Cell damage and tissue repair in the central nervous system : electron mi[c]roscopy study of neuronal death and cell replacement

Abstract

The central nervous system is vulnerable to various insults and particularly to ischemia. To mimic ischemia, a photochemical or compression lesion was induced in the right sensory motor cortex of rat brains. We studied the time course of ultrastructural changes in cortical neurones after lesioning, and the occurrence of different types of neuronal death was examined with respect to a potential therapeutic window. The lesion's appearance was documented by magnetic resonance imaging (MRI). At 0.5, 1, 3, 6, 12, 24, 48 and 72 hours post-lesion, cortical neurones were examined by electron microscopy (EM). Following a photochemical lesion, the neuropil in the lesioned area appeared disorganised at 0.5 h, while necrotic and apoptotic cells were identified as separate bodies. Three hours later the tissue was disintegrated. On the contralateral side, ruptured membranes were found at 3 h, which is a sign of irreversible cell death. Following a compression lesion, apoptotic cell death was most frequent at 12 h in the lesioned area, and signs of secondary delayed cell death, e.g. an enlarged endoplasmatic reticulum, were found at 3 h.

Following a cortical photochemical lesion, neurogenesis was studied after beam-walking and fluoxetine pre-treatment. Dividing cells, confirmed by bromodeoxyuridine staining and EM, migrated to the border of the lesion, and their number was enhanced after fluoxetine treatment. Embryonic stem cells and bone marrow stromal cells, labelled with the iron-oxide nanoparticle Endorem®, were implanted into rat brains following a cortical photochemical lesion or a spinal cord compression lesion. Iron-containing cells, confirmed by Prussian blue staining and EM, were injected either into the contralateral hemisphere or intravenously into the femoral vein. The fate of labelled cells was tracked in vivo using MRI, which at seven and 14 days post-injection showed labelled cells migrating to the injury site.

The time course of ultrastructural changes in spinal cord neurones following a compression lesion was studied. EM showed at 0.5-6 h apoptotic and at 12-72 h necrotic cell death in the vicinity of the lesion.

The studies demonstrate that the chosen models are useful when studying ultrastructural changes in injured cells. As the morphology drastically changed at 3 h, the cellular alterations at this time point might represent a breakpoint at which cells either progress towards cell death or recover. Fluoxetine enhances stem cell migration towards a lesion. Endorem®-labelled stem cells remain viable and migrate to a lesion site; thus, Endorem® can be used for MRI tracking of implanted stem cells in animals and humans.