The Chorioallantoic Membrane (CAM), also known as the chorioallantois, is a highly vascularized

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

found in the

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

of certain

amniotes Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are dis ...

birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

and

reptiles Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchoceph ...

. It is formed by the fusion of the

mesodermal The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embry ...

layers of two extra-embryonic membranes – the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitell ...

and the

allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...

. It is the avian homologue of the

mammalian Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

. It is the outermost extra-embryonic membrane which lines the non-vascular egg shell membrane.

Structure

The chorioallantoic membrane is composed of three layers. The first is the chorionic

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

that is the external layer present immediately below the shell membrane. It consist of epithelial cells that arise from chorionic

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

. The second is the intermediate mesodermal layer that consists of

mesenchymal Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...

tissue formed by the fusion of the mesodermal layer of the chorion and the mesodermal layer of the allantois. This layer is highly vascularized and rich in

stromal Stromal cells, or mesenchymal stromal cells, are differentiating cells found in abundance within bone marrow but can also be seen all around the body. Stromal cells can become connective tissue cells of any organ, for example in the uterine mucos ...

components. The third is the allantoic epithelium that consists of epithelial cells arising from the allantoic ectoderm. It forms a part of the wall of the allantoic sac. Both the epithelial layers are separated from the mesodermal layer by

basement membranes The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between epithelial tissues including mesothelium and ...

Function

The Chorioallantoic membrane performs the following functions: The CAM functions as the site of

gaseous exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a b ...

for

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

between the growing embryo and the environment. Blood

capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

and

sinuses Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the spheno ...

are found in the intermediate mesodermal layer allows close contact (within 0.2 μm) with air found in pores of the shell membrane of the egg. The chorionic epithelial layer contains the calcium transporting region of the CAM, and thus is responsible for the transport of calcium ions from the egg shell into the embryo for the purpose of

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in ...

of the bones of the developing

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

. The CAM also helps in maintaining the acid-base homeostasis in the embryo. Finally the allantoic epithelium serves a barrier to the allantoic cavity, and acts in a selectively permeable manner by permitting the absorption of water and

electrolytes An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...

, as well as maintains a barrier against the

toxins A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849� ...

and waste materials stored inside the allantoic cavity.

Development

The development of the CAM is similar to that of the allantois in mammals. Its growth starts from day 3 of embryonic development. Development of the allantois occurs extra embryonically from the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

wall of the endodermal

hindgut The hindgut (or epigaster) is the posterior ( caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal ju ...

. Partial fusion of the chorion and allantois occurs between days 5 and 6. By day 10, there is an extensive formation of capillary network. The complete differentiation of the CAM is complete by day 13.

Cultivation protocols

Chorioallantoic membranes can be cultivated either outside (ex-ovo) or inside of the shell (in-ovo).

Ex-ovo

Here, the embryo is grown outside of the shell. In this method, the eggs are first kept in inside a humidified

incubator An incubator is anything that performs or facilitates various forms of incubation, and may refer to: Biology and medicine * Incubator (culture), a device used to grow and maintain microbiological cultures or cell cultures * Incubator (egg), a de ...

for up to a period of 3 days, to ensure that the position of the embryo is opposite to the position where the egg will be subsequently cracked. A small hole is made on the side of the air chamber to equilibrate the pressure, followed by the cracking of the egg on a petri-dish. This method is ideal for visualizing the growing embryo and their manipulation without limitations in accessing the embryo during the different stages of development. However the process requires

aseptic Asepsis is the state of being free from disease-causing micro-organisms (such as pathogenic bacteria, viruses, pathogenic fungi, and parasites). There are two categories of asepsis: medical and surgical. The modern day notion of asepsis is der ...

conditions. There are also problems associated with the handling of the embryo, as the

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example ...

membrane is prone to rupture both during and after the culture.

In-ovo

Here, the embryo is grown within the confines of the egg shell. In this method, fertilized eggs are rotated inside an incubator for three days in order to prevent the embryo from sticking to the membranes of the shell. A hole is then created on the eggshell and wrapped with a film to prevent

dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...

and

infections An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable di ...

. The egg is then maintained in a static position until further use. This step prevents the CAM from sticking to the shell membrane. At day 7 post-

fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...

, the hole is extended in order to access the CAM. This method offers several advantages over the ex-vivo method as the physiological environment for the developing embryo remains virtually unchanged. It is easier to maintain sterility as well the integrity of the CAM and the embryo when they are present inside the shell. However good technical skills are required for this method. The presence of the shell around the developing embryo makes access to the embryo difficult. There are also limitations in the observing and imaging of the developing embryo.

Applications

CAM provides several features such as ease of access, and the rapid development of the membrane structure, presence of an immunodeficient environment, ease of visualization for imaging techniques ranging from microscopic to PET scans. Thus, it makes for a suitable model for a number of research applications in the field of biological and biomedical research: * Vascular development and angiogenesis. *

Xenograft Xenotransplantation (''xenos-'' from the Greek meaning "foreign" or strange), or heterologous transplant, is the transplantation of living cells, tissues or organs from one species to another. Such cells, tissues or organs are called xenograf ...

studies. * Study of

tumour A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

growth and differentiation. *

Wound repair Wound healing refers to a living organism's replacement of destroyed or damaged tissue by newly produced tissue. In undamaged skin, the epidermis (surface, epithelial layer) and dermis (deeper, connective layer) form a protective barrier again ...

studies. *

Toxicology Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating e ...

studies. *

Nanomaterials * Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale). Nanomaterials research takes a materials science-based approach to n ...

assessments. *

Drug delivery Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to dr ...

. * Study of molecules affecting with

angiogenic Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splittin ...

and anti-angiogenic activities. * Culturing of

viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's ...

like Herpes Simplex Virus, etc. * Drug screening studies. *

Radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Rad ...

related studies. * Allergenicity and toxicity studies. *

Helminth Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as ...

cultivation. *

Oncological Oncology is a branch of medicine that deals with the study, treatment, diagnosis and prevention of cancer. A medical professional who practices oncology is an ''oncologist''. The name's etymological origin is the Greek word ὄγκος (''ó ...

investigations.

Advantages

The advantages of using CAM are: * It is easy to use as compared to other

animal models A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the working ...

. * Assays can be visualized real time using very simple to highly complex visualizing techniques. * Rapid vascular growth. * Cost effective, easy to access. * The

circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

is completely accessible making the delivery of intravenous molecules easy. * Assays take relatively less time. * Easily reproducible and reliable.

Disadvantages

Despite the numerous advantages, there are a number of disadvantages associated with the use CAMs: * Sensitivity to modifications in environmental conditions. * Limited availability of reagents like antibodies due to avian origin. * Non-specific inflammatory reaction after 15 days of development. * Difficulty of distinguishing the formation of new capillaries from the already existing vascular network. * Differences in metabolism of drugs as compared to mammals.

References

{{reflist Vertebrate developmental biology Membrane biology Birds