Table 1.

Summary of the contributions of key mucins, secretory gel forming and membrane associated, to the physical properties and stability of human tear film, the mucin alterations in dry eye pathologies, and possible mucin oriented therapies of DES. Detailed review of the biochemistry and immunology of the diverse mucin types present in ocular surface tissues and of the different DES subtypes can be found elsewhere [6,7,30,54,74,75].

Type of Mucins	Contribution to TF Physical Properties and Stability	Alterations in Pathology	Mucin Oriented Therapeutics
Secreted gel forming mucins: MUC5AC (most abundant), MUC5B, MUC2, and MUC19	The mucus gel ensures the shear thinning property of tears, facilitates the spreading of the lipid layer and acts as surface chemistry trap and barrier against invasion of lipids and exogenous agents towards the corneal surface.	Quantitative abnormalities of MUC5AC are observed in as diverse range of DES subtypes as aqueous tear deficient DE and evaporative DE	Diquafosol tetrasodium, lacritin
Membrane associated mucins: MUC1, MUC4, MUC16	MAM ensure ideal wettability and high lubricity of the ocular surface with MUC16 (having the longest hydrophilic ectodomain) considered especially critical. By ensuring no slip condition of the cornea and viscosity gradient across the TF, MAM might cooperate with secretory mucins on the shear thinning properties of tears.	Altered in decreased wettability DE subtype, characterized with normal AT volume and TFLL, but rapid breakup immediately at or <5 s after eye opening	Diquafosol tetrasodium, rebamipide