Modeling the Architecture of a Clot. A thrombus is composed of distinct regions including a fibrin cloud, an inner core, and an outer shell. The fibrin cloud is juxtaposed to the endothelium and penetrates the extravascular space in the setting of endothelial compromise. The inner core is the site of active thrombin generation and consists of activated, tightly packed platelets. The outer shell is composed of loosely packed platelets recruited to the thrombus by soluble small molecules such as ADP and thromboxane A2 (TxA2).Stalker TJ et al. Blood. 2013;121:1875-1885.