IMPLEMENTATION OF INTUSSUSCEPTIVE MICROVASCULAR GROWTH IN THE CHICKENCHORIOALLANTOIC MEMBRANE (CAM) .1. PILLAR FORMATION BY FOLDING OF THECAPILLARY WALL

Intussusceptive microvascular growth is a new mode of capillary networ k growth originally described in the lungs of rabbits and rats. It con stitutes an alternative to endothelial sprouting. The capillary networ k grows by insertion of new intercapillary meshes with dimensions arou nd 1.5 mu m called tissue pillars or posts. In a recent investigation, growth by intussusception was demonstrated in the chicken chorioallan toic membrane (CAM). In the present study the first of several modes o f its implementation can now be presented in the CAM by in vivo video microscopy and analyses of light and electron microscopic serial secti ons: Cores of tissue pillars containing collagen fibrils ensheathed by extensions of endothelial-like cells will form within the tips of ver tically running tissue folds that project into the capillary lumen. Du e to retraction of tissue toward the intercapillary space the fold is thinning. Finally, the pillar's core is connected to its fold by a ver y slender extension of a single endothelial cell. Cell membrane fusion within that slender membrane-like structure causes subsequent separat ion of the pillar from its fold throughout an increasing vertical dist ance. This mechanism allows for expansion of the capillary network int o the surrounding tissue, leaving behind tissue pillars as remnants of folds. (C) 1996 Academic Press, Inc.