Animal experimentation: the chicken model in myopia research

The following article is presented to you by Sandra Gisbert Martinez
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.

 

Animal experiments are essential for the progress of basic and applied biological and medical research since many fundamental questions cannot be solved purely by epidemiological approaches in human studies. In particular in myopia, which represents the result of complex interactions of environmental and genetic factors, basic questions cannot be answered only in human studies, like “which visual cues make eyes myopic?” or “which biochemical changes occur in the fundal layers when the eye starts growing longer?” or “why and how do certain drugs inhibit myopia development and could they be used to inhibit myopia in humans?”

What is animal experimentation?

The Animal Welfare Act ensures the protection of animals, which are predetermined to use in animal experiments or whose tissue or organs shall be used for scientific purposes. Any procedure carried out on animals for scientific purposes must be monitored and approved by the competent authority. The approval process requires accurate information about the purpose of using animals as well as their living and care conditions are examined. As you can observe, the aim of the law is to protect the life and well-being of the animal.

What is 3Rs principle?

For those who know the topic of animal experimentation, have probably heard of the 3Rs principle which came from the English terms “Reduction – Replacement – Refinement” defined by W. Rusell and R. Burch. It is very important to keep this principle in mind and take it as a guideline because any researcher planning to use animals in their research must justify that there are good ethical, scientific, legal and economic reasons for making sure that animals are looked after properly and used in minimum numbers.

It is necessary fully satisfying the meaning of the 3Rs:

  • Reduction: reduce the number of animals used to a minimum but as many as necessary are used.
  • Replacement: it seeks to substitute animal experiments with alternative methods as much as possible, or to avoid them completely.
  • Refinement: refine the way experiments are carried out, to make sure animals suffer as little as possible. This includes better housing and improvements to procedures which minimize pain and suffering and/or improve animal welfare.
What about experimental animals in myopia research?

It has been reported that genetic factors together with environmental interaction contribute to increase the probability of becoming myopic, nevertheless it is possible that the genetic explanation of myopia would still dominate at that days if animal experiments had not introduced new evidence about mechanisms involved in emmetropisation. The most striking finding showed that placing in front of the eyes spectacle lenses (positive or negative), they can alter their refractive state compensating the defocus imposed by increasing or slowing their rates of axial elongation, and this was observed in chickens, fish, tree shrews, marmosets, rhesus monkeys and guinea pigs (1-5). This fact provides the evidence that there existed a homeostatic mechanism that regulated refractive error. Whereby, it arises a central question as how visual feedback influences axial eye growth.

Why chicken model is suitable for myopia research?

Chicken model was introduced by Wallman, Turkel and Trachtman and today still remains one of the major models for myopia (6). Many of its characteristics are what make it an attractive model for myopia research. Within these features are included (7):

  • Eye size, relatively large (8 to 14mm).
  • Eye growth faster than in other animal species studied (about 100µm/day).
  • Highly sensitive control of refractive state by retinal image quality and focus.
  • Excellent optics.
  • Active accommodation (about 17D).
  • High visual acuity (7cycles/degree).
  • Easy to deliver drugs by intravitreal injection.
  • Easy to handle them.
  • Inexpensive and easy cares.

 

Nevertheless, they also have some disadvantages such as the lack of a fovea, differences in scleral composition or a different mechanism of accommodation compared to mammals.

In spite of that, chicks provided fundamental information on the mechanisms of emmetropization, and still many experiments are carried out on them in order to try to understand which variables in the eye really determine growth rates during myopia progression.


Stay up-to-date, Keep on reading and…

Myopia in Science!

 

  • References.
1) Schaeffel F, Glasser A, Howland HC. (1988) Accommodation, refractive error and eye growth in chickens. Vision Res 28: 639–657.

2) Hung LF, Crawford ML, Smith EL. (1995) Spectacle lenses alter eye growth and the refractive status of young monkeys. Nature Med 1: 761–765.

3) Whatham AR, Judge SJ. (2001) Compensatory changes in eye growth and refraction induced by daily wear of soft contact lenses in young marmosets. Vision Res 41: 267–273.

4) Shen W, Sivak JG. (2007) Eyes of a lower vertebrate are susceptible to the visual environment. Invest Ophthalmol Vis Sci 48: 4829–4837.

5) Metlapally S, McBrien NA. (2008) The effect of positive lens defocus on ocular growth and emmetropization in the tree shrew. J Vis 8(1): 1–12. 12. Howlett M, McFadden S. (2009) Spectacle lens compensation in the pigmented guinea pig. Vision Res 49: 219–227.

6) Wallman J, Turkel J, Trachtman J. (1978) Extreme myopia produced by modest change in earlyvisual experience. Science 201(4362): 1249-51.

7) Schaeffel F, Feldkaemper M. (2015) Animal models in myopia research. Clin Exp Optom;98(6):507-17
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