Quantitative study of intussusceptive capillary growth in the chorioallantoic membrane (CAM) of the chicken embryo.

In an attempt to sort out the respective contributions of sprouting and intussusceptive microvascular growth (IMG) during chicken chorioallantoic membrane (CAM) development, we analyzed the morphology and the quantitative growth of the capillary bed of the CAM by light microscopy. By perfusing the CAM microvasculature with highly concentrated colloidal gold particles, the capillaries could be unambiguously distinguished from the surrounding unlabelled tissue. This allowed us to identify, count and measure the intercapillary tissue profiles. By means of morphometric analysis we could show that CAM angiogenesis undergoes three phases of development. In an early phase, from Day 5 to Day 7, the major mechanism of capillary network growth is sprouting. In an intermediate phase, from Day 8 to Day 12, IMG is prevailing, and at Days 13 and 14, CAM structure is undergoing expansion with only a small increase in complexity. These findings are important in view of experimental protocols using the CAM as a model for testing angiogenetic factors. Indeed, care has to be taken not to misinterpret normal age-dependent alterations of the CAM vascular architecture as specific responses to tested agents.