The functional capacity of guinea pig megakaryocytes. II. The uptake of particles and macromolecules and the effect of rabbit antiguinea pig platelet antiserum.

The capacity of megakaryocytes to take up particles and macromolecules was tested by exposing them to homologous erythrocytes, latex particles, Micrococcus lysodeikticus, and horseradish peroxidase. Uptake of particles 1.1 to 7 mum. in size was extensive under the experimental conditions employed. After uptake of each of the three particles the addition of tannic acid and calcium to the fixative produced electron-dense deposits within the vacuole surrounding the particle; therefore, the vacuole was not closed off. This finding indicates that many of the particles could be actually trapped within the preformed elements of the demarcating membrane system. Horseradish peroxidase was concentrated within cytoplasmic vacuoles after 2 hours of exposure to 0.1 mg. per ml. The presence of surface membrane receptors was determined by the sheep cell rosette method. The presence of the C3b receptor for complement was determined by the sheep cell rosette method. The presence of the C3b receptor for complement produced rosette formation. The functional capacity of megakaryocytes to react like platelets was also tested by observing cell lytic changes and determining that there was approximately 40% release of 3H-serotonin produced by specific rabbit antiguinea pig platelet antiserum in the presence of complement. These studies demonstrate several functional capacities of megakaryocytes: the uptake of particles, pinocytosis, and the cell lytic response to specific antibody.