Ultrastructural and histochemical studies of the rat iris: identified neuronal inputs and supportive glia

A detailed ultrastructural description was made of the rat iris with special emphasis on nerve fibre populations and their supportive glial cells. The location and identity of different autonomic (sympathetic and parasympathetic) and sensory nerves was further studied by monoamine histofluorescence, acetylcholinesterase histochemistry, immunohistochemistry for substance P and neurofilament, and Linder's silver staining. Schwann cells were defined with immunohistochemical techniques using antiserum to glial fibrillary acidic protein. By correlation of these morphological techniques, the relative proportion of different neuronal inputs and their glial constituents in various parts of the rat iris could be determined. The sympathetic and parasympathetic unmyelinated fibres showed the well-known preferential localization in the posterior part, approaching the smooth muscle cells and chromatophores in the dilator muscle. Larger arterioles in the anterior loose stroma of the iris were in close proximity to sympathetic fibres as indicated by monoamine histofluorescence. Sensory trigeminal nerves were visualized both with Linder's silver staining and neurofilament immunohistochemistry. Large myelinated axon bundles in the anterior part of the iris were clearly seen, and thin unmyelinated fibres were scattered throughout the anterior and posterior parts. Unmyelinated substance P-containing fibres were scattered preferentially in the anterior part of the iris, without close association with blood vessels. Distribution of supportive cells, indicated by means of immunofluorescence with antiserum against glial fibrillary acidic protein, appeared largely similar to that of neurofilament-positive nerve fibres. In the sphincter muscle, cholinesterase-positive nerve fibres were densely packed and adrenergic fibres as well as sensory, neurofilament- and substance P-containing fibres were sparse but distributed throughout the muscle. Accompanying glial cells positive for glial fibrillary acidic protein were also found.