discovering dry eye
The "Slippery" Ocular Surface
BY JASON J. NICHOLS, OD, MS, MPH

Mucus is a slimy substance that coats many of the epithelial surfaces of the body. It's mainly composed of mucins, inorganic salts and water. Mucins are a family of large, heavily glycosylated proteins. Human mucins are designated as MUC1 (ocular), MUC2, MUC3, MUC4 (ocular), MUC5AC (ocular), MUC5B and MUCs6-8. The purposes of mucus are to act as a barrier to noxious substances and to act as a lubricant to minimize stresses between tissues (for example, between the ocular surface and palpebral conjunctiva).

In addition to its presence on the ocular surface, mucus is also prominent on the epithelia of the respiratory, gastrointestinal and genital tracts. In this regard, its presence throughout the body is vital to appropriate physiological function.

Figure 1. Lissamine green conjunctival staining of the nasal conjunctiva in a contact lens wearer.

The Mucus Layer

The mucus layer of the tear film is the most posterior portion of the tears. It's produced primarily by the goblet cells of the bulbar conjunctiva but also by the epithelial cells of the ocular surface (which contain mucus secretory vesicles) and crypts of Henle in the fornices.

The goblet cells are specialized apocrine cells that number about 1.5 million on the ocular surface and are most concentrated over the nasal conjunctiva. Mucus secretion isn't well understood but is probably controlled neurally. Recent studies are showing that P₂Y₂, which is a purine-derivative neuromodulator, stimulates the goblet cells, resulting in mucus production. In this regard, contact lens wear, which has been shown to be associated with a reduction in sensitivity of the ocular surface, may influence neurally stimulated mucus secretion.

The mucus layer of the tear film is composed of various glycosylated proteins that are about 50 percent carbohydrate by mass. MUC5AC is thought to be the major "mucus gel" of the tear film, while MUC1 is thought to aid in the spreading of MUC5AC; the function of MUC4 in relation to the ocular surface is not presently known.

Both dry eye disease and contact lens wear have been shown to be associated with a reduction in goblet cell density and thus, potentially, with an alteration to this layer.

Identifying Damaged Areas

I often find significant conjunctival epithelial staining (using lissamine green) located on the nasal portion of the conjunctiva in many of my lens wearing patients (Figure 1). The cause of this staining isn't clear, and it might be because of mechanical rubbing from the lens, a tear deficiency, inflammation or an interaction of any number of these factors. Regardless, it seems reasonable to suggest that these changes might also reflect concurrent damage to the mucus secretory system as goblet cell density is greatest over the nasal conjunctiva.

Drawing Conclusions

If indeed lens wear decreases goblet cell density, then the ability of the mucus to protect and lubricate the ocular surface will also decrease. Thus, it's wise to recommend the use of a lubricant rewetting drop for wearers who have these signs of staining, as the assumption of underlying problems is probably true.

Dr. Nichols is a research scientist at The Ohio State University College of Optometry.