Alpha-actinin arcs in megakaryocyte spreading

Cultured megakaryocytes, isolated from guinea pig bone marrow, were treated with buffer or adenosine diphosphate (ADP) (10 microM) on plain or coated glass surfaces. Control cells were rounded and non-adherent. The nucleotide induced the cells to spread to several times the initial diameter, and to become flattened and markedly adherent to the substratum. 'Cytoskeletons' were examined by transmission electron microscopy (TEM). Those from unspread cells contained only rare microfilaments and no filament bundles; those from spread cells contained large numbers of microfilaments in nets and many filament bundles, which were largely oriented circumferentially. Interference reflection microscopy demonstrated that the spread cells were attached to the substratum in arc-shaped regions, which corresponded to arcs containing alpha-actinin as seen by specific immunofluorescence of the same cells. However, other arcs of alpha-actinin staining did not correspond to arcs of tight attachment. We conclude that fibrous arcs, which appear to assemble as part of the spreading process, play a role in what are probably transient surface attachment sites. A survey of observations of spreading in other cell types suggests that similar arcs are more widespread than has been realized.