Nerve endings with structural characteristics of mechanoreceptors in the human scleral spur

The innervation of the scleral spur region was investigated to learn whether mechano-receptors are present in this region.

Serial tangential sections and whole-mount preparations of the scleral spur region of 18 human eyes of different ages were investigated with electronmicroscopic and immunohistochemical methods. For immunohistochemistry antibodies against neurofilament-proteins, synaptophysin, substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), tyrosine-hydroxylase, dopamine-beta-hydroxylase, and acetylcholinesterase were used.

Club- or bulb-shaped nerve endings with a diameter of 5 microns to 25 microns were identified in the scleral spur region throughout the whole circumference of the eyes. The terminals derive from myelinated axons with a diameter of approximately 3 microns and stain with antibodies against neurofilament-proteins and synaptophysin but do not stain for tyrosine-hydroxylase, dopamine-beta-hydroxylase, acetylcholinesterase, NPY, VIP, SP, or CGRP. Electronmicroscopically, the endings contain abundant neurofilaments, granular and agranular vesicles of different sizes, numerous mitochondria, and lysosome-like lamellated structures. The endings are incompletely ensheathed by Schwann cells. Those areas of the cell membrane of the endings that are not covered by Schwann cells are in intimate contact with the fibrillar connective tissue elements of the scleral spur.

These structural features are highly characteristic for mechanoreceptive nerve endings in other tissues of the human body. The authors therefore hypothesize that the club-or bulb-shaped nerve endings in the human scleral spur are afferent mechanoreceptors that measure stress or strain in the connective tissue elements of the scleral spur. Such changes might be induced by ciliary muscle contraction and/or by changes in intraocular pressure.