The mucin component of the mucoaqueous layer coats the microplicae of the superficial corneal epithelial cells and forms a fine network over the conjunctival surface. In addition to mucin, it contains proteins, electrolytes, water, and carbohydrates in a polar glycocalyx. Mucins are glycoproteins; they have a protein backbone modified by the covalent addition of multiple long carbohydrate chains composed of repeating sugar molecules strung end to end (see Figs 7-2, 7-3).
Two main types of mucins are produced within the body: secreted and membrane-spanning. Secreted mucins are
divided into gel-forming mucins and soluble mucins
released into the extracellular environment
secreted principally by the goblet cells of the conjunctiva
Membrane-spanning mucins (also called membrane-anchored, membrane-bound, membrane-tethered mucins) are
Some think that the membrane-spanning mucins help spread the secreted mucins across the ocular surface. Both are minimally secreted by the main lacrimal gland. Goblet cells produce mucin at a rate of 2–3 μL/day, which contrasts with the 2–3 mL/day of aqueous tear production.
Tear dysfunction may result when tear mucins are deficient in number (eg, in vitamin A deficiency and conjunctival destruction), excessive in number (eg, in hyperthyroidism; foreign-body stimulation; and allergic, vernal, and giant papillary conjunctivitis), or biochemically altered (eg, in keratoconjunctivitis).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.