Functional anatomy of human lacrimal duct epithelium

Resorption of tear fluid in the lacrimal ducts has hitherto been controversial; one reason for this has been insufficient knowledge of the anatomical structure and function of the lacrimal duct epithelium. The present study analyzes the structure of lacrimal duct epithelium by means of histological, histochemical, immunohistochemical and electronmicroscopical methods and draws a conclusion about its physiological function regarding its role in immunodeficiency. Investigations were performed on 31 lacrimal systems of 17 male and 14 female individuals (aged 54-88 years). Lacrimal ducts are surrounded by a wide-ranging cavernous system, which is embedded in an osseous canal between the maxilla and the lacrimal bone. The internal wall of the lacrimal canaliculi is lined by a stratified epithelium. The lacrimal sac and nasolacrimal duct contain a double-layered epithelium, which rests on a broad basement membrane. In their apical part epithelial cells contain large lipid droplets and secretory vacuoles. Epithelial cells are faced by microvilli and some tufts of kinociliae are also visible. Goblet cells are integrated in the epithelium as solitary cells or in a characteristic arrangement of several cells. The secretory product of these cells contains carbohydrates including fucose and sialic acid. Inside the surrounding cavernous system serous glands are found that open their excretory ducts into the lacrimal sac and nasolacrimal duct. Some T- and B-lymphocytes and macrophages may be demonstrated immunohistochemically in the submucosa partly penetrating the epithelium. Synthesized mucins of goblet cells form a specialized protective layer on the epithelium of the lacrimal ducts, which functionally serves for a simplified drainage of tear fluid into the inferior meatus of the nose. Together with immunocompetent cells, the protective layer plays a role in antigen defense and prevents invasion of pathogenic agents. The facing of epithelial cells by microvilli gives hints of re-absorption of lacrimal fluid inside the lacrimal ducts.