To examine the effect of flow on megakaryocytes (megas) exposed to extracellular matrix
(ECM), we subjected megas on ECM to laminar flow in a chamber similar to that used
to study platelets circulated over aortic subendothelium. Megas, harvested from guinea
pig bone marrow by centrifugal elutriation and velocity sedimentation, were allowed
to adhere to cover slips coated with ECM, which were then placed in a perfusion chamber.
Medium was circulated over the megas on this surface for up to 18 hrs. The cells were
examined during the experiments by phase contrast and afterwards by scanning electron
microscopy. With time, many attached megas developed a single, prominent, elongated
pseudopod resembling the flagelliform processes observed in situ protruding into marrow
sinusoids and also observed in circulating megas. In other experiments megas were
introduced into the flowing medium and exposed to new ECM-coated cover slips mounted
in the chamber. At shear rates of 10-200 sec−1, megas from the flowing suspension started to adhere to the ECM within 1 to 2 minutes.
The number of attached cells continued to increase for several hours. Adhesion of
megas under flow to the ECM was specific, since there was no adherence to glass, to
glutaraldehyde-fixed ECM coated cover slips or to endothelial cells cultured on ECM
coated cover slips. With time many of the megas developed the same type of pseudopods
which formed in megas attached to ECM prior to flow exposure. The responses of megas
to flow over ECM illustrate two aspects of mega behavior: the acquisition, even before
platelet shedding, of the adhesive capacity of circulating platelets, and the possible
roles of ECM and flow as anchor and inducer of platelet shedding.
