Orientation and diameter distribution of rabbit blood platelets flowing in small arterioles

Spatial orientation of blood platelets flowing in small arterioles of the rabbit mesentery was investigated using intravital fluorescence microscopy. Only platelets were studied that could be localized objectively within a thin optical section around the median plane of the vessel. The orientation of a platelet was assessed from its microscopic image and described by an angle pair, assuming the platelet to be an ellipsoidal body and using an empirical frequency distribution for its thickness to diameter ratio. The orientation of the platelets was not random; they tended to align themselves with their equatorial planes parallel to the wall. The degree of alignment increased from the center of the vessel toward the wall. As a corollary a frequency distribution of their diameter (3.15 +/- 0.72 micron (mean +/- sd) was obtained in vivo.