What happens to tears inside the efferent lacrimal passage?

Abstract 

Background: To determine whether absorption of protein components of the tear fluid occurs in normal efferent tear ducts, an animal experiment was carried out. Methods: Iodinated albumin was dropped into eyes of female rats. After 10, 20 or 60 min the rats were killed, blood collected and the heads embedded for histological examination. 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: Uptake of radioactivity into the serum was low, but increased with time. After 60 min maximal incorporation of the applied radioactivity into the blood was 0.13%; most (70–80%) of the incorporated radioactivity was 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). Histologically the rat efferent lacrimal tear ducts showed a multilayered lining epithelium with integrated goblet cells in a characteristic 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. A higher proportion 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 the iodinated preparation.