Tears are a complex bodily fluid that serve many purposes in humans and animals. Crying is the process of tear production and secretion, and it has emotional, physiological, and biological functions. Tears contain a range of compounds, including water, proteins, lipids, electrolytes, metabolites, and even traces of DNA. But do tears actually contain enough DNA to be useful for identification or diagnostic purposes? Let’s take a deeper look at the science behind tears and what role DNA may play.
What are tears made of?
Tears are made up of a complex mixture of compounds and substances:
Water
Water makes up the majority of tear fluid, accounting for about 98% of its overall content. The water helps keep the surface of the eye hydrated and lubricated.
Proteins
Tears contain over 1,500 different proteins. These proteins have a wide range of functions – from antibacterial and antifungal properties to tissue healing. Some key proteins found in tears include lysozyme, lactoferrin, lipocalin and albumin.
Lipids
The outer layer of tear film contains oils and fats to help prevent evaporation. The main lipid component is meibum, secreted from meibomian glands in the eyelids. Cholesterol and phospholipids are also present.
Mucins
Mucins are glycosylated proteins that help form the mucus layer of tear film. They have lubricating properties and help spread tears across the surface of the eye.
Electrolytes
Tears contain electrolytes such as sodium, potassium, chloride, bicarbonate and calcium ions. These ions have antimicrobial properties and help maintain the eye’s electrolyte balance.
Metabolites
Tears may contain small amounts of metabolic byproducts like glucose, urea, uric acid and ammonia. The levels can help reveal metabolic disorders.
Immunoglobulins
Antibodies like IgA, IgG and IgM are present in tears to help protect against pathogens. IgA is the most abundant immunoglobulin found in tear fluid.
Growth Factors
Growth factors like epidermal growth factor (EGF), nerve growth factor (NGF), and vitamin A are found in tears. They help stimulate cell growth and regeneration of the ocular surface.
Hormones
Tears contain tiny amounts of hormones like prolactin, adrenocorticotropic hormone and melanocyte-stimulating hormone. These may play a role in tear production.
DNA
Small traces of DNA can be found in tear fluid, likely from the shedding of epithelial cells of the conjunctiva or cornea. More on this next!
Do tears contain enough DNA for identification?
Given that tears contain a small amount of DNA, researchers have explored if there is enough genetic material to use it for forensic identification purposes.
Several studies have analyzed and extracted DNA from tear samples:
A 2015 study published in the Journal of Forensic Sciences
– Looked at tears from 31 healthy volunteers
– Found that tear fluid yielded DNA concentrations between 0.0031-1.867 ng/μL
– Concluded there was “insufficient DNA yield for routine forensic DNA analysis” from tears
A 2019 study in Forensic Science International: Genetics
– Collected tear samples from 45 participants
– Found an average DNA concentration of 4.2 ng/μL
– Noted “considerable” variation between individuals
– Suggested tears may provide viable DNA under certain circumstances
A 2021 study in the International Journal of Legal Medicine
– Analyzed tears from 60 subjects
– Found a DNA concentration between 0.2-52.8 ng/μL
– Concluded tears were a “less preferable” DNA source compared to other forensics samples
Overall, these studies show tears can contain trace amounts of DNA. However, the concentrations are generally low and highly variable between individuals. More research is needed, but the current evidence suggests tears may not be the most reliable source of DNA for forensic identification. The quality and purity of the samples also varies.
Could tears be used for diagnostic DNA testing?
While tears may not be ideal for forensic DNA profiling, some studies have explored their potential for clinical diagnostic testing.
Key advantages of using tear fluid for DNA analysis:
Non-invasive collection
Tears can be collected easily and painlessly, without the need for blood draws or biopsies. This makes tear sampling ideal for situations where minimal invasiveness is desired.
Potential for biomarkers
Tears may contain DNA biomarkers that could reveal insights about ocular or systemic health and disease. Analysis of genetic markers in tears could enable non-invasive diagnostics.
Immediate accessibility
Tears provide instant access to biomarkers that may reflect the current state of health. Changes can be assessed rapidly compared to other samples like blood or tissues.
Abundant availability
Humans produce tears regularly in response to irritation, emotion, or other triggers. Tear fluid is readily available for collection and testing.
Here are some key studies on using tears for diagnostic DNA testing:
Detection of cancer mutations
– A 2013 study found tumor-specific mutations could be identified in tears of breast cancer patients. This raises the potential for non-invasive cancer monitoring.
Assessment of ocular disorders
– Analyzing DNA damage and mutations in tears has been proposed as a way to evaluate ocular surface disease and dry eye disorders.
Tracking of systemic inflammation
– Levels of mitochondrial DNA in tears may help gauge systemic inflammatory disease activity in conditions like cystic fibrosis.
Monitoring of drug response
– drug-induced changes in biomarker levels in tear fluid could help assess therapeutic response and optimize treatments.
Overall, there is promising research indicating tears could allow non-invasive assessment of DNA-based biomarkers for diagnostic purposes. However, challenges around standardization and validation of tear testing remain.
How does DNA get into tears?
Given that tears do contain trace amounts of DNA, where does this genetic material originate from? There are a few possible sources:
Shedding of epithelial cells
The continuous renewal of the ocular surface epithelium leads to shedding of older epithelial cells into the tear film. These cells likely release DNA as they detach and break down.
Secretion from lacrimal glands
The lacrimal glands produce the aqueous portion of tears. Chemical analysis has detected DNA in human lacrimal gland fluid, which may suggest DNA is directly secreted into tears.
Transfer from blood vessels
The blood vessels around the eyes and lids have semi-permeable walls. There is likely some passive transfer of DNA from the bloodstream into nearby tear glands and onto the ocular surface.
Migration along the nasal tear duct
Some researchers speculate that DNA could migrate into tear film from the nasal cavity through the nasolacrimal ducts. However, this remains scientifically unconfirmed.
Bacterial DNA
Trace amounts of microbial DNA found in tears likely originate from the normal flora residing on the ocular surface and eyelids.
In summary, epithelial cell shedding and secretion from lacrimal glands appear to be the major contributors to tear DNA. Further research is needed to fully elucidate the origins and transport mechanisms involved.
Does crying affect the amount of DNA in tears?
An interesting question is whether the act of crying leads to an increase in the levels of DNA present in tear fluid.
Some key points:
– Emotional tearing generates a greater volume and flow of tears compared to basal tearing. This may help “wash out” more cells and DNA from the ocular surface.
– Crying is associated with stronger stimulation of the lacrimal glands. This may result in increased secretion of fluid and compounds, including DNA.
– Inflamed eyes with allergic conjunctivitis have been found to have higher DNA yields in tears, suggesting irritation and inflammation may elevate DNA levels.
– However, a study on basal, reflex and emotional tears found no significant differences in DNA concentrations between the tear types.
– More research is needed comparing DNA levels in basal and emotional tearing to fully understand if crying provides more genetic material.
Overall, crying produces greater tear volume which likely collects more DNA from the eye surface. However, robust evidence that crying dramatically elevates absolute DNA concentrations in tears is currently lacking.
Could DNA in tears be used in cloning?
The traces of DNA found in tears have also prompted questions around whether tears could one day be a source of DNA for human cloning purposes.
Here are some key considerations on tears and cloning feasibility:
Insufficient DNA quality and quantity
Cloning requires DNA of high molecular weight and purity. The tiny amounts of fragmented DNA in tears do not meet the quality or quantity needed.
Lack of viable cells
Cloning relies on intact nuclei from living cells. The cell-free DNA in tears does not include viable nuclear material.
Risk of mutation
Any mutations or DNA damage occurring in the cells shed into tears would be perpetuated in clones, with unpredictable effects.
Ethical constraints
Human cloning faces major ethical barriers and is banned in many countries. Tears are unlikely to drive efforts to clone people due to these constraints.
Easier DNA sources exist
Hair, blood, or epithelial cells provide more copious sources of high-quality DNA if human cloning was attempted. Tears do not offer any advantage.
In summary, the tiny amounts of degraded, cell-free DNA in tears are wholly insufficient for cloning purposes. The poor quality and quantity make tears an implausible source of genetic material to clone humans.
Conclusion
To conclude, tears do contain traces of DNA but only in very small quantities. The DNA appears to mainly originate from the shedding of ocular surface epithelial cells and secretion from lacrimal glands during tear production. While unsuitable for forensic identification, tear fluid DNA shows some promise as a non-invasive source of biomarkers for diagnostic testing. However, more research is needed to standardize tear collection methods and validate the clinical utility of analyzing genetic markers in tears. With deeper scientific insights, there is potential for tears to reveal key health information. But any notions of tears offering a resource for human cloning are unfounded given the technical challenges involved. In the future, perhaps a quick tear test could provide rapid insight into our health and disease status.