The Dense Bodies of Human Platelets: Inherent Electron Opacity of the Serotonin Storage Particles

Platelets fixed in glutaraldehyde and then in osmic acid or potassium dichromate contain dense bodies which are known to be rich in serotonin. Opacity of the particles is believed to be due to a highly specific reaction between glutaraldehyde, serotonin and osmic acid. The present study has examined dense bodies of platelets in negatively stained whole mount preparations, unstained whole mounts, and glutaraldehyde fixed, plastic embedded cells not exposed to osmium or dichromate. The results demonstrate that dense bodies in platelet hyaloplasm are innately electron opaque. Osmic acid and potassium dichromate may deposit in dense bodies, but the metal containing oxidizing fixatives are not the primary cause of opacity to the electron beam. It is suggested that the inherent electron opacity may be due to a nucleation of heavy metals in the matrix of dense bodies.