A clear understanding and improvement of the process of healing or eradication of disease requires significant studies and biomedical experimentation.  A large number of these experiments are of a test-tube type carried out in the laboratory.  However, before they can be studied in humans, safety and efficacy of drugs, devices (implants), and surgical procedures are evaluation in experimental animals.

          Outer surfaces of the body are exposed constantly to environmental insults and are very sensitive to irritants.  FDA (Federal Drug Administration) requires that all consumer products, food additives, and drugs have to be evaluated for skin and eye irritation.

          Eyes are particularly sensitive to irritation and damage by chemicals.  The front part of the eye, the cornea, has to be perfectly transparent so that the light may be transmitted to the retina and processed into images by the brain.  Because it is the most exposed part of the eye, damage to the cornea can interrupt this light path and cause blindness.

          A major protection mechanism of the cornea is sensory nerve ends; it has the highest number of nerve ends of all body surfaces.  Because these nerve ends are very close to the surface and because there are hundreds of thousands of them, most “stimulation” of the cornea (changes in pH, temperature and pressure) is perceived as pain.  This protection mechanism, as advanced warning system (alarm), signals that something is “irritating” the eye and turns on the rapid blinking and tearing.

          The “gold standard” eye irritation test is the Rabbit Eye test (Draze’s Test), in which the materials are introduced into the rabbit eye, and the eye is observed for different consequences of irritation (redness, clouding of the cornea).  Unfortunately, the rabbits cannot tell us how painful the test is and when the discomfort changes into stinging and then into pain.  This type of animal testing easily becomes inhumane.  The test is also not accurate and frequently when a particular material is tested on human subjects it is found that there is discomfort, stinging or even pain.  Furthermore, nerve sensitivity varies from animal to animal and human to human.  Discovering adverse irritation at such a late stage in development of a new drug or consumer product represents a very considerable financial loss.  An added disadvantage is that the purchase and upkeep of test animals is very expensive.

          There is a great need for a laboratory model of the corneal innervation that could sense irritation and translate into a measurable number which could then be used to devise an objective numerical irritation index.  A test needs to be developed which would, without the use of animal or human subject, register an electronic “ouch”.  Techniques are not available to culture neurons and detect and measure their responses as small electrical currents.  We have already developed a tissue model of the human skin and cornea, and have submitted a patent application on the invention.  Innervation of these artificial tissue would provide the model for which there has been a need for a number of years.

Such a model would be accurate, cost effective, and would avoid the use of animals and therefore inhumane experiments.

© Cardiovascular Research Institute, February 2000
An Institute of Discovery at the University of North Texas Health Science Center at Fort Worth