NERVE-ENDINGS WITH STRUCTURAL CHARACTERISTICS OF MECHANORECEPTORS IN THE HUMAN SCLERAL SPUR

Purpose. The innervation of the scleral spur region was investigated t o learn whether mechanoreceptors are present in this region. Methods. 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 immunohistoc hemistry antibodies against neurofilament-proteins, synaptophysin, sub stance P (SP), calcitonin gene-related peptide (CGRP), vasoactive inte stinal polypeptide (VIP), neuropeptide Y (NPY), tyrosine-hydroxylase, dopamine-beta-hydroxylase, and acetylcholinesterase were used. Results . Club- or bulb-shaped nerve endings with a diameter of 5 mu m to 25 m u m were identified in the scleral spur region throughout the whole ci rcumference of the eyes. The terminals derive from myelinated axons wi th a diameter of approximately 3 mu m and stain with antibodies agains t neurofilament-proteins and synaptophysin but do not stain for tyrosi ne-hydroxylase, dopamine-beta-hydroxylase, acetylcholinesterase, NPY, VIP, SP, or CGRP. Electronmicroscopically, the endings contain abundan t neurofilaments, granular and agranular vesicles of different sizes, numerous mitochondria, and lysosome-like lamellated structures. The en dings are incompletely ensheathed by Schwann cells. Those areas of the cell membrane of the endings that are not covered by Schwann cells ar e in intimate contact with the fibrillar connective tissue elements of the scleral spur. Conclusion. These structural features are highly ch aracteristic for mechanoreceptive nerve endings in other tissues of th e human body. The authors therefore hypothesize that the club- or bulb -shaped nerve endings in the human scleral spur are afferent mechanore ceptors that measure stress or strain in the connective tissue element s of the scleral spur. Such changes might be induced by ciliary muscle contraction and/or by changes in intraocular pressure.