Platelet Density and Size: the Interpretation of Heterogeneity

Abstract

Platelets were separated according to buoyant density using a colloidal silica-polyvinylpyrrolidone system and subjected to electronic sizing. All density populations were heterogeneous in size, the most dense platelets ranging from less than 3 fl [fluoroliter] to greater than 21 fl in both man and rat. Light platelet fractions contained no platelets greater than 13 fl in either species. Cohort labeling with [75Se]selenomethionine showed no indication of significant change in platelet buoyant density with aging; greater specific activity found in young, dense platelets appears to be related to increased protein synthetic activity shown in vitro and likely to occur also in their precursor megakaryocytes. Dense, intermediate and light platelets may be released synchronously by the 3 different ploidy classes of megakaryocyte and varying density could indicate differing structural characteristics and presumably differences in function. The present findings do not deny the possibility that platelets decrease in size with aging but if such occurs, it is not associated with a significant change in platelet buoyant density.