The innervation of the splenic capsule in the guinea pig: an immunohistochemical and ultrastructural study.

The innervation of the capsule of the guinea pig spleen was studied by light microscopy using an indirect fluorescent-labelled antibody technique, as well as by electron microscopy. A dense network of nerve fibres immunoreactive to the general neuronal marker, protein gene product 9.5 was observed in tangential sections through the capsule corresponding to the subcapsular compartment. The PGP 9.5-immunoreactivity in the fibres appeared to a large extent to be colocalised with tyrosine hydroxylase and neuropeptide Y (NPY) immunoreactivities as well as with synaptophysin immunoreactivity. Only very occasional fibres with substance P or calcitonin-gene-related peptide immunoreactivity were observed in tangential sections of the capsular region. By electron microscopy unmyelinated nerve fibres in the capsule were found to contain a large number of small dense-cored as well as clear vesicles and large dense-cored vesicles in varicose parts of the axons. The axolemma of the varicose regions was often naked, devoid of Schwann cells, and sometimes appeared denser than the nonspecialised parts of the membrane. These naked regions were observed in single sections to be apposed to splenic cells with variable intervals of extracellular space and interposed basal lamina material. Another type of contact was characterised by a very close association with splenic cells with no basal lamina interposed between the plasma membranes of the axon and the splenic cell. An intimate ultrastructural relationship was often also seen between varicose vesicle-containing axons and neighbouring axons in the nerve fibre bundles. The results show that the splenic capsule and its immediate neighbouring regions are innervated by catecholaminergic, NPY-containing fibres, which appear to establish different types of relations with the splenic cells as well as with one another.