Proplatelets and stress platelets.

The process of platelet formation by the fragmentation of megakaryocyte pseudopodia, termed proplatelets, demonstrable in the marrow sinusoids is poorly understood. "Stress" platelets produced under conditions of stimulated platelet production differ from normal circulating platelets with respect to volume and a number of functional characteristics. To clarify the relationship of stress platelets to proplatelets, rats were injected with heterologous platelet antiserum. Nondiscoid platelet forms, some characteristically beaded in appearance, strongly resembling bone marrow proplatelets, can be recovered in the circulation of normal rats. During the early period of recovery from acute thrombocytopenia, there was a substantial increase in the proportion of these elongated platelets in the citrated platelet rich plasma. Exposure to EDTA rendered them spherical. Circulating proplatelets may contribute significantly to the prompt increase in platelet volume during recovery from acute thrombocytopenia at a time prior to significant increase in megakaryocyte size and ploidy.