Role of Fibrin Thrombi in Fat Necrosis

With regard to the pathogenesis of pancreatitis, Schmitz-Moormann [11] reported that acute pancreatitis begins with fat necrosis around and within the pancreas. In the second step, acinar cell necrosis, as well as vascular destruction and thrombosis, arises in the immediate vicinity of the fat necrosis. Klöppel et al. [12] also noted that the earliest autodigestive lesions are found in the peri-pancreatic fatty tissue. If the initial fat necrosis is extensive and effusion of

Fig. 7. Fat necrosis encroached on the acini in the intimate vicinity of the necrosis (a) with dissolution of the basement membrane (b). a HE. X200. b Immunostaining for type IV collagen. X200.

enzymes from the neighboring acinar cells continues, the full spectrum of fatal pancreatitis will develop. Hence, fat necrosis is most likely the lesion which definitively marks the beginning of acute pancreatitis. However, causal relationships between fat necrosis and advancement to the next stage of fatal pancreatitis, such as parenchymal necrosis and hemorrhage, have not been fully explained. We found that fat necrosis was present in all 9 patients with acute pancreatitis, as shown in table 2. Fat necrosis in the agonal stage often encroaches on the acini in the intimate vicinity of the necrosis (fig. 7). This is similar to the findings of acute pancreatitis described by Schmitz-Moormann [11] and Klöppel et al. [12]. However, fat necrosis is accompanied by either hemorrhage or hemosiderin deposition in all cases of acute pancreatitis, whereas it is seldom seen in those in the agonal stage. These findings suggest that hemorrhage occurs in the fat necrosis in the course of the illness.

Escape of elastase from the ductal system causes dissolution of blood vessel elastic fibers and probably causes hemorrhage [13, 14]. However, the cause of the hemorrhage in fat necrosis is thought to be as follows: fibrin thrombi are frequently found in fat necrosis either at the agonal stage (fig. 8) or in patients with acute pancreatitis. Hemorrhage or hemosiderin deposition in fat necrosis are seldom observed in that of the agonal stage, whereas they are found in all patients with acute pancreatitis as mentioned above (fig. 9). In a certain type of fat necrosis lesion which is accompanied by simultaneous hemorrhage and fibrin thrombi, fibrin thrombi are found in the necrosis except for areas of hemorrhagic foci. Hence, the fibrin thrombi in fat necrosis are formed shortly after occurrence of the fat necrosis, and they are then probably resolved by the action of plasmin resulting eventually in hemorrhage. Therefore, we emphasize that hemorrhage appears to occur after the resolution of fibrin thrombi in fat necrosis due to a reperfusion-injury mechanism [15].

Fig. 9. Fat necrosis with hemorrhage in a patient with acute pancreatitis of 4 days' duration. Hemorrhage is found among fat necrosis at left (a) and fibrin thrombi (arrows) were found except in areas of hemorrhage (b). a HE. X32. b PTAH. X32.

Fig. 9. Fat necrosis with hemorrhage in a patient with acute pancreatitis of 4 days' duration. Hemorrhage is found among fat necrosis at left (a) and fibrin thrombi (arrows) were found except in areas of hemorrhage (b). a HE. X32. b PTAH. X32.

In conclusion, the role of fat necrosis may be explained as follows: when extensive fibrin thrombi in fat necrosis are resolved before the reparative change is complete, hemorrhage might extend into the surrounding tissue.

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Koichi Suda

Department of Human Pathology, Juntendo University School of Medicine 2-1-1, Hongo, Bunkyo-ku Tokyo 113-8421 (Japan)

Tel. +81 3 5802 1036, Fax +81 3 3812 1056, E-Mail [email protected]

Suda K (ed): Pancreas - Pathological Practice and Research. Basel, Karger, 2007, pp 56-66

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