Mucins are large proteins decorated with sugar molecules. This “sugar coating” allows them to hold a significant amount of water and form gel-like secretions. These slippery, viscous secretions create lubricating and protective barriers across biological surfaces. Produced by epithelial tissues in most animals, mucins maintain hydration and defend against physical stress and pathogens. Their structure also makes them resistant to breakdown by enzymes, which protects underlying tissues.
Mucin in the Digestive and Respiratory Systems
The digestive and respiratory tracts are lined with mucous membranes that use mucin for protection. In the digestive system, this begins in the mouth, where mucins in saliva lubricate food for easier swallowing. The stomach has a thick mucus layer rich in mucins like MUC5AC and MUC6, which provides a barrier protecting the lining from its own corrosive acid. In the intestines, mucin lubricates food passage and forms a barrier that limits contact between bacteria and intestinal cells.
In the respiratory system, from the nasal passages to the lungs, mucin is a component of mucus. Goblet cells and submucosal glands secrete gel-forming mucins, primarily MUC5AC and MUC5B, which create a sticky layer to trap inhaled particles, allergens, and microorganisms. This layer is propelled upward by the beating of cilia, a process known as the mucociliary escalator, which moves trapped debris out of the lungs.
Mucin in Other Bodily Systems
Mucin is also found in specialized locations, such as on the surface of the eye. The stability of the tear film that coats the eye depends on a mixture that includes mucins. Cells of the cornea and conjunctiva produce membrane-bound mucins (MUC1, MUC4, MUC16) that form a protective scaffold. Another mucin, MUC5AC, is secreted into the tear film to lubricate the eye, allowing the eyelid to blink without abrasive stress. This mucin layer ensures tears adhere evenly to the corneal surface, preventing dry spots and maintaining clear vision.
In the female reproductive tract, the consistency of mucin-rich cervical mucus changes throughout the menstrual cycle. These changes can create a permeable environment that helps the movement of sperm toward the egg during ovulation. At other times, the mucus can form a thick, impassable plug that blocks sperm and protects the uterus from pathogens.
Mucin Beyond the Human Body
Mucin’s function extends into the natural world, with snails and slugs being a clear example. The mucin-rich slime they produce provides lubrication for movement and acts as an adhesive for climbing. This external mucus also provides defense against microbial threats and keeps their delicate bodies from drying out. The properties of snail mucin, with its high concentration of beneficial compounds like hyaluronic acid, glycolic acid, and antioxidants, have led to its widespread use in skincare products.
This substance is harvested and purified for use in serums and creams for its hydrating and regenerative capabilities. Many species of fish are covered in a layer of mucus that serves a similar protective role. This coating acts as a barrier against parasites and pathogens in their aquatic environment, reduces drag while swimming, and can aid in wound healing.