scholarly journals The actin filament content of hair cells of the bird cochlea is nearly constant even though the length, width, and number of stereocilia vary depending on the hair cell location.

1988 ◽  
Vol 107 (6) ◽  
pp. 2563-2574 ◽  
Author(s):  
L G Tilney ◽  
M S Tilney
Keyword(s):  
Hair Cell ◽  
Actin Filament ◽  
Cross Sections ◽  
Hair Cells ◽  
Actin Filaments ◽  
Membrane Surface ◽  
Scanning Electron Micrographs ◽  
Length Width ◽  
Filament Content ◽  
Filament Bundle

By direct counts off scanning electron micrographs, we determined the number of stereocilia per hair cell of the chicken cochlea as a function of the position of the hair cell on the cochlea. Micrographs of thin cross sections of stereociliary bundles located at known positions on the cochlea were enlarged and the total number of actin filaments per stereocilium was counted and recorded. By comparing the counts of filament number with measurements of actin filament bundle width of the same stereocilium, we were able to relate actin filament bundle width to filament number with an error margin (r2) of 16%. Combining this data with data already published or in the process of publication from our laboratory on the length and width of stereocilia, we were able to calculate the total length of actin filaments present in stereociliary bundles of hair cells located at a variety of positions on the cochlea. We found that stereociliary bundles of hair cells contain 80,000-98,000 micron of actin filament, i.e., the concentration of actin is constant in all hair cells with a range of values that is less than our error in measurement and/or biological variation, the greatest variation being in relating the diameters of the stereocilia to filament number. We also calculated the membrane surface needed to cover the stereocilia of hair cells located throughout the cochlea. The values (172-192 micron 2) are also constant. The implications of our observation that the total amount of actin is constant even though the length, width, and number of stereocilia per hair cell vary are discussed.

scholarly journals Actin filaments, stereocilia, and hair cells of the bird cochlea. I. Length, number, width, and distribution of stereocilia of each hair cell are related to the position of the hair cell on the cochlea.

1983 ◽  
Vol 96 (3) ◽  
pp. 807-821 ◽  
Author(s):  
L G Tilney ◽  
J C Saunders
Keyword(s):  
Hair Cell ◽  
Surface Area ◽  
Hair Cells ◽  
Actin Filaments ◽  
Hexagonal Lattice ◽  
Frequency Discrimination ◽  
Sensory Epithelium ◽  
Scanning Microscopy ◽  
Apical Surface ◽  
Length Width

Located on the sensory epithelium of the sickle-shaped cochlea of a 7- to 10-d-old chick are approximately 5,000 hair cells. When the apical surface of these cell is examined by scanning microscopy, we find that the length, number, width, and distribution of the stereocilia on each hair cell are predetermined. Thus, a hair cell located at the distal end of the cochlea has 50 stereocilia, the longest of which are 5.5 microns in length and 0.12 microns in width, while those at the proximal end number 300 and are maximally 1.5 microns in length and 0.2 micron in width. In fact, if we travel along the cochlea from its distal to proximal end, we see that the stereocilia on successive hair cells gradually increase in number and width, yet decrease in length. Also, if we look transversely across the cochlea where adjacent hair cells have the same length and number of stereocilia (they are the same distance from the distal end of the cochlea), we find that the stereocilia of successive hair cells become thinner and that the apical surface area of the hair cell proper, not including the stereocilia, decreases from a maximum of 80 microns2 to 15 microns2. Thus, if we are told the length of the longest stereocilium on a hair cell and the width of that stereocilium, we can pinpoint the position of that hair cell on the cochlea in two axes. Likewise, if we are told the number of stereocilia and the apical surface of a hair cell, we can pinpoint the location of that cell in two axes. The distribution of the stereocilia on the apical surface of the cell is also precisely determined. More specifically, the stereocilia are hexagonally packed and this hexagonal lattice is precisely positioned relative to the kinocilium. Because of the precision with which individual hair cells regulate the length, width, number, and distribution of their cell extensions, we have a magnificent object with which to ask questions about how actin filaments that are present within the cell are regulated. Equally interesting is that the gradient in stereociliary length, number, width, and distribution may play an important role in frequency discrimination in the cochlea. This conclusion is amplified by the information presented in the accompanying paper (Tilney, L.G., E.H. Egelman, D.J. DeRosier, and J.C. Saunders, 1983, J. Cell Biol., 96:822-834) on the packing of actin filaments in this stereocilia.

scholarly journals Actin filaments, stereocilia, and hair cells of the bird cochlea. II. Packing of actin filaments in the stereocilia and in the cuticular plate and what happens to the organization when the stereocilia are bent.

1983 ◽  
Vol 96 (3) ◽  
pp. 822-834 ◽  
Author(s):  
L G Tilney ◽  
E H Egelman ◽  
D J DeRosier ◽  
J C Saunder
Keyword(s):  
Actin Filament ◽  
Hair Cells ◽  
Actin Filaments ◽  
Fine Tuning ◽  
Thin Sections ◽  
The Cross ◽  
Diffraction Patterns ◽  
Cross Bridges ◽  
Different Parts ◽  
Filament Bundle

A comparison of hair cells from different parts of the cochlea reveals the same organization of actin filaments; the elements that vary are the length and number of the filaments. Thin sections of stereocilia reveal that the actin filaments are hexagonally packed and from diffraction patterns of these sections we found that the actin filaments are aligned such that the crossover points of adjacent actin filaments are in register. As a result, the cross-bridges that connect adjacent actin filaments are easily seen in longitudinal sections. The cross-bridges appear as regularly spaced bands that are perpendicular to the axis of the stereocilium. Particularly interesting is that, unlike what one might predict, when a stereocilium is bent or displaced, as might occur during stimulation by sound, the actin filaments are not compressed or stretched but slide past one another so that the bridges become tilted relative to the long axis of the actin filament bundle. In the images of bent bundles, the bands of cross-bridges are then tilted off perpendicular to the stereocilium axis. When the stereocilium is bent at its base, all cross-bridges in the stereocilium are affected. Thus, resistance to bending or displacement must be property of the number of bridges present, which in turn is a function of the number of actin filaments present and their respective lengths. Since hair cells in different parts of the cochlea have stereocilia of different, yet predictable lengths and widths, this means that the force needed to displace the stereocilia of hair cells located at different regions of the cochlea will not be the same. This suggests that fine tuning of the hair cells must be a built-in property of the stereocilia. Perhaps its physiological vulnerability may result from changes of stereociliary structure.

scholarly journals Actin filaments, stereocilia, and hair cells of the bird cochlea. V. How the staircase pattern of stereociliary lengths is generated

1988 ◽  
Vol 106 (2) ◽  
pp. 355-365 ◽  
Author(s):  
LG Tilney ◽  
MS Tilney ◽  
DA Cotanche
Keyword(s):  
Hair Cell ◽  
Hair Cells ◽  
Actin Filaments ◽  
Developmental Time ◽  
Final Phase ◽  
Set Up

The stereocilia on each hair cell are arranged into rows of ascending height, resulting in what we refer to as a "staircase-like" profile. At the proximal end of the cochlea the length of the tallest row of stereocilia in the staircase is 1.5 micron, with the shortest row only 0.3 micron. As one proceeds towards the distal end of the cochlea the length of the stereocilia progressively increases so that at the extreme distal end the length of the tallest row of the staircase is 5.5 micron and the shortest row is 2 micron. During development hair cells form their staircases in four phases of growth separated from each other by developmental time. First, stereocilia sprout from the apical surfaces of the hair cells (8-10-d embryos). Second (10-12-d embryos), what will be the longest row of the staircase begins to elongate. As the embryo gets older successive rows of stereocilia initiate elongation. Thus the staircase is set up by the sequential initiation of elongation of stereociliary rows located at increased distances from the row that began elongation. Third (12-17-d embryos), all the stereocilia in the newly formed staircase elongate until those located on the first step of the staircase have reached the prescribed length. In the final phase (17-d embryos to hatchlings) there is a progressive cessation of elongation beginning with the shortest step and followed by taller and taller rows with the tallest step stopping last. Thus, to obtain a pattern of stereocilia in rows of increasing height what transpires are progressive go signals followed by a period when all the stereocilia grow and ending with progressive stop signals. We discuss how such a sequence could be controlled.

scholarly journals Nerve growth cone lamellipodia contain two populations of actin filaments that differ in organization and polarity.

1992 ◽  
Vol 119 (5) ◽  
pp. 1219-1243 ◽  
Author(s):  
A K Lewis ◽  
P C Bridgman
Keyword(s):  
Actin Filament ◽  
Growth Cone ◽  
Actin Filaments ◽  
Membrane Surface ◽  
Leading Edge ◽  
Growth Cones ◽  
Differential Interference Contrast Microscopy ◽  
Nerve Growth ◽  
Filament Bundles ◽  
Two Populations

The organization and polarity of actin filaments in neuronal growth cones was studied with negative stain and freeze-etch EM using a permeabilization protocol that caused little detectable change in morphology when cultured nerve growth cones were observed by video-enhanced differential interference contrast microscopy. The lamellipodial actin cytoskeleton was composed of two distinct subpopulations: a population of 40-100-nm-wide filament bundles radiated from the leading edge, and a second population of branching short filaments filled the volume between the dorsal and ventral membrane surfaces. Together, the two populations formed the three-dimensional structural network seen within expanding lamellipodia. Interaction of the actin filaments with the ventral membrane surface occurred along the length of the filaments via membrane associated proteins. The long bundled filament population was primarily involved in these interactions. The filament tips of either population appeared to interact with the membrane only at the leading edge; this interaction was mediated by a globular Triton-insoluble material. Actin filament polarity was determined by decoration with myosin S1 or heavy meromyosin. Previous reports have suggested that the polarity of the actin filaments in motile cells is uniform, with the barbed ends toward the leading edge. We observed that the actin filament polarity within growth cone lamellipodia is not uniform; although the predominant orientation was with the barbed end toward the leading edge (47-56%), 22-25% of the filaments had the opposite orientation with their pointed ends toward the leading edge, and 19-31% ran parallel to the leading edge. The two actin filament populations display distinct polarity profiles: the longer filaments appear to be oriented predominantly with their barbed ends toward the leading edge, whereas the short filaments appear to be randomly oriented. The different length, organization and polarity of the two filament populations suggest that they differ in stability and function. The population of bundled long filaments, which appeared to be more ventrally located and in contact with membrane proteins, may be more stable than the population of short branched filaments. The location, organization, and polarity of the long bundled filaments suggest that they may be necessary for the expansion of lamellipodia and for the production of tension mediated by receptors to substrate adhesion molecules.

scholarly journals Actin filament content and organization in unstimulated platelets.

1984 ◽  
Vol 98 (6) ◽  
pp. 1985-1991 ◽  
Author(s):  
J E Fox ◽  
J K Boyles ◽  
C C Reynolds ◽  
D R Phillips
Keyword(s):  
Actin Filament ◽  
Actin Filaments ◽  
Actin Polymerization ◽  
Gel Filtration ◽  
Structural Reorganization ◽  
Filamentous Form ◽  
Thin Sections ◽  
Cell Extracts ◽  
Triton X ◽  
Filament Content

The extent of actin polymerization in unstimulated, discoid platelets was measured by DNase I inhibition assay in Triton X-100 lysates of platelets washed at 37 degrees C by gel filtration, or in Triton X-100 lysates of platelets washed at ambient temperatures by centrifugation in the presence of prostacyclin. About 40% of the actin in the discoid platelets obtained by either method existed as filaments. These filaments could be visualized by electron microscopy of thin sections. Similar results were obtained when the actin filament content of discoid platelets was measured by sedimentation of filaments from Triton X-100 lysates at high g forces (145,000 g for 45 min). However, few of these filaments sedimented at the lower g forces often used to isolate networks of actin filaments from cell extracts. These results indicate that actin filaments in discoid cells are not highly crosslinked. Platelets isolated by centrifugation in the absence of prostacyclin were not discoid, but were instead irregular with one or more pseudopodia. These platelets also contained approximately 40-50% of their actin in a filamentous form; many of these filaments sedimented at low g forces, however, indicating that they were organized into networks. The discoid shape of these centrifuged platelets could be restored by incubating them for 1-3 h at 37 degrees C, which resulted in the reversal of filament organization. High g forces were then required for the sedimentation of the actin. Approximately 80-90% of the actin in platelets washed at 4 degrees C was filamentous; this high actin filament content could be attributed to actin polymerization during the preparation of the platelets at low temperatures. These studies show that platelet activation involves mechanisms for the structural reorganization of existing filaments, in addition to those previously described for mediating actin polymerization.

scholarly journals Structural organization of actin in the sea urchin egg cortex: microvillar elongation in the absence of actin filament bundle formation

1982 ◽  
Vol 93 (1) ◽  
pp. 24-32 ◽  
Author(s):  
DA Begg ◽  
LI Rebhun ◽  
H Hyatt
Keyword(s):  
Sea Urchin ◽  
Actin Filament ◽  
Actin Filaments ◽  
Calcium Ionophore ◽  
Ionophore A23187 ◽  
Acid Activation ◽  
Cytoplasmic Ph ◽  
Sea Urchin Egg ◽  
Filament Bundles ◽  
Filament Bundle

We have investigated the relationship between the formation of actin filament bundles and the elongation of microvilli (MV) after fertilization in sea urchin eggs. In a previous study (1979, J Cell Biol. 83:241-248) we demonstrated that increased pH induced the formation of actin filaments in isolated sea urchin egg cortices with the concomitant elongation of MV. On the basis of these results we suggested that increased cytoplasmic pH after fertilization causes a reorganization of cortical actin, which in turn provides the force for MV elongation. To test this hypothesis, we compared the morphology of microvilli in eggs activated with and without the release of fertilization acid. Activation of eggs in normal sea water with the calcium ionophore A23187 causes the release of fertilization acid and the elongation of MV containing core bundles of actin filaments. Eggs activated with A23187 in NA(+)-free water do not undergo normal fertilization acid release but develop elongated, flaccid MV. These MV contain an irregular network of actin filaments rather than the parallel bundles of filaments found in normal MV. The addition of 40 mM NaCl to these eggs results in the release of H(+) and the concomitant conversion of flaccid MV to erect MV containing typical core bundles of actin filaments. Identical results are obtained when 10 mM NH(4)Cl is substituted for NaCl. The induction of cytoplasmic alkalinization in unactivated eggs with NH(4)Cl does not cause either MV elongation or the formation of actin filament bundles . These results suggest that: (a) the elongation of MV is stimulated by a rise in intracellular free Ca(++) concentration; (b) actin filament bundle formation is triggered by an increase in cytoplasmic pH; and (c) the formation of actin filament bundles is not necessary for MV elongation but is required to provide rigid support for MV.

scholarly journals Actin filaments elongate from their membrane-associated ends

1981 ◽  
Vol 90 (2) ◽  
pp. 485-494 ◽  
Author(s):  
LG Tilney ◽  
EM Bonder ◽  
DJ DeRosier
Keyword(s):  
Plasma Membrane ◽  
Actin Filament ◽  
Actin Filaments ◽  
The Other ◽  
Dense Material ◽  
Thin Sections ◽  
Vacuole Membrane ◽  
Filament Bundles ◽  
Filament Bundle

In limulus sperm an actin filament bundle 55 mum in length extends from the acrosomal vacuole membrane through a canal in the nucleus and then coils in a regular fashion around the base of the nucleus. The bundle expands systematically from 15 filaments near the acrosomal vacuole to 85 filaments at the basal end. Thin sections of sperm fixed during stages in spermatid maturation reveal that the filament bundle begins to assemble on dense material attached to the acrosomal vacuole membrane. In micrographs fo these early stages in maturation, short bundles are seen extending posteriorly from the dense material. The significance is that these short, developing bundles have about 85 filaments, suggesting that the 85-filament end of the bundle is assembled first. By using filament bundles isolated and incubated in vitro with G actin from muscle, we can determine the end "preferred" for addition of actin monomers during polymerization. The end that would be associated with the acrosomal vacuole membrane, a membrane destined to be continuous with the plasma membrane, is preferred about 10 times over the other, thicker end. Decoration of the newly polymerized portions of the filament bundle with subfragment 1 of myosin reveals that the arrowheads point away from the acrosomal vacuole membrane, as is true of other actin filament bundles attached to membranes. From these observations we conclude that the bundle is nucleated from the dense material associated with the acrosomal vacuole and that monomers are added to the membrane-associated end. As monomers are added at the dense material, the thick first-made end of the filament bundle is pushed down through the nucleus where, upon reaching the base of the nucleus, it coils up. Tapering is brought about by the capping of the peripheral filaments in the bundle.

scholarly journals Movement of the actin filament bundle in Mytilus sperm: a new mechanism is proposed

1987 ◽  
Vol 104 (4) ◽  
pp. 981-993 ◽  
Author(s):  
LG Tilney ◽  
Y Fukui ◽  
DJ DeRosier
Keyword(s):  
Plasma Membrane ◽  
Actin Filament ◽  
Cell Elongation ◽  
Actin Filaments ◽  
Blue Mussel ◽  
Optical Diffraction ◽  
Acrosomal Reaction ◽  
Filament Bundle

An actin filament bundle approximately 2-5 microns in length is present in the sperm of the blue mussel, Mytilus. In unfired sperm this bundle extends from the midpiece through a canal in the center of the nucleus to terminate on the membrane limiting the inside of the cone-shaped acrosomal vacuole. The bundle is composed of 45-65 actin filaments which are hexagonally packed and regularly cross-bridged together to form an actin paracrystal so well ordered that it has six nearly equal faces. Upon induction of the acrosomal reaction, a needle-like process is formed in a few seconds. Within this process is the actin filament bundle which appears unchanged in filament number and packing as determined by optical diffraction methods. Using fluorescein-conjugated phalloidin we were able to establish that the bundle does not change length but instead is projected anteriorly out of the midpiece and nuclear canal like an arrow. Existing mechanisms to explain this extension cannot apply. Specifically, the bundle does not increase in length (no polymerization), does not change its organization (no change in actin twist), does not change filament number (no filament sliding), and cannot move by myosin (wrong polarity). Thus we are forced to look elsewhere for a mechanism and have postulated that at least a component of this movement, or cell elongation, is the interaction of the actin filament bundle with the plasma membrane.

Hair cell regeneration and recovery of the vestibuloocular reflex in the avian vestibular system

1996 ◽  
Vol 76 (5) ◽  
pp. 3301-3312 ◽  
Author(s):  
J. P. Carey ◽  
A. F. Fuchs ◽  
E. W. Rubel
Keyword(s):  
Hair Cell ◽  
Cell Density ◽  
Hair Cells ◽  
Cell Damage ◽  
Cell Loss ◽  
Average Density ◽  
Cell Regeneration ◽  
Vestibuloocular Reflex ◽  
Scanning Electron Micrographs ◽  
Hair Cell Regeneration

1. Although auditory and vestibular hair cells are known to regenerate after aminoglycoside intoxication in birds, there is only sparse evidence that the regenerated hair cells are functional. To address this issue, we examined the relation of hair cell regeneration to recovery of the vestibuloocular reflex (VOR), whose afferent signal originates at hair cells in the vestibular epithelium. Hair cell damage was produced by treating white Leghorn chicks (Gallus domesticus, 4–8 days posthatch) with streptomycin sulfate in normal saline (1,200 mg.kg-1.day-1 im) for 5 days. 2. In the 1st wk after treatment, the VOR gain was essentially 0, and hair cell density as assessed by light microscopy was approximately 40% of normal. Between the 1st and 3rd wk after treatment, the VOR was present. Although VOR gain varied considerably from one chick to another, it increased, on average, between the 1st and 3rd wk, as did the average hair cell density. At the end of 8–9 wk, the gain and phase of the VOR had returned to normal values, as had the average density of hair cells. 3. Therefore, despite the catastrophic initial effect of hair cell loss on the VOR, recovered hair cells appeared to restore the VOR completely. Average hair cell density increased with average VOR gain. VOR gain correlated better with recovery of type 1 hair cells than with recovery of type II hair cells. 4. In contrast to hair cell density, the appearance of the vestibular epithelia as assessed by hair cell stereocilia in scanning electron micrographs was a poor indicator of VOR gain. In both treated and control birds, epithelia with the same appearance could have quite different VOR gains, suggesting a variation in the functional viability of the hair cells. 5. This observation suggests that several factors, such as the repair of stereocilia, the efficacy of hair cell synapses on afferent fibers, and the extent of compensation by central vestibular pathways, may affect the recovery of VOR gain. However, our data suggest that hair cell regeneration plays an important role in this recovery.

scholarly journals Three different actin filament assemblies occur in every hair cell: each contains a specific actin crosslinking protein.

1991 ◽  
Vol 112 (4) ◽  
pp. 641-651 ◽  
Author(s):  
D Drenckhahn ◽  
K Engel ◽  
D Höfer ◽  
C Merte ◽  
L Tilney ◽  
...  
Keyword(s):  
Actin Filament ◽  
Hair Cells ◽  
Actin Filaments ◽  
Supramolecular Assembly ◽  
Spatial Proximity ◽  
Cross Linking ◽  
Supporting Cells ◽  
Cuticular Plate ◽  
Auditory Organ ◽  
Close Spatial Proximity

The apex of hair cells of the chicken auditory organ contains three different kinds of assemblies of actin filaments in close spatial proximity. These are (a) paracrystals of actin filaments with identical polarity in stereocilia, (b) a dense gellike meshwork of actin filaments forming the cuticular plate, and (c) a bundle of parallel actin filaments with mixed polarities that constitute the circumferential filament belt attached to the cytoplasmic aspect of the zonula adhaerens (ZA). Each different supramolecular assembly of actin filaments contains a specific actin filament cross-linking protein which is unique to that particular assembly. Thus fimbrin appears to be responsible for paracrystallin packing of actin filaments in stereocillia; an isoform of spectrin resides in the cuticular plate where it forms the whisker-like crossbridges, and alpha actinin is the actin crosslinking protein of the circumferential ZA bundle. Tropomyosin, which stabilizes actin filaments, is present in all the actin filament assemblies except for the stereocilia. Another striking finding was that myosin appears to be absent from the ZA ring and cuticular plate of hair cells although present in the ZA ring of supporting cells. The abundance of myosin in the ZA ring of the surrounding supporting cells means that it may be important in forming a supporting tensile cellular framework in which the hair cells are inserted.

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