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Biomedical subjects

M E Holwill

Publications and source records attributed to M E Holwill.

At least 19 recordsLinked to original sources
PubMed1991

Computer modelling of Tetrahymena axonemes at macromolecular resolution. Interpretation of electron micrographs.

A computer-generated model of the structural arrangement of the complete 9+2 ciliary axoneme of Tetrahymena at macromolecular resolution (4 nm) is presented. The model reconciles detailed information about subcomponents from negative-stained, thin-section and freeze-fracture electron micrographs, integrating the images into a consistent three-dimensional picture. This illuminates problems such as the requirement for compaction of dynein to form the arm, difficulties in visualization of the circumferential links, construction of the central sheath, and the comparative periodicities of the inner and outer arms. The model is pragmatic in that it is flexible and easily changed, as new information becomes available. It is also useful in the development of dynamic concepts, such as a spatial description of the dynein cross-bridge cycle, which is illustrated, or relationships between adjacent doublets during sliding and bending.

Animals↗
PubMed1990

A physical model of microtubule sliding in ciliary axonemes.

Ciliary movement is caused by coordinated sliding interactions between the peripheral doublet microtubules of the axoneme. In demembranated organelles treated with trypsin and ATP, this sliding can be visualized during progressive disintegration. In this paper, microtubule sliding behavior resulting from various patterns of dynein arm activity and elastic link breakage is determined using a simplified model of the axoneme. The model consists of a cylindrical array of microtubules joined, initially, by elastic links, with the possibility of dynein arm interaction between microtubules. If no elastic links are broken, sliding can produce stable distortion of the model, which finds application to straight sections of a motile cilium. If some elastic links break, the model predicts a variety of sliding patterns, some of which match, qualitatively, the observed disintegration behavior of real axonemes. Splitting of the axoneme is most likely to occur between two doublets N and N + 1 when either the arms on doublet N + 1 are active and arms on doublet N are inactive or arms on doublet N - 1 are active while arms on doublet N are inactive. The analysis suggests further experimental studies which, in conjunction with the model, will lead to a more detailed understanding of the sliding mechanism, and will allow the mechanical properties of some axonemal components to be evaluated.

Animals↗
PubMed1988

Flagellar wave reversal in the kinetoplastid flagellate Crithidia oncopelti.

Living Crithidia oncopelti cells swim through their environment by means of tip-to-base waves on their single flagellum. The cells are able to re-orient themselves by using a short burst of asymmetrical base-to-tip waves. All points on a flagellum are capable of initiating waves. Placing a population of cells in a medium of high viscosity initially produces a large number of organisms beating in the reverse mode. An individual cell has a random "switching" behavior. Viscosity affects the frequency of forward and reverse waves in different ways. The concentration of free Ca++ ions determines the direction of wave propagation in reactivated axonemes. Calmodulin may play a role in mediating the Ca++ dependence of wave direction.

Animals↗
PubMed1986

Structures attached to doublet microtubules of cilia: computer modeling of thin-section and negative-stain stereo images.

With a single set of positional coordinates for longitudinal and transverse attachment of the inner and outer rows of dynein arms with respect to the doublet microtubules of Tetrahymena ciliary axonemes, a computer model has been constructed at 4-nm resolution that reconciles negative-stain en face stereo images of arm and spoke positions to traditional images of tannic acid/glutaraldehyde-fixed sections. In this model, inner and outer arms correspond in substructure; both repeat with a 24-nm periodicity without stagger between rows, and a pair of arms is in exact alignment with the first spoke (S1) in each doublet spoke group. The model and the supporting micrographs suggest that each arm cycles in three dimensions and that, during cycling, the inner and outer arms move in opposite directions with respect to the center of subfiber A of the doublet (N). Attachment is off-center with respect to subfiber B of the adjacent doublet (N + 1), causing the sliding doublets to skew with respect to one another.

Adenosine Triphosphatases↗
PubMed1980

Motile flagellar axonemes with a 9 + 1 microtubule configuration.

Electron microscope (EM) studies of the eukaryotic flagellum reveal that the organelle contains a 9 + 2 arrangement of microtubules, the axoneme, with nine doublets surrounding two singlets enveloped by a membrane which is continuous with that of the cell; various linkages and projections are associated with the microtubules. Strong experimental evidence supports the idea that the forces required for bend formation on eukaryotic flagella are derived from active relative sliding of the peripheral doublets. Dynein arms, which project from each peripheral microtubule and possess ATPase activity, interact with a neighbouring doublet and undergo conformational changes which induce sliding. To form and propagate coordinated bends along a flagellum the sliding must be resisted in a controlled manner by structures within the axoneme. The regulatory mechanism responsible for the control of inter-doublet sliding is not known in detail, but ultrastructural studies suggest that interactions between the radial spokes attached to each doublet and the central complex of the axoneme may be involved. We report here the treatment of flagella with a 9 + 2 microtubular structure from the trypanosomid flagellate Crithidia oncopelti to produce motile axonemes with only one central microtubule. We conclude that the complete central complex is not involved in the conversion of microtubule sliding into axonemal bending, but may be both associated with the control of wave propagation and essential for bend initiation.

Animals↗
PubMed1979

A sliding microtubule model incorporating axonemal twist and compatible with three-dimensional ciliary bending.

1. Equations are developed to calculate the relative displacements of the doublet microtubules at the tip of a cilium when the microtubules twist about the axis of the organelle. 2. Displacements measured from electron micrographs show asymmetry (or skew) which can be matched quantitatively by the theoretical model with the appropriate selection of twist angle and orientation of the axoneme with respect to the plane of beat. 3. For Elliptio cilia the experimental results are consistent with a planar effective stroke and a recovery stroke involving a three-dimensional bend. The plane of the effective stroke is not normal to a surface containing the central pair of microtubules but contains microtubule 2 to produce the observed skew. 4. This model for the beat also explains the range of orientations of axoneme observed in sections through the metachronal wave.

Animals↗
PubMed1976

Effects of calcium on flagellar movement in the trypanosome Crithidia oncopelti.

1. The effects of calcium on the motility of different preparations of flagella from Crithidia oncopelti were studied using stroboscopic and high-speed cine photographic techniques. 2. By varying the concentration of calcium in suspensions of chemically treated samples of the organism it was found that changes occurred in bend shape, wave direction and frequency. 3. Waves on the flagellum of the organisms in vivo possess the unusual ability to propagate from tip to base, but reverse in direction during an avoiding response. In chemically extracted and reactivated preparations tip to base propagation was observed only at low concentrations (less than 10(-4) mol m-3) of calcium ion; at high concentrations base to tip propagation only was seen. In cells treated with ion across membranes, tip to base propagation was seen only in the presence of EGTA; when calcium was added the majority of organisms propagated waves from base to tip. 4. At certain values (ca. 10(-3) mol m-3) of the calcium concentration the wave shape had meander-like characteristics, whereas at higher and lower concentrations it was more sinsoidal. At high calcium concentrations only one wave appeared on the flagellum whereas at low values two or three were observed. 5. A reduction in frequency at high calcium concentrations was probably due to competitive inhibition of magnesium ions. 6. The results suggest that wave reversal in living Crithidia is induced by the release of calcium ions within the flagellum following stimulation of the membrane. In terms of the sliding filament model of flagellar activity the effects of calcium suggest that the ion is effective in modifying the interaction between the spoke head and central sheath and may control the relative direction of microtubular sliding.

Animals↗
PubMed1974

Dynamics of the hispid flagellum of Ochromonas danica. The role of mastigonemes.

High speed cinephotographic techniques were used to determine the pattern of fluid flow about the hispid flagellum of Ochromonas danica and to investigate the behavior of this flagellum in media of increased viscosity. The fluid currents are consistent with the hypothesis that the mastigonemes are passive, rigid, remain normal to the flagellar surface, and lie in the plane of flagellar undulation during motility.

Animals↗