Figure 3.

Localisation of FXIII-A in platelet subpopulations. Within resting platelets, Ca²⁺ ions and cellular FXIII-A reside within the cytoplasm. Following weak stimulation, with agonists such as ADP, PS-negative platelets expose FXIII-A on the external membrane. The integrin αIIbβ3 is exposed in this subpopulation of platelets. Stimulation with dual agonists, such as thrombin and collagen, generate procoagulant platelets which expose FXIII-A within the phosphatidylserine (PS)-rich cap. In this subpopulation αIIbβ3 is inactive [70]. The cap is also rich in other coagulation factors, such as factor X, factor IX and prothrombin, and substrates for FXIII-A including fibrinogen, thrombospondin and factor Va [64].