Hyperthermia induces ultrastructural changes in mouse pial microvessels

Background: Pial microvessels' responses to local hyperthermia revealed the development of in vivo spontaneous thrombosis. The cellular and subcellular changes which contribute to such events remained unexplored. Therefore, the effect of regional hyperthermia (43°C) on mouse pial microvessels was studied at the ultrastructural level. Methods: A simple cranial window assembly, including an artificial cerebrospinal fluid delivery and heating system to ensure a precise brain regional temperature, was used. The animal core body temperature was maintained at 37°C. Topical and transvessel bimodal fixation of microvessels was done with a phosphate buffered mixture of glutaraldehyde and paraformaldehyde, followed by a standard electron microscopy procedure. Results: When the pial microvessels of control (37°C) animals were examined, no evidence of cellular damage was discerned. Endothelial. cells including luminal membrane were unchanged. Degranulated platelets or platelet aggregates were not seen. However, numerous platelets in association with scattered red blood cells and occasional white blood cells could be observed in a close proximity, but not adhered, to the endothelial wall of hyperthermic (43°C) brains. Platelets displayed a variety of forms consistent with the onset of platelet activation. Discoid platelets containing granules and spheroid degranulated platelets and those with large pseudopodia were recognized. The venular endothelial surface revealed conspicuous endothelial change, with the presence of endothelial denudation. The site of platelet aggregation in both venules and arterioles was accompanied by focal endothelial lucency and denudation vacuole formation, luminal membrane rupture, and swelling of the nuclear envelope. Conclusions: These findings demonstrate the extent of damage to the pial microvasculature in response to a local hyperthermic exposure. The results emphasize that changes in the endothelium may represent the earliest signs of oncoming vascular pathology. © 1995 Wiley‐Liss, Inc.