Tears, or lacrimal fluid, are a complex biological secretion serving to lubricate, nourish, and protect the delicate tissue. Tears are a mixture of water, electrolytes, proteins, and lipids, forming a three-layered film across the eye. Tears contain DNA, but only in trace amounts due to biological processes occurring at the ocular surface. This genetic material originates not from the tear fluid itself, but from the cells that are naturally shed into it.
The Cellular Source of DNA in Tears
DNA is present in tear fluid as a consequence of the constant cellular turnover that takes place on the eye’s surface. DNA is housed within the nucleus of nucleated cells. The primary source of this genetic material is sloughed-off epithelial cells of the cornea and conjunctiva that are regularly shed and float in the tear film.
Tears can contain both live and dead nucleated cells. Occasional leukocytes, or white blood cells, may also enter the tear fluid from the immune system, particularly during inflammation or infection. The DNA derived from these cells can be categorized as nuclear DNA (nDNA), which contains the unique genetic profile used for identification, and mitochondrial DNA (mtDNA), which is present in hundreds of copies per cell but is inherited only from the mother.
The amount of DNA recoverable from tears is relatively low. Adding to the challenge of recovery is the presence of enzymes called endonucleases within the tear fluid. These enzymes, such as tear lipocalin, act to degrade foreign or unwanted genetic material, limiting the overall quantity and quality of intact DNA available for analysis.
Tears and Forensic Identification
The use of tears as a source of DNA for forensic identification presents a significant challenge. Unlike blood or saliva, the cellular concentration in tear fluid is extremely low, often falling into the category of “trace DNA” or “low-copy number” evidence. The total yield of genetic material is typically insufficient for standard DNA profiling techniques.
Forensic scientists rely on a technique called Polymerase Chain Reaction (PCR) to amplify the minute quantities of DNA to a detectable level. However, working with a sample that has such a low initial DNA concentration increases the risk of profile failure or incomplete genetic profiles. Furthermore, the small volume of a typical tear stain makes collection and preservation difficult, as the DNA can be easily degraded by environmental factors.
Despite these hurdles, modern forensic studies have shown promising results, demonstrating that successful Short Tandem Repeat (STR) profiles can be obtained from tear stains deposited on common substrates. Trace DNA analysis from items like used contact lenses or tissue paper has yielded complete genetic profiles. The potential for contamination or secondary transfer of DNA is a major concern, though, as genetic material from skin cells or other body fluids can easily mix with the tear sample, complicating the identification of the true source.
Diagnostic Biomarkers in Tear Fluid
While the use of whole nuclear DNA from tears for personal identification is limited, the fluid is proving to be a valuable, non-invasive source for medical diagnostics. Tear fluid is rich in various biomarkers that reflect the body’s physiological state, including proteins, metabolites, and nucleic acids. Analyzing these components can offer real-time insights into both ocular and systemic health.
Research is advancing in using tear analysis to diagnose and monitor conditions beyond just the eye, such as dry eye syndrome or infections. For example, specific protein levels are associated with the severity of dry eye disease. Changes in the concentration of glucose or other metabolites in tears are being explored as a method for monitoring diabetes without the need for a blood draw.
Furthermore, tears contain various forms of genetic material beyond whole nuclear DNA, including microRNAs and fragmented DNA. These small nucleic acid molecules can carry information about disease processes, such as certain cancers or neurological disorders, making tear fluid a promising medium for early and convenient disease detection.