THE ROLE OF FACTOR VII IN HAEMOSTASIS: A DETAILED MICROSCOPIC ANALYSIS OF THE MORPHOLOGY OF THE EVOLVING HAEMOSTATIC PLUG IN NORMAL AND VII DEFICIENT DOGS
The role of F.VII in haemostasis remains controversial, both in terms of the functional consequences of the deficiency state and the activation pathways to which it makes its principal contribution In vivo. We have developed a cuticle bleeding time (CBT) model in dogs and used this to investigate the functional consequence of both congenital and acquired F.VII deficiency (SD) (Blood 65:1197, 1985). There was no significant difference between the CBT of these animals when compared to controls. However, the CBT prolonged at a significantly lower Heparin level than that observed in controls. F.VIIa was also infused into F.VIII deficient and normal dogs and FPA measured as an indicator of thrombin generation. Significant change in FPA level occurred in the latter but not the former, suggesting that activation of F.IX rather than F.X was favoured. We have now performed detailed morphological studies of the evolving haemostatic plug (HP) in the injured cuticle of F.VII and normal animals by light (LM) and electron microscopy (EM). Quantification of the EM changes noted were performed by morphometric analysis. The tightness of the intravascular component of the HP was assessed by random measurement of intraplatelet distance. The degree of platelet activation was measured by comparing the area of the open canalicular system (OCS) in comparison to the total platelet area. The appearance of fibrin in the plug was also noted. Qualitative LM revealed little difference between the two sets of animals. The appearance of fibrin at the periphery of HP plug was delayed in SD and was reduced in quantity. However, by morphometry although the pattern was identical in both groups, there was a significant delay in the changes noted in SD. These results suggest that the extrinsic pathway may play an important role in triggering the intrinsic pathway, either by providing for activation of the cofactors V and VIII or pulse generation of F.IXa. This may play a critical role in haemostasis when the vessel injured is larger than those in the nail cuticle of the dog (50 - 150 μm) or when other components of haemostatic mechanism are compromised