Labelling of amoeboid microglial cells in the supraventricular corpus callosum following the application of horseradish peroxidase in the cerebrum and spinal cord in rats.

Following an injection of horseradish peroxidase solution into the cervical or lumbosacral segments of the spinal cords, or into the parietal cortices, of postnatal (1-10 days), developing (15-20 days) and mature (2 months and older) rats, only postnatal rats demonstrated peroxidase-positive amoeboid microglial cells in the supraventricular part of the corpus callosum. The amoeboid microglial cells were bilaterally labelled following a unilateral injection into the parietal cortex or the spinal cord. They were, however, not labelled after 15 minutes, but were definitely labelled 1 hour after the injection of the solution into the spinal cord. Also, they were not labelled when the solution was injected into the central canal or the subarachnoid space. The results suggest an ascending diffusion of the injected horseradish peroxidase in the spinal cord via the wide interstitial spaces to reach the cerebrum where it is engulfed by the amoeboid microglial cells. An enhanced activity of endogenous peroxidase was also indicated by the observation of peroxidase product in the lysosomal granules of the amoeboid microglial cells.