The down-side of platelet interaction with surfaces.

Events occurring on the exposed luminal surface of blood platelets during interaction with foreign surfaces have been evaluated in detail, but the down-side has received less attention. The present study has examined the interaction of platelets with glass precoated with fibrinogen-gold (Fgn/Au), latex spherules, or cationized ferritin (CF) and compared their organization on the ventral surface with the fate of the same particulates when added to platelets already spread on glass or formvar grids. Platelets spread readily on the particle-coated glass surface. During early stages of interaction, particles of Fgn/Au, CF and latex were taken up by channels of the open canalicular system (OCS). Fgn/Au and CF often reached the dorsal surface through OCS channels where they were moved to platelet centers. In contrast, layers of Fgn/Au, CF and latex remained evenly dispersed from edge to edge on the down-side of spread platelets. Clearance of particulates from peripheral and intermediate to central zones under spread platelets was not observed. Particulates did not move from the up-side around platelet margins to the down-side, or from the ventral surface around edges to the up-side, unless spread cells were in contact. Thus, particulates use the OCS as a two-way street. The persistence of evenly dispersed layers of Fgn/Au, CF or latex on precoated glass under spread or spreading platelets indicates a significant difference in the way particulates are handled on the free upside and bound down-side. The results support the concept that the main purpose of mobile receptors is to facilitate spreading, and that the down-side is a protected zone of adhesion.