Three-dimensional microanatomy of perineuronal proteoglycan nets enveloping motor neurons in the rat spinal cord

Spinal motor neurons possess reticular coats of extracellular matrix proteo glycans on their somata and proximal dendrites. In order to define the anat omical background of the network, spatial relationships of the perineuronal proteoglycans with synaptic boutons and astrocyte processes were analyzed in rat motor neurons by TEM after histochemical detection of the substances with cationic iron colloid, and by SEM after exposure of the cytoarchitect ure with NaOH maceration. Narrow intercellular channels filled with proteog lycan were found to extend along the surface of the neurons to form a homog eneous network of a mesh size of about 1 mu m. The system of perineuronal c hannels consisted of two parts: a primary intervaricose net which meandered among synaptic boutons on the surface of the motor neuron, and secondary s ubvaricose nets which irrigated interfaces between larger boutons and the n euron. No elements in the perineuronal cytoarchitecture coincided with the meshwork of proteoglycan, indicating the involvement of postsynaptic factor s in the distribution of the substance. Thin astrocyte processes surroundin g the neurons formed a distinct network with heterogeneous meshes correspon ding to boutons of various sizes. The perineuronal glial nets extended thei r surface area in contact with the intervaricose nets of proteoglycan by co mplex cellular interdigitations. The subvaricose nets of proteoglycan compa rtmentalized multiple synapses on large boutons, suggesting an involvement in the division of the synapses during development.