Demonstration of nerve fibers in human accessory lacrimal glands.

To provide morphologic evidence for the innervation of accessory lacrimal glands, glands were biopsied and examined using standard transmission electron microscopic techniques. Non-myelinated nerve fibers were found in the connective tissue between the glandular epithelia where they made contact with glandular epithelial cells, myoepithelial cells, vascular endothelial cells, plasma cells and fibroblasts. The distances measured between axons and target cells ranged from 30 to 130 nm. Where nerve fibers approached cells sustaining a basement membrane, their basement membranes fused to form a discrete unit resembling so-called 'synapses à distance'. Cells with no basement membrane were situated in direct contact with the basement membrane of a nerve fiber. Single axons were identified between glandular epithelial cells and cells of intralobular ducts. Most of these axons contained many small clear vesicles and a few large, dense core vesicles, a finding considered typical of cholinergic parasympathetic nerve fibers. In addition, one of the axons identified contained small dense core vesicles typical of sympathetic nerve fibers. Human accessory lacrimal glands are therefore definitely innervated, with parasympathetic structures morphologically prevailing over sympathetic structures.