ABSTRACT
The findings of an erythroid progenitor cell proliferation factor on the surface of lymphocytes and a Schwann cell mitogen on the surface of neurites indicate that the plasma membrane plays a more active role in cell proliferation events than had been previously recognized. Lymphocytes were isolated from the plateletpheresis residues, following depletion of platelets and monocytes by centrifugation on Ficoll Paque and adherence to polystyrene. Virtually all of the hematopoietic growth factors which have been described are released into liquid culture medium by eukaryotic cells. Overnight incubation of lymphocytes in liquid medium imparts growth factors to the spent medium. When burst-promoting activity-containing lymphocyte conditioned medium is centrifuged, a pellet is obtained which contains a heterogeneous population of unilamellar membrane vesicles.
