Figure 5. At chronic time points neurons are found in close proximity to fibers.

Three sections from zebra finches implanted with optical microfibers, collected at least ten weeks post-implant. Sections are near the tip of the implant, within the basal ganglia (area X, depth 2.9 mm). (a) and (b) show implants in the basal ganglia (bundle sizes of 4,500 and 1,125 fibers respectively), (c) shows unimplanted basal ganglia, and (d) shows a corresponding brightfield image used to confirm fiber locations. Red is NeuN (neurons), blue is DAPI (nuclei) and green dots are manual annotations that reveal fiber locations. The immunohistochemistry shows NeuN-stained cells in close proximity to fibers. In some cases, we observe a dense circle of DAPI stained cells in close proximity to the fibers (arrow), suggesting either bleeding (in birds, red blood cells have DNA) or a reactive tissue response (such as glia or astrocytes). This most frequently occurs at locations where multiple fibers are in close proximity (this can occur with bundles of over 4,000 fibers or when fibers are wet prior to insertion). The length scale of reactive tissue response is approximately an order of magnitude smaller than for silicon electrode shanks with a 50 μm profile [13].