Chicken as biological research model
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Chicken The chicken (''Gallus gallus domesticus'') is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster or cock is a term for an adult m ...
s (''Gallus gallus domesticus'') and their eggs have been used extensively as research models throughout the history of biology. Today they continue to serve as an important model for normal human biology as well as pathological disease processes.


History


Chicken embryos as a research model

Human fascination with the chicken and its egg are so deeply rooted in history that it is hard to say exactly when avian exploration began. As early as 1400 BCE, ancient Egyptians artificially incubated chicken eggs to propagate their food supply. The developing chicken in the egg first appears in written history after catching the attention of the famous Greek philosopher,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...
, around 350 BCE. As Aristotle opened chicken eggs at various time points of incubation, he noted how the organism changed over time. Through his writing of '' Historia Animalium'', he introduced some of the earliest studies of embryology based on his observations of the chicken in the egg. Aristotle recognized significant similarities between human and chicken development. From his studies of the developing chick, he was able to correctly decipher the role of the placenta and umbilical cord in the human. Chick research of the 16th century significantly modernized ideas about human physiology. European scientists, including Ulisse Aldrovandi, Volcher Cotier and William Harvey, used the chick to demonstrate tissue differentiation, disproving the widely held belief of the time that organisms are "preformed" in their adult version and only grow larger during development. Distinct tissue areas were recognized that grew and gave rise to specific structures, including the
blastoderm A blastoderm (germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the for ...
, or chick origin. Harvey also closely watched the development of the heart and blood and was the first to note the directional flow of blood between veins and arteries. The relatively large size of the chick as a model organism allowed scientists during this time to make these significant observations without the help of a microscope. Expanding use of the microscope coupled with a new technique in the late 18th century unveiled the developing chick for close-up examination. By cutting a hole in the eggshell and covering it with another piece of shell, scientists were able to look directly into the egg while it continued to develop without dehydration. Soon studies of the developing chick identified the three embryonic germ layers:
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 ...
,
mesoderm 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 E ...
and
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
, giving rise to the field of
embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, '' -logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and ...
.
Host versus graft Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between donor and recipient a ...
response was first described in the chicken embryo. James Murphy (biologist) (1914) found that rat tissues that could not grow in adult chickens survived in the developing chick. In an
immunocompetent In immunology, immunocompetence is the ability of the body to produce a normal immune response following exposure to an antigen. Immunocompetence is the opposite of immunodeficiency (also known as ''immuno-incompetence'' or being ''immuno-comprom ...
animal, like the mature chicken, the host immune cells attack the foreign tissue. Since the immune system of the chick is not functional until about day 14 of incubation, foreign tissue can grow. Eventually, Murphy showed that the acceptance of tissue grafts was host-specific in immunologically competent animals. Culturing virus was once technically difficult. In 1931, Ernest Goodpasture and
Alice Miles Woodruff Alice Miles Woodruff (November 29, 1900 – November 24, 1985), born Alice Lincoln Miles, was an American virologist. She developed a method for growing fowlpox outside of a live chicken alongside Ernest William Goodpasture. Her research greatly ...
developed a new technique that used chicken eggs to propagate a pox virus. Building on their success, the chick was used to isolate the
mumps virus The mumps virus (MuV) is the virus that causes mumps. MuV contains a single-stranded, negative-sense genome made of ribonucleic acid (RNA). Its genome is about 15,000 nucleotides in length and contains seven genes that encode nine proteins. The g ...
for vaccine development and it is still used to culture some viruses and parasites today. The ability of chicken embryonic nerves to infiltrate a mouse tumor suggested to Rita Levi-Montalcini that the tumor must produce a diffusible growth factor (1952). She identified Nerve Growth Factor (NGF) leading to the discovery of a large family of growth factors which are key regulators during normal development and disease processes including cancer.


Adult chicken as a research model

The adult chicken has also made significant contributions to the advancement of science. By inoculating chickens with
cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...
bacteria (Pasteurella multocida) from an overgrown, and thereby attenuated, culture
Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, microbial fermentation and pasteurization, the latter of which was named afte ...
produced the first lab-derived attenuated vaccine (1860s). Great advances in immunology and oncology continued to characterize the 20th century, for which we indebted to the chicken model.
Peyton Rous Francis Peyton Rous () (October 5, 1879 – February 16, 1970) was an American pathologist at the Rockefeller University known for his works in oncoviruses, blood transfusion and physiology of digestion. A medical graduate from the Johns Hopki ...
(1879-1970) won the Nobel prize for discovering that viral infection of chicken could induce sarcoma (Rous, 1911). Steve Martin followed up on this work and identified a component of a chicken retrovirus, Src, which became the first known oncogene. J. Michael Bishop and Harold Varmus with their colleagues (1976) extended these findings to humans, showing that cancer causing oncogenes in mammals are induced by mutations to proto-oncogenes. Discoveries in the chicken ultimately divided the adaptive immune response into antibody (B-cell) and cell-mediated (T-cell) responses. Chickens missing their
bursa ( grc-gre, Προῦσα, Proûsa, Latin: Prusa, ota, بورسه, Arabic:بورصة) is a city in northwestern Turkey and the administrative center of Bursa Province. The fourth-most populous city in Turkey and second-most populous in the ...
, an organ with an unknown function at the time, could not be induced to make antibodies. Through these experiments, Bruce Glick, correctly deduced that bursa was responsible for making the cells that produced antibodies. Bursa cells were termed B-cells for Bursa to differentiate them from thymus derived T-cells.


Cancer

The chicken embryo is a unique model that overcomes many limitations to studying the biology of cancer in vivo. The chorioallantoic membrane (CAM), a well-vascularized extra-embryonic tissue located underneath the eggshell, has a successful history as a biological platform for the molecular analysis of cancer including viral oncogenesis,
carcinogenesis Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abnor ...
, tumor xenografting, tumor
angiogenesis 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 splitting ...
, and cancer
metastasis Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...
. Since the chicken embryo is naturally immunodeficient, the CAM readily supports the engraftment of both normal and tumor tissues. The avian CAM successfully supports most cancer cell characteristics including growth, invasion, angiogenesis, and remodeling of the microenvironment.


Genetics

The ''Gallus gallus'' genome was sequenced by Sanger
shotgun sequencing In genetics, shotgun sequencing is a method used for sequencing random DNA strands. It is named by analogy with the rapidly expanding, quasi-random shot grouping of a shotgun. The Sanger sequencing#Method, chain-termination method of DNA sequencin ...
and mapped with extensive BAC contig-based physical mapping. There are significant, fundamental similarities between the human and chicken genomes. However, differences between human and chicken genomes help to identify functional elements: the genes and their regulatory elements, which are most likely to be conserved through time. Publication of the chicken genome enables expansion of transgenic techniques for advancing research within the chick model system.


References

{{Reflist Embryology Chickens Animal testing by animal type