Expression of Surfactant Proteins in the Human Canaliculus: Evidence and Potential Insights Into the Tear Flow Dynamics

Purpose: To investigate the presence and distribution patterns of 6 surfactant proteins (SPs) in the human lacrimal canaliculus. Methods: The study was performed on fresh frozen cadaveric samples of canaliculi. Immunohistochemical labeling was performed for assessing the presence and distribution of SP: SP-A, SP-B, SP-C, SP-D, SP-G/SFTA2, and SP-H/SFTA3. Immunofluorescence double staining was performed using the respective fluorescein-conjugated antibodies and the results were scored as positive or negative and the distribution pattern within the canalicular system was assessed. Western blot analysis was performed on the protein content which was resolved by reducing 15% sodium dodecyl sulfate-polyacrylamide electrophoresis and bands were studied following staining with primary and secondary antibodies. Human lung tissues were used as controls. Results: Fluorescence double staining with 4,6-diamidino 2-pheynlindole and SPs showed strong immunostaining for SP-A, SP-B, SP-C, SP-D, and SP-H/SFTA3. The positive immunofluorescence was noticed across all the layers of the epithelium but not the subepithelial structures. The expression was noted on the surfaces and superficial cytoplasm of the superficial and deep epithelial cells. There was no expression of SP-G/SFTA2 across the canalicular system. Western blot analysis of the proteins confirmed and concurred with the immunofluorescence findings. Conclusions: This study provides a proof of principle for the presence of SPs known from lungs in the canalicular system and hypothesizes their possible functions and also their potential role in the tear flow dynamics between the ocular surface and the lacrimal drainage system. Accepted for publication April 24, 2018. M.J.A. received support from the Alexander von Humboldt Foundation for his research and he also receives royalties from Springer for the textbook Principles and Practice of Lacrimal Surgery and treatise Atlas of Lacrimal Drainage Disorders and is a registered PhD student at the University of Hyderabad. F.P. was supported by Deutsche Forschungsgemeinschaft (DFG) grants PA738/1-1 to 1–5 as well as PA738/2-1. He receives royalties from Elsevier for the anatomy atlas Sobotta and the Sobotta Textbook of Anatomy. The authors have no financial or conflicts of interest to disclose. Address correspondence and reprint requests to Mohammad Javed Ali, F.R.C.S., Institute of Anatomy II, Friedrich-Alexander-University of Erlangen-Nürnberg, Erlangen, Germany. E-mail: drjaved007@gmail.com © 2018 by The American Society of Ophthalmic Plastic and Reconstructive Surgery, Inc., All rights reserved.