Age-Dependent Differential Staining of Fibrin in Blood Clots and Thrombi

Abstract

It has been known for a long time that histologically fibrin can be visualized as a fibrous structure of variable colors, even when stained with the same histochemical technique. The reason for this phenomenon called metachromasia remains unknown. We hypothesized that metachromasia is related to fibrin structural maturation and age. To establish a link between the color of fibrin in histological preparations and the age of blood clots and thrombi. Using the Picro-Mallory staining technique, we studied changes in fibrin color in histological preparations of in vitro human plasma clots at various time points within 48 h after formation in the absence and presence of platelets. Also, we used the same stain to visualize fibrin in histological sections of ex vivo human cerebral thrombi of different maturity. In histological preparations of plasma clots formed either in the absence or in the presence of platelets, fibrin was distinctively polychromic depending on the time lapse between formation and chemical fixation of the clot. In the 30-min and 6-h clots (“young” clots), fibrin was red; after 6–12 h (“mature” clots), fibrin was purple or violet; at 24 or 48 h (“old” clots), fibrin was blue. In thrombi removed from cerebral arteries of ischemic stroke patients, fibrin also stained differentially. The colors generally corresponded to the time from the onset of stroke to the time of intravital thrombectomy, such that fibrin in the younger thrombi was red or purple, while in the older thrombi, fibrin was blue. The Picro-Mallory stain can be used to assess histologically the maturity of fibrin in blood clots, thrombi, and thrombotic emboli based on the age-dependent differential staining of fibrin. A color-temporal scale is proposed that can help pathologists to estimate the age of a blood clot or thrombus.