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First published online 29 June 2004
doi: 10.1242/jcs.01215

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Microvilli appear to represent the first step in actin bundle formation in Drosophila bristles

Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018, USA

View larger version (123K): [in a new window]   Fig. 1. Thin section through the apical end of a bristle shaft cell (B) from a 32-hour pupa just prior to the emergence of the bristle. The bristle shaft cell anticipates this event by extending long microvilli. Surrounding the bristle shaft cell is its socket cell (SC).

View larger version (131K): [in a new window]   Fig. 2. Jasplakinolide treatment accentuates microvilli growth on elongating bristle tips. (a) Thin section through the apical portion of an untreated newly emerging bristle shaft cell (B) from a 33-hour pupa. A portion of the neuron (N) that innervates the bristle is present in this section. Note that pimples are present extending from the surface of this emerging bristle shaft cell (arrowheads) as well as from the surface of the adjacent socket cell (SC). (b-d) 33-hour pupal thoraces were cultured in jasplakinolide for 1 hour before fixation and thin sections of emerging tips are shown here. In b-d microvilli are seen extending from the bristle tips. In panels c and d (which are adjacent sections) microvilli have extended from the pimples from adjacent epithelial cells (E). (e) Higher magnification of the microvillus indicated by the squares in panels c and d. As these are adjacent sections we have pasted the squares together to show the entire length of the microvillus. Within this microvillus one can easily see the core of actin filaments. There are transverse stripes on this core bundle (lines) which indicate the presence of crossbridges between the filaments.

View larger version (161K): [in a new window]   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.

View larger version (192K): [in a new window]   Fig. 4. Bristles lacking fascin crossbridges still form microvilli. (a,b) Transverse sections of newly emerged bristle tip from 34-hour-old singedX2 mutant pupa. A smaller version appeared in Tilney et al., (1998) and is included here at sufficient magnification in panel b so that the aggregated core bundles (arrows) of the microvilli can be clearly visualized. Panel b, published with permission by Rockefeller University Press. The size of these bundles and the number of filaments per bundle are similar to those in microvilli at the bristle tip (e.g. Fig. 2e) or to those in longitudinal section (panel d). (c,d) Grazing longitudinal section through a singedX2 bristle tip at the same magnification as in panels a and b, respectively. The bundles are indicated by arrows. Note that the diameters of the bundles here are the same as in panel b and in Fig. 2e.

View larger version (122K): [in a new window]   Fig. 5. Forked proteins are present in newly emerging bristles but absent from surrounding socket cells. Confocal image of a sprouting thoracic macrochaete from a 33-hour pupa stained with anti-forked antibody (green) and phalloidin (red). Signal overlap is seen as yellow. The perimeter of the socket cell (SC) is outlined with a dashed line. Bar, 5 µm.