The chorioallantoic capillary plexus of the chicken egg: a microvascular corrosion casting study.

The chorioallantoic membrane of the avian egg serves as the principal organ of respiratory gaseous exchange for the embryo until close to hatching. It lies closely apposed to the inner shell membrane and contains an extremely dense capillary plexus supplied by the allantoic blood vessels. This study applied the microvascular corrosion casting technique to investigate the three-dimensional arrangement of the plexus at various stages of incubation. Casts were produced between days 6 and 14 of incubation, and their appearances were compared with those obtained from traditionally sectioned material and from freeze-cleaved specimens. By day 6 the capillary network was remarkably profuse but showed considerable regional variation in vessel density. In some areas there were only short capillary buds whereas in other areas fusion had taken place so that a true plexus was formed. By day 10 the capillaries had become confluent to such a degree that the cast consisted of a thin sheet of resin perforated only by an array of small irregularly shaped orifices. These corresponded closely in size to the intervening columns of chorionic epithelial cells seen in the sectioned material. It is clear from the appearances of the casts that the capillary surface density becomes maximal at approximately day 10 of incubation. From then on in incubation any increase in the diffusing capacity of the chorioallantoic membrane must be the result of either an increase in its overall surface area, or a decrease in the thickness of the air-blood barrier.