The chorioallantoic membrane (CAM) is a structure that develops outside the embryo within the eggs of amniotes like birds and reptiles. This temporary membrane is a feature of development within a shelled egg, serving as a bridge between the growing organism and the external environment. It serves its purpose during incubation and ceases to function once the animal hatches.
Formation and Structure
The chorioallantoic membrane originates from the joining of two separate tissues: the chorion and the allantois. The chorion is the outermost membrane, while the allantois begins as an extension from the embryo’s hindgut. In the chicken egg, this fusion process starts when the allantois expands and makes contact with the chorion, around the fourth day of embryonic development. Over the next several days, these two structures completely merge their tissues.
This fusion creates a unified membrane with three layers: an outer chorionic epithelium, a middle mesodermal layer, and an inner allantoic epithelium. The middle layer, formed from the mesoderm of both original membranes, becomes extensively vascularized. This results in a dense network of blood vessels that spreads throughout the CAM.
By approximately day 12 of incubation in a chick embryo, this highly vascularized membrane fully lines the inside of the eggshell. This proximity to the shell is important for its function. The CAM’s structure acts as a living, vascular lining for the egg, positioning the embryo’s circulatory system close to the air that permeates the porous shell.
Biological Functions in Embryonic Development
A primary role of the chorioallantoic membrane is to act as the embryo’s respiratory organ. The eggshell contains thousands of microscopic pores that allow for the passage of gases. The CAM’s rich blood supply is positioned directly beneath these pores, allowing for efficient gas exchange. Oxygen from the outside environment diffuses across the shell and membrane into the embryonic bloodstream, while carbon dioxide produced by the embryo’s metabolism diffuses out. This system functions as the embryo’s lung until it develops its own pulmonary respiration just before hatching.
The allantoic portion of the membrane also serves as a storage site for metabolic waste. As the embryo develops, it generates nitrogenous wastes, primarily uric acid. This substance is shunted into the allantoic sac, a cavity within the CAM, where it is sequestered away from the embryo to prevent toxic buildup.
Another function of the CAM is transporting calcium to the embryo for skeletal development. The eggshell is a major source of this calcium. Specialized cells in the CAM’s chorionic epithelium secrete protons that dissolve the inner eggshell, allowing the liberated calcium ions to be absorbed and transported to the embryo via its vascular network.
The CAM Model in Scientific Research
The chorioallantoic membrane has become a widely used model in biomedical research due to its unique biological properties. The model is cost-effective, provides relatively quick results compared to other animal models, and is useful for:
- Xenograft studies: The CAM is naturally immunodeficient, as the embryonic immune system is not fully developed. This means it will not reject foreign tissues or cells grafted onto it, making it an effective platform for studying implanted human or other animal tissues.
- Angiogenesis research: Its extensive and rapidly developing vascular network makes the CAM an ideal system for studying the formation of new blood vessels. Researchers can apply substances to the membrane and directly observe their effects on blood vessel growth in real-time.
- Cancer research: Cancer cells can be grafted onto the membrane, where they form tumors and invade surrounding tissue. Scientists can then observe these processes and test the efficacy of anti-cancer drugs on a living tumor within a complex biological system.
- Toxicology and biomaterial testing: The toxicity of new drugs or nanoparticles can be assessed by observing their impact on the membrane and the embryo. Similarly, the biocompatibility of new materials for medical implants can be evaluated by placing them on the CAM.
Comparison to Human Placental Development
Humans and other eutherian mammals do not form a chorioallantoic membrane like birds and reptiles. Instead, the homologous structures, the chorion and allantois, follow a different path to form the placenta. While the CAM is an external system, the human placenta is an organ that develops inside the mother’s uterus.
In human development, the chorion expands to form the fetal portion of the placenta. It develops structures called chorionic villi, which invade the uterine wall to establish a close connection with the maternal blood supply. This interface allows for nutrient, gas, and waste exchange between mother and fetus.
The allantois in humans has a much-reduced role and does not expand into a large sac for waste storage. Instead, its blood vessels contribute to forming the umbilical cord, creating the vascular link between the fetus and the placenta. Parts of the allantois are also involved in the development of the urinary bladder.