sprouts. These observations indicate that elastic fibers are used for endothelial and pericyte migration during capillary sprouting in rat mesentery. The composition of this elastic fiber matrix may provide important clues for the development of tissue-engineered scaffolds that support and directionally guide angiogenesis.