Background: Microparticles (MPs) are small vesicles released from cells of different origin, bearing surface antigens from parental cells. Elevated numbers of blood MPs have been reported in (cardio)vascular disorders and cancer. Most of these MPs are derived from platelets.

Objectives: To investigate whether atomic force microscopy (AFM) can be used to detect platelet-derived MPs and to define their size distribution.

Methods: Blood MPs isolated from seven blood donors and three cancer patients were immobilized on a modified mica surface coated with an antibody against CD41 prior to AFM imaging. AFM was performed in liquid-tapping mode to detect CD41-positive MPs. In parallel, numbers of CD41-positive MPs were measured using flow cytometry. Mouse IgG1 isotype control was used as a negative control.

Results: AFM topography measurements of the number of CD41-positive MPs were reproducible (coefficient of variation=16%). Assuming a spherical shape of unbound MPs, the calculated diameter of CD41-positive MPs (dsph) ranged from 10 to 475 nm (mean: 67.5+/-26.5 nm) and from 5 to 204 nm (mean: 51.4+/-14.9 nm) in blood donors and cancer patients, respectively. Numbers of CD41-positive MPs were 1000-fold higher than those measured by flow cytometry (3-702x10(9) L(-1) plasma vs. 11-626x10(6) L(-1) plasma). After filtration of isolated MPs through a 0.22-microm filter, CD41-positive MPs were still detectable in the filtrate by AFM (mean dsph: 37.2+/-11.6 nm), but not by flow cytometry.

Conclusions: AFM provides a novel method for the sensitive detection of defined subsets of MPs in the nanosize range, far below the lower limit of what can be measured by conventional flow cytometry.