Innervation of the cavernous body of the human efferent tear ducts and function in tear outflow mechanism.

Abstract

The lacrimal sac and nasolacrimal duct are surrounded by a wide cavernous system of veins and arteries comparable to a cavernous body. The present study aimed to demonstrate the ultrastructure of the nervous tissue and the localisation of neuropeptides involved in the innervation of the cavernous body, a topic not previously investigated. Different S-100 protein antisera, neuronal markers (neuron-specific enolase, anti-200 kDa neurofilament), neuropeptides (substance P, neuropeptide Y, calcitonin gene-related peptide, vasoactive intestinal polypeptide) and the neuronal enzyme tyrosine hydroxylase were used to demonstrate the distribution pattern of the nervous tissue. The ultrastructure of the innervating nerve fibres was also examined by means of standard transmission electron microscopy. The cavernous body contained specialised arteries and veins known as barrier arteries, capacitance veins, and throttle veins. Perivascularly, the tissue was rich in myelinated and unmyelinated nerve fibres in a plexus-like network. Small seromucous glands found in the region of the fundus of the lacrimal sac were contacted by nerve fibres forming a plexus around their alveoli. Many nerve fibres were positive for S-100 protein (S 100), neuron-specific enolase (NSE), anti-200 kDa neurofilament (RT 97), calcitonin gene-related peptide (CGRP), substance P (SP), tyrosine hydroxylase (TH), and neuropeptide Y (NPY). Vasoactive intestinal polypeptide (VIP) immunoreactivity was only demonstrated adjacent to the seromucous glands. Both the density of nerve fibres as well as the presence of various neuropeptides emphasises the neural control of the cavernous body of the human efferent tear ducts. By means of this innervation, the specialised blood vessels permit regulation of blood flow by opening and closing the lumen of the lacrimal passage as effected by the engorgement and subsidence of the cavernous body, at the same time regulating tear outflow. Related functions such as a role in the occurrence of epiphora related to emotional responses are relevant. Moreover, malfunction in the innervation of the cavernous body may lead to disturbances in the tear outflow cycle, ocular congestion or total occlusion of the lacrimal passages.