By Michelle Elizondo González
Contributing author Jorge Valdez García
Every time we blink, an almost imperceptible but essential process takes place: a thin layer spreads across the surface of the eye, protecting it, keeping it moist, and allowing us to see clearly. But the most interesting part of this system is not the water in our tears. It is mucin, a nearly invisible molecule that helps keep the tear film in place.
The stability of the tear film depends on the interaction of three main components: an outer lipid layer that reduces evaporation; an aqueous layer containing water, proteins, and electrolytes; and a mucin-rich layer that interacts with the surface of the eye and helps keep it hydrated.
For a long time, these three components were thought to function largely independently. It is now understood that mucin plays a central role in organizing and stabilizing the entire system.
What Is Mucin?
Mucin is a large glycoprotein, meaning it consists of a protein attached to multiple chains of sugars. This structure gives mucin distinctive properties, including the ability to retain water and form elastic, sticky structures. In many ways, it acts as the “glue” that allows tears to adhere to the surface of the eye.
This is important because the eye’s surface is hydrophobic, meaning it naturally repels water. Mucin helps make the surface more water-friendly, allowing the liquid portion of the tear film to spread evenly. As a result, the tear film remains more stable after each blink rather than forming dry spots on the eye’s surface.
In addition to its structural role, mucin can retain water and help keep the corneal epithelium hydrated. Its viscoelastic properties also reduce the mechanical friction generated by blinking. Some membrane-bound mucins—MUC1, MUC4, and MUC16—form a protective barrier against microorganisms and external particles and play an important role in the eye’s immune defense.
When the Eye Becomes Diseased
Not all mucins are the same. The ocular surface contains two main types: secreted mucins, such as MUC5AC, which are produced by goblet cells in the conjunctiva, and transmembrane mucins, which are attached to epithelial cells. Secreted mucins form gels that help spread tears across the ocular surface, while transmembrane mucins help stabilize and protect the epithelium.
When this system is disrupted, the consequences can be significant. Reduced mucin production or impaired mucin function can destabilize the tear film, increasing evaporation and leaving the ocular surface exposed. This process is closely associated with dry eye disease, one of the most common eye conditions today.
Dry eye is more than a minor irritation. It is a complex condition that can cause inflammation and damage to the surface of the eye.
People with dry eye may experience burning, a sensation that something is stuck in the eye, sensitivity to light, and blurred vision. The condition can have several causes, including autoimmune diseases such as Sjögren’s syndrome, vitamin A deficiency, aging, hormonal changes, and certain medications.
Other diseases that affect the outer surface of the eye, including trachoma and Stevens-Johnson syndrome, can reduce the number of goblet cells responsible for producing mucins and helping keep the eye moist. Conditions such as diabetes and nervous system disorders can also affect tear production and worsen the problem.
Mucin has attracted growing scientific interest not only as a structural component of the tear film, but also as a potential biomarker and therapeutic target. Understanding its role in the tear film could open new avenues for studying and treating eye diseases, particularly those associated with dryness.
Current evidence indicates that maintaining a healthy mucin layer is essential for ocular health. Although it may seem simple, this glycoprotein plays a critical role in the interaction between the eye and its environment. It is another example of how even the smallest components can have a significant impact on human physiology.
Referencias
- Morales-Mancillas, Nallely R.1; Garza-Garza, Lucas A.1,2; Hernandez-Camarena, Julio C.1; Castrejón-Perez, Gabriela1; Valdez-García, Jorge E.1. Correlation between the Ocular Surface Disease Index and dry eye functional parameters measured with the OCULUS Keratograph 5M in a Hispanic population. The Pan-American Journal of Ophthalmology 6(3):108, October 2024.
- Rebeka Goldmann, Carlos A. Rodríguez-Barrientos, Jorge E Valdez; Tear Meniscus: a comparative study between meniscometry and tear meniscus height. Invest. Ophthalmol. Vis. Sci. 2025;66(8):5659.
- Raul Trejo Treviño, Daniel Bastán-Fabián, José Carlos Guerrero-Acosta, Jan Moisés Guillén-Ortiz, Jorge Eugenio Valdez-García; Meibomian gland dysfunction treated with a novel device: A case series. Invest. Ophthalmol. Vis. Sci. 2024;65(7):6581.
Author
Michelle Elizondo González is a sixth-semester biotechnology engineering student at Tecnológico de Monterrey. She collaborates on academic projects focused on using microorganisms and biotechnological tools to develop innovative and sustainable solutions.
This article was supervised by Jorge Valdez García, head of the Educational Research and Innovation Unit in Health Sciences at the Institute for the Future of Education (IFE) at Tecnológico de Monterrey. He is a member of Mexico’s National System of Researchers and of the Mexican Academies of Surgery and Sciences.
