Anatomy of the chick chorioallantoic membrane relevant to its use as a substrate in bioassay systems

Scanning electron microscopy was used in this investigation to characterize and contrast the chorionic surface of 11 day chick chorioallantoic membranes (CAM) prepared by either the classical dropping technique or the Zwilling technique. The structure of the chorionic blood sinus and its relationship to the chorionic ectoderm were also examined. The Zwilling CAM has been proposed as the substrate in an assay being developed by investigators in this laboratory, an assay which is intended as an alternative to the Draize Rabbit Eye Irritancy Test. The purpose of this study was to define the substrate. The results show considerable uniformity of the Zwilling CAM surface in comparison to the dropped CAM. Broad areas of exposed basement membrane were an unexpected finding on the surface of the dropped CAM. It appears that this basement membrane is made up of more than just the thin discontinuous basal lamina seen with the transmission electron microscope. The findings of this study are pertinent to chick embryo physiology as well as to toxicologic assays and tumor cell assays which utilize the CAM.