Figure 7. Ultrastructural features of irradiated rat and human brain tissue.

Electron microscopy of rat (upper panel) and human (lower panel) tissues in normal controls (A, D), an early/intermediate time point post radiation (B, E, 11 months and 7 months, respectively) and a late time point (15 months in the rat (C) and 7 years in the human (F)). Ultrastructural analysis of the myelin sheaths demonstrates normally compacted lamellae in the normal brains. At about 7 months post XRT, myelin sheaths in both human and rats (B and E and insets) acquire an irregular appearance with segmental loss of lamellar compaction associated with separation at the intraperiod line. These changes are more prevalent in larger myelin sheaths and are often mixed with normal myelinated fibers (arrows in (B) and (E)). Later times post XRT are associated with an increasing frequency and severity of myelin sheath degradation and vesiculation. Insets are magnifications of representative areas of myelin sheaths in each panel. Semithin toluidine sections of irradiated human tissue are shown in (G) and (H). Evidence of myelin sheath fragmentation is seen in the early/intermediate time point (14 months post XRT in (G)) as well as cytoplasmic lipid debris (white arrows) suggestive of active myelin degradation. Abnormal myelin sheaths persist and occur at higher frequency at late time points (7 years in H). Scale bar in (F) corresponds to 0.85 µm in (A), (D) and (E), 0.3 µm in (B) and (F) and 0.5 µm in (C). Scale bar in (H) corresponds to 10 µm in (G) and (H).