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Robin Kelleher Davis, Nadja Knop, Erich Knop, David A. Sullivan, Pablo Argueso; The Marine Mammal Tear Film Has Unique Attributes. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3723.
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In a previous study we found that pinnipeds lack the lipid layer of the tear film, leading us to hypothesize that mucin-type glycoproteins are the major protective components in the ocular surface of marine mammals. The purpose of this study was to characterize mucin-type O-glycans in marine mammal tears, and to determine whether pinnipeds have meibomian glands.
Protein and carbohydrate concentrations of tears collected from humans and marine mammals (with IRB & ACUC approvals) were determined using bicinchoninic acid and sulfuric methods respectively. Samples normalized for protein were electrophoresed on 1% SDS-agarose gels, transferred to nitrocellulose membranes by vacuum blotting, and probed for the T-antigen carbohydrate using Arachis hypogea agglutinin (PNA), followed by chemiluminescence detection. Normal-phase high pressure liquid chromatography (HPLC) was used to analyze O-glycans released by ammonia-based beta-elimination. For histology, ocular tissues from a sea lion necropsy were fixed in formalin, examined using a stereo magnifier, embedded in paraffin, sectioned to 10-20 um thickness using a rotary microtome, and stained.
Biochemical analysis revealed PNA binding to proteins of high molecular weight, consistent with mucins, in human, sea lion, and dolphin tear samples. The size distribution of PNA-positive bands was different for each species analyzed, with dolphin migrating at the highest molecular weight. Separation of O-glycans by HPLC revealed differences in chromatograms of tears from humans, sea lions, and dolphins. In stereo magnifier analysis, a distinct tarsus was missing. The lid consisted mainly of muscular tissue and underneath the epidermis, there was a whitish granular layer consisting of hair follicles and sebaceous glands. Bundles of holocrine sebaceous acini were arranged between hair follicles. Towards the lid margin, the number of hair follicles decreased and the relative volume of sebaceous glands increased, forming several solitary gland bodies at the margin. The acini drained via ductules into a straight duct with a multi-layered stratified squamous epithelium, which opened onto the outer lid skin.
Our results indicate that as in humans, mucin-type O-glycans are present in marine mammal tears, with differences in size distribution and O-glycan profiles across species. Pinnipeds have sebaceous glands at the eyelid margin, resembling human meibomian glands in structure, but different in orientation and size.
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