IMPLEMENTATION OF INTUSSUSCEPTIVE MICROVASCULAR GROWTH IN THE CHICKENCHORIOALLANTOIC MEMBRANE (CAM) .2. PILLAR FORMATION BY CAPILLARY FUSION

Intussusceptive microvascular growth (IMG) is a new mechanism of capil lary growth: The vascular network expands by insertion of newly formed columns of interstitial tissue (interstitial tissue structures) into the vascular lumen called tissue pillars or posts (diameter: 0.5-2.5 m u m). IMG has so far been described during organ development and growt h and in tumor angiogenesis. Different modes of its implementation cou ld be demonstrated in the rat lung and the chicken chorioallantoic mem brane (CAM). In the present investigation a further mechanism of IMG i s reported in the chicken CAM: tissue pillars form by splitting of lar ger interstitial tissue structures and intercapillary walls located be tween neighboring capillary segments which will consecutively fuse. Sp litting is dependent on the existence of a pillar's core composed of a bundle of collagen fibrils ensheathed by extensions of endothelial-li ke cells inside these structures. Pillar cores thus represent the smal lest unit of interstitial tissue around which the vascular lumen might expand. This mode of IMG is obviously connected to physiological remo deling of the capillary network and appears to be dominant during late r stages of CAM development. (C) 1997 Academic Press.