The two- and three-dimensional structure of the microcirculation of the chick chorioallantoic membrane

The chick chorioallantoic membrane (CAM) is a common model for studying biological processes, but descriptions of the CAM circulatory system and especially experimental preparations of the CAM in shell-less eggs are both scant and controversial. We studied the CAM structure and the three-dimensional spatial configuration of the CAM vessels using five methods: in vivo stereoscopic observations, whole-mount preparations, histological sections, corrosion cast microinjection techniques, and the reconstruction of a three-dimensional wax model. Our findings show that the CAM consists of a superficial two-dimensional layer composed of a network of a very dense capillary mesh, floating over and enclosing a deeper three-dimensional space in which medium and larger free-floating vessels are seen to supply and drain the superficial layer. It is interesting to note that no tips or sprouts of blood vessels were observed during the development of the CAM vessels. In a shell-less egg preparation, the capillaries were found in the mesoderm layer of the CAM and not in or superficial to the ectoderm as is the case in the CAM of the intact egg which adheres to the shell membrane.