In terrestrial mammals, the preocular tear film plays an essential role in maintaining ocular surface integrity, protecting against toxic challenges in the external environment, and preserving visual acuity. These protective functions are critically dependent upon the composition and stability of the tear film structure, which includes an underlying glycoprotein or mucin foundation (derived from goblet and epithelial cells), a protein-rich aqueous component (secreted primarily by main and accessory lacrimal glands), and an overlying lipid layer (originating from the meibomian glands). However, it is unclear whether the tear film serves an analogous role in protecting the eyes of marine mammals. In a previous study1, we determined that tears of marine mammals do contain proteins, but that the tear film, examined using interferometry, appears to lack the lipid layer found in terrestrial mammals. This finding is quite significant in that it indicates that in sea mammals an alternative component, in lieu of lipid, promotes the stability of the tear film. This led us to hypothesize that, in the absence of a lipid layer, a mucin foundation, similar to that found in terrestrial mammals, may be critical to the integrity of the tear film of marine mammals. If this were true, mucins would be likely to be present in the tears secreted by marine mammals. The purpose of this study was to determine whether marine mammals express tear glycoproteins similar to those found in terrestrial mammals. In order to assess whether glycoproteins were present in tears, samples from bottlenose dolphins and sea lions were analyzed for lectin binding. Lectin binding to high molecular weight proteins (greater than 250 Kd) would be strongly indicative of the presence of large glycoproteins, or mucins.
We thank Dr. Sam Ridgway (U.S. Navy Marine Mammal Program, San Diego, CA) for generously providing ocular secretion samples from bottlenose dolphins. We also appreciate the collaboration, technical support, and expertise of the staff at the New England Aquarium (Boston, MA) in helping us to collect tear samples from their resident seals and sea lions. This work was supported by grants from Arey's Pond Boat Yard, Inc. and N.I.H. [EY05612].
1. Davis RK, Richards SM, Doane MG, DA Sullivan. 2005. Characteristics of Ocular Secretions in Marine Mammals. Presented at the 36th Annual Conference of the International Association for Aquatic Animal Medicine, Seward, Alaska.