Research into head and facial development is an important use of quails. In one study, the embryos from both quails and ducks were used to implant neural crest cells (simple cells that arise very early in development) from one species into the other. The result was quails with duckbills or 'duails', and ducks with quail beaks or 'qucks'. This suggests that head and facial diversification is due to neural crest cells, and further work should help to unravel the underlying causes of craniofacial defects, which are among the most common birth defects. Quails are also used to test the safety of agrochemicals in the environment.
In 1910, Peyton Rous filtered cells from some malignant connective tissue that had appeared spontaneously in a chicken. This filtrate was injected into healthy chickens, which then developed similar tumours. His experiments showed that the filtrate contained the cancer-causing agent, which eventually became known as the Rous sarcoma virus – the first known virus to transform normal cells into cancer cells.
Chickens have been used to discover the molecular basis of limb development, a process similar in humans and birds. They have also helped the understanding of many limb disorders.
Since chickens are vertebrates, their developmental process has a great deal in common with humans, despite the many differences. They have provided valuable insights into the development of the nervous system, showing how cells migrate and differentiate.
Current research studying the development of the chick embryo is revealing clues about the heart condition atrial septal defect, also known as ‘hole in the heart’. The wall between the left and the right atria of the heart does not close completely during development, making it more difficult for the heart to pump blood around the body. Chick embryos are being used to model the condition because like humans they have 4 chambered hearts.
While the discovery of penicillin was due to chance observation, streptomycin was identified after many years of meticulous searching, and the development of tests which enabled the discovery of other antibiotics. More than 10,000 different soil microbes were studied before streptomycin was discovered. It had low toxicity in animals and protected chicks against many disease-causing bacteria including those causing tuberculosis, plague and pneumonia.
Traditionally, Influenza vaccines are produced in fertilized chicken eggs. 11 days after fertilization, an influenza virus is injected into the eggs and infects the embryo, where it multiplies. After several days of incubation, machines open the eggs to harvest the virus, which is then purified and chemically inactivated.
The overall capacity of this production process is limited as one to two eggs are needed to produce only one dose of vaccine and the whole process takes around 6 months. The process is not suitable to emergencies, as it is impractical to keep millions of 11 day old eggs on standby and the length of time to produce a vaccine is prohibitive. Alternatively, vaccines can be produced in tissue culture.
In 1911, the chicken provided some clues to one of the earliest models of cancer growth and spread, especially from cancer causing viruses. Studies showed how viruses cause tumours and subsequent use of hormone treatments could limit growth (oncogenesis) in chickens among others.
In 1951, Max Theiler was awarded the Nobel Prize of Physiology and Medicine for his works leading up to the production of a yellow fever vaccine. He experimented on mince, monkeys and chick embryos to develop the vaccine. Although yellow fever manifests differently in animal models, these experiments played a vital role in developing a successful vaccine.
Chickens are an extremely important model for many fields of biology, including Nobel Prize winning advances in immunology, virology, and development. It is only fitting that the chicken be the first bird to have its genome sequenced in 2005.
Proteins sequenced from dinosaurs confirm that the closest living relative to the Tyrannosaurus rex is the chicken. Many molecules show a remarkable similarity in both animals. There is thus molecular evidence that birds are the modern-day descendants of dinosaurs.