Actin filament bundles in Drosophila wing hairs: hairs and bristles use different strategies for assembly.

Actin filament bundles can shape cellular extensions into dramatically different forms. We examined cytoskeleton formation during wing hair morphogenesis using both confocal and electron microscopy. Hairs elongate with linear kinetics (approximately 1 microm/h) over the course of approximately 18 h.

Actin filament turnover regulated by cross-linking accounts for the size, shape, location, and number of actin bundles in Drosophila bristles.

Drosophila bristle cells are shaped during growth by longitudinal bundles of cross-linked actin filaments attached to the plasma membrane. We used confocal and electron microscopy to examine actin bundle structure and found that during bristle elongation, snarls of uncross-linked actin filaments and small internal bundles also form in the shaft cytoplasm only to disappear within 4 min. Thus, formation and later removal of actin filaments are prominent features of growing bristles.

Long continuous actin bundles in Drosophila bristles are constructed by overlapping short filaments.

The actin bundles essential for Drosophila bristle elongation are hundreds of microns long and composed of cross-linked unipolar filaments. These long bundles are built from much shorter modules that graft together. Using both confocal and electron microscopy, we demonstrate that newly synthesized modules are short (1-2 microm in length); modules elongate to approximately 3 microm by growing over the surface of longitudinally adjacent modules to form a graft; the grafted regions are initially secured by the forked protein cross-bridge and later by the fascin cross-bridge; actin bundles are smoothed by filament addition and appear continuous and without swellings; and in the absence of grafting, dramatic alterations in cell shape occur that substitutes cell width expansion for elongation.

Actin filament turnover removes bundles from Drosophila bristle cells.

Drosophila bristle cells form enormous extensions that are supported by equally impressive scaffolds of modular, polarized and crosslinked actin filament bundles. As the cell matures and support is taken over by the secreted cuticle, the actin scaffold is completely removed. This removal begins during cell elongation and proceeds via an orderly series of steps that operate on each module.