Analysis of microvascular changes in frostbite injury

Reported here is the characterization of the hairless mouse ear as a model system for defining the microvascular effects of minimal frostbite injury by means of gross, in vivo microscopic, histologic, and electron microscopic analysis. Initial efforts, using controlled temperatures and time of freezing, defined the minimum conditions necessary to produce consistent tissue necrosis to be -4 degrees C for 3 min. In vivo observation, after rapid thaw, showed a return of blood flow to apparent normal prefreeze rates, followed by a gradual sludging of blood 15 to 20 min post-thaw and eventually in cessation of blood flow. No vascular spasm was observed. Histologic and electron microscopic examination, unexpectedly, did not reveal evidence of early platelet or fibrin thrombi. However, marked vasodilatation and circulatory congestion began 2-3 hr after thawing. Subsequently, prominent discontinuities between damaged endothelial cells and breaks in the microvascular basement membranes were found at the light and ultrastructural level. Interstitial edema and extravasated erythrocytes occurred shortly thereafter, preceding gross tissue necrosis evident at post-freeze Day 3. We conclude that in this animal system thrombus formation is not an initial event, but rather that vascular injury in the form of endothelial cell damage predominates in early frostbite injury.