[Tear outflow. Impact of mucins and TFF-peptides]

The epithelial lining of the lacrimal sac and the nasolacrimal duct consists of pseudo-stratified, columnar epithelia rich in goblet cells. Major secretory products of the epithelial cells are mucins together with TFF peptides. Expression and distribution of several mucins and TFF peptides in the human efferent tear ducts was investigated by means of reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. mRNAs for all the mucins investigated, MUC1, MUC2, MUC4, MUC5AC, MUC5B and MUC7, were detected in healthy human lacrimal sacs and nasolacrimal ducts. Both MUC5AC and MUC5B were detected in goblet cells forming intraepithelial mucous glands. MUC7 together with TFF3 occurred only in columnar epithelial cells of the efferent tear duct system. The mucin diversity of the efferent tear ducts could enhance tear transport and antimicrobial defense. The absence of some mucins in non-functioning although patent segments of the lacrimal passage, suggests that mucins ease tear flow through the efferent tear ducts because these conditions are associated with epiphora. Disorders in the balance of single mucins could be of importance with regard to dacryostenosis, dacryocystitis and dacryolith formation.