Accelerated delimitation of the infarct zone by capillary-derived nestin-positive cells in aged rats.

An important cellular event associated with reduced structural and functional recovery after stroke in aged animals is the early formation of a scar in the infarcted region that impairs neural recovery and repair. Despite the detrimental impact of infarct scar formation, the brain regions and cell types that supply the components of the scar are not well characterized. We hypothesized that premature cerebral scar formation in aged animals is associated with an altered cellular response to cerebral ischemia. Focal cerebral ischemia was produced by reversible occlusion of the right middle cerebral artery in 3 month- and 20 month-old male Sprague Dawley rats. After 3, 7, 14, and 28 days, brain tissue was subjected to real-time reverse-transcriptase-PCR (RT-PCR) and immunostaining for 1) a cellular proliferation marker (BrdU); 2) a neuroepithelial marker (nestin); 3) an astrocytic marker (glial fibrillary acidic protein [GFAP]); 4) a neuronal marker, doublecortin; and 5) a basal lamina marker (laminin), and analyzed using 3D-reconstruction of confocal images. In this model the infarct was localized primarily in the parietal cortex. By RT-PCR there was a robust increase in nestin mRNA transcripts shortly after stroke, and this increase was particularly intense in aged rats. Accordantly, we found in aged rats a rapid delimitation of the infarct area by nestin-positive cells and an early incorporation of these cells into the glial scar. The capillaries of the corpus callosum were the major source of proliferating, nestin-positive cells, many of which were also immunoreactive for doublecortin, although a smaller population of nestin cells were associated with the ventricular walls. Despite the proliferation of nestin cells, they did not make a significant contribution to neurogenesis in the infarcted cortex, possibly because the corpus callosum impedes the migration of subventricular zone-derived nestin-positive cells into the lesioned area. We conclude that: (i) the aged brain has the capability to mount a cytoproliferative response to injury, but the timing of the cellular and genetic reaction to cerebral insult is accelerated in aged animals; (ii) the proliferating cells contribute to the formation of the glial scar, but few of the cells appear to become neurons; and (iii) the vasculature plays a hitherto unrecognized role as a source of proliferating cells after stroke. Because capillary-derived cells help to form the glial scar, elucidating the molecular basis of this phenomenon and its acceleration in the aging brain could yield novel approaches to enhancing neurorestoration in the elderly.