Mucins are the glycoproteins which form the viscous, gel-like mucus la
yer of the tear film. Molecular characterization of mucins has been sl
ow due to their heavy glycosylation and high molecular weight, but rec
ent cloning of human gut and trachea, mammary gland, and salivary glan
d mucins has begun to shed light on the primary structure of these imp
ortant protective molecules. To date nine human mucin genes, designate
d MUC1-8, have been cloned. The presence in the protein backbone of ta
ndem repeats of series of amino acids rich in serine and/or threonine
is a feature common to all. Numbers and sequences of amino acids in ea
ch tandem repeat varies with each mucin. Numbers of amino acids per re
peat vary from 169 (MUC6) to 8 (MUC5AC). Until recently little has bee
n known regarding the expression of these genes by the ocular surface
epithelium. This review summarizes recent work from our laboratory aim
ed at determining the molecular character of mucins expressed by conju
nctival and corneal epithelium. Using northern blot analysis and in si
tu hybridization techniques, we have demonstrated that the stratified
epithelium of both cornea and conjunctiva express the transmembrane mu
cin, MUC1. This widely expressed mucin is the best characterized of th
e cloned mucins. Using similar methodologies we demonstrated that the
conjunctival goblet cell expresses MUC5AC, a gel-forming mucin, which
has a cysteine-rich domain responsible for the disulfhydryl bonding be
tween mucin molecules. Conjunctival stratified epithelium expresses MU
C4, a relatively uncharacterized mucin, whose function is not known. A
re there additional and/or unique mucins expressed by the ocular surfa
ce epithelium? We summarize our recent work on characterization of a m
ucin isolated from rat and human corneal epithelium. Antibodies to the
se mucins localize to the glycocalyx of the corneal epithelial apical
cells. Since MUC1 appears to be the only one of the cloned mucins expr
essed by the corneal epithelium. it is a candidate for comparison to o
ur mucin isolate. Current efforts are directed toward this comparison.
Identification of mucin genes expressed by the ocular surface epithel
ium opens many new avenues for investigation into expression, regulati
on, and glycosylation of mucins in ocular surface pathologies and into
the specific character of these molecules which enhance protection fr
om pathogen invasion. Copyright (C) 1996 Elsevier Science Ltd.