97th DOG Annual Meeting 1999
ABSORPTION OF TEAR FLUID COMPONENTS IN THE EFFERENT TEAR DUCTS
F. Paulsen, A. Thale and R. Mentlein
In 1964 Rohen suggested absorption of tear fluid through the lining epithelium of the human efferent lacrimal passage. Recent investigation could confirm this hypothesis. To determine if absorption of protein components in the tear fluid really occurrs in normal efferent tear ducts an animal experiment was carried out.
Methods: Iodinated albumin was dropped into eyes of femal rats. After 10, 20 and 60 min the rats were killed, blood collected and the heads embedded for histology. Serum was obtained from the clotted blood and the radioactivity in a protein sediment and the combined supernatants counted. In a second approach, serum was fractionated by molecular mass and radioactivity in the fractions measured. Furthermore autoradiographs of rat head sections were performed.
Results: Low uptake of radioactivity into the serum was observed, but increased with time. After 60 min a maximal incorporation of 0.13% of the applied radioactivity into the blood was measured that was mostly (70-80%) not protein bound. Gel chromatographic separation according to molecular mass yielded fractionated peaks of radioactivity corresponding to albumin with maximal 4.8 Bq/ml serum, iodinated tyrosine (5.5 Bq/ml), and free iodine (237 Bq/ml, each after 60 min). Histology of the rat efferent lacrimal tear ducts showed a multilayered lining epithelium with integrated goblet cells in a characteristical arrangement of several cells. In autoradiographs of rat head sections no transport of radioactivity could be visualized.
Conclusion: In rats only traces of iodinated albumin are incorporated from the efferent lacrimal tear ducts into the blood. An even higher part of the radioactivity is taken up as the proteolytic degradation product of bovine serum albumin to free amino acids, and 96% of the radioactivity incorporated was free iodine probably as a contaminant of iodinated preparation. The rat is unfit for studying absorptive processes in the efferent tear ducts. Nevertheless recent studies could show that the epithelium of the human lacrimal sac and nasolacrimal duct plays an important role in tear outflow, microbial defence, receptor-mediated binding of bacteria and by the protective mucus layer. Further investigations by comparative anatomy will be necessary to evaluate an animal which can serve for absorption experiments in the lacrimal passage.
Department of Anatomy, University of Kiel, Olshausenstraße 40, D-24098 Kiel