Fig. 3. Jasplakinolide treatment results in actin filament elongation from the surface projections on epithelial cells. The parallel dense lines above the plasma membrane represent an early stage in cuticle deposition (C). (a) Untreated 32-hour pupa. At this stage bristles begin to erupt from the surface of the thorax (not depicted). Extending from the apical surface of this epithelial cell are tiny projections we call pimples. Attached to the cytoplasm surface of the plasma membrane in each pimple is some electron-dense material (vertical arrowheads). (b) Untreated 41-hour pupa. Pimples are present on the apical surface. By this time actin filaments extend from the dense material of some of the pimples into the cortical cytoplasm. Microtubules (MT) are found running parallel to the cell surface. (c,d) Portions of epithelial cells from 41-hour pupal thoraces cultured in jasplakinolide for 4 hours. Note that actin filaments are now commonly seen extending from the electron-dense material at the pimple tips into cortical cytoplasm as a rootlet (horizontal arrowheads). These filaments are three or four times longer than those seen in untreated pupae and are clustered together into discrete core bundles. The pimples in panel c have not elongated outward although they possess long rootlets. The pimples shown in panel d have elongated into short microvilli; the actin core bundles extend from the dense material at the pimple tips into the cortical cytoplasm as rootlets (arrowheads) where actin filaments often fray apart. (e) Epithelial cells from a 41-hour pupa lacking both the forked and fascin proteins cultured in jasplakinolide for 4 hours. Pimples are present and form short microvilli on the surface of the epithelial cells. Extending from the tips of the pimples are actin filaments that are clustered together into bundles (horizontal arrowheads) and then fray apart in the cortical cytoplasm. (f) Transverse section through microvilli lacking both forked and fascin crossbridges. Dots in the center of each are the actin filaments of these core bundles cut in transverse section. Panels a-e are printed at the same magnification.