The preocular tear film of terrestrial mammals plays an essential role in maintaining ocular surface integrity, protecting against microbial challenge, and preserving visual acuity. These functions, in turn, are critically dependent upon the composition and stability of the tear film structure, which includes an underlying mucin foundation (glycoprotein 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 fact, the protein and lipid composition of ocular surface secretions of sea mammals has not been previously studied. The purpose of this study was 2-fold: first, to determine whether marine mammals express tear proteins similar to those of humans; and second, to examine whether the tear film of sea mammals has an outer lipid layer.
Ocular surface secretions collected from Atlantic bottlenose dolphins, harbor seals, sea lions, and humans were processed for protein analysis by using an Agilent Technologies Protein 200 Plus Assay Kit. Protein electrophoresis was performed using a Bioanalyzer 2100. Evaluation of the seal and sea lion tear film for lipid content was conducted by using thin film interferometry. This non-invasive technique involved imaging of the ocular surface using a handheld interferometer connected to a video monitor.
We found that ocular surface secretions of marine mammals contain multiple proteins. Prominent proteins were identified in sea lions (kDa ranges: 14-23; 22.2-24.1; 59.6-60.1; and 92.6-99), dolphins (kDa bands: 13.2; 15.8; and 54.8), and seals (kDa band: 15.1). For comparison, human tears analyzed in the same way were found to contain proteins in a similar molecular weight range (kDa bands: 16.4; 21.5; 24.5; 29.7; 56.4; 61.4; and 99.1). In contrast to what is known for terrestrial mammals, we found no evidence of a lipid layer in seal or sea lion tear film.
In the present study we have demonstrated that the tear film of marine mammals contains a variety of proteins that fall in a similar size range to those of humans. However, unlike terrestrial mammals, seals and sea lions do not appear to have an outer lipid layer on their tear film. This finding is quite significant in that it suggests that sea mammals use an alternative approach, in lieu of lipid, to promote the stability and prevent the evaporation of the tear film.
We thank Dr. Sam Ridgway (Naval Ocean Systems Center, 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 and perform interferometry with their resident harbor seals and sea lions.
(Supported by grants from Arey's Pond Boat Yard, Inc. and NIH [EY05612])