Lamellipodia have been induced to form within the reticulopodial networks of Allogromia laticollaris by being plated on positively charged substrata . Video-enhanced, polarized light, and differential interference contrast microscopy have demonstrated the presence of positively birefringent fibrils within these lamellipodia . The fibrils correspond to the microtubules and bundles of microtubules observed in whole-mount transmission electron micrographs of lamellipodia . Microtubular fibrils exhibit two types of movements within the lamellipodia : lateral and axial translocations . Lateral movements are often accompanied by reversible lateral associations between adjacent fibrils within a lamellipodium . This lateral association-dissociation of adjacent fibrils has been termed 'zipping' and 'unzipping' . Axial translocations are bidirectional . The axial movements of the microtubular fibrils can result in the extension of filopodia by pushing against the plasma membrane of the lamellipodia . Shortening, or complete withdrawal, of such filopodia is accomplished by the reversal of the direction of the axial movement .The bidirectional streaming characteristic of the reticulopodial networks also occurs within the lamellipodia . In these flattened regions the streaming is clearly seen to occur exclusively in association with the intracellular fibrils . Transport of both organelles and bulk hyaline cytoplasm occurs bidirectionally along the fibrils .Our previous studies (37) of the ultrastructure of the reticulopodial network of Allogromia demonstrated that the highly branched and anastomosed reticulopodium contains an extensive microtubule-based cytoskeleton. The microtubules were frequently bundled, forming a cytoskeletal system of far greater spatial organization than previously thought (22). By using sensitive (extinction factor of^-2,000) differential interference contrast optics, fibrillar elements were visualized within the most favorable areas of the reticulopodial networks of living organisms (37). The ultrastructural data indicated that these pseudopodial fibrils most certainly represented bundles of microtubules . The spatial distribution of microtubules within the reticulopodia reflected accurately the pattern of filopodial branching and anastomoses . This suggested that the microtubules might have a causal role in the formation and maintenance of pseudopodial form. In addition, the fibrils were oriented parallel to the direction ofthe cytoplasmic streaming, suggesting that they participated in some way in the intracellular transport process. 1668Recent advances in video-enhanced contrast microscopy (5, 6) have provided the means with which to study the cytoskeletal dynamics of living Allogromia. The video microscopic studies reported here were performed in part on flattened lamellipodial regions of the reticulopodial network of Allogromia . The results of the present studies clearly demonstrate that the microtubule-based cytoskeleton of the reticulopodial network is itself motile . The motility o...

One-day old chicks were exposed to one of two pure tone stimuli (0.9 kHz at 120 or 125 dB SPL) for 48 h. Three major results arose from a variety of tests that assessed the structural and functional consequences of the exposure on the peripheral auditory system at either 0 days or 12 to 15 days recovery. First, brainstem response data showed that the 120 and 125 dB groups had maximum evoked potential threshold shifts of 57 and 71 dB immediately after removal from the sound. Fifteen days post-exposure, the thresholds in the 120 dB group returned to near-normal levels, while in the 125 dB group, recovery was within 19 dB of control thresholds. Second, scanning electron microscopic measurements of hair cell density within the lesion showed that at 0 days recovery, the 120 and 125 dB groups had a 30% and 59% short hair cell loss, respectively, but by 15 days no differences could be identified between the exposed and control animals, regardless of exposure level. Finally, at 0 days of recovery, micromechanical stimulation data did not reveal any significant difference in stiffness between the control and surviving hair cells in the lesion area. Although the more intense exposure induced greater structural and functional damage in the chick cochlea, the birds retained or even enhanced their ability to replace lost hair cells and had partial hearing recovery by 15 days post-exposure.

First branchial cleft anomalies may involve the tympanic membrane or middle ear. A complete otologic examination must be performed in any patient with a suspected first branchial cleft anomaly. A surgeon treating such a defect must be prepared to perform both the excision of the lesion and reconstructive otologic surgery.