The movement of bound ligands over cell surfaces

The long range movements of membrane-bound ligands into surface caps and into the pseudopods of phagocytizing cells, the uropods of motile cells and the cleavage furrow of dividing cells appear to be analogous processes. A common mechanism to explain these movements must take into account several new and central observations: ligand-receptor complexes can migrate to regions of existing microfilament accumulation; laser photobleaching studies with fluorescent Con A indicate that ligand-receptor movement occurs unidirectionally; video computer analyses of Con A redistribution show that movement may exceed the maximum rates measured for protein diffusion in membranes. These observations are not consistent with models in which ligand-receptor movement occurs by diffusion or by direct interaction with contractile microfilaments. However, they can be satisfied by a new model that proposes the entrainment of selected membrane determinants on membrane waves directed towards regions such as caps, pseudopodia, uropods or cleavage furrow. These oriented waves are initiated by tension due to asymmetric microfilamentmembrane interaction.     

Analogous ultrastructure and surface properties during capping and phagocytosis in leukocytes

Ultrastructural analyses have revealed striking similarities between Concanavalin A capping and phagocytosis in leukocytes. Both processes involve extensive membrane movement to form a protuberance or pseudopods; a dense network of microfilaments is recruited into both the protuberance and the pseudopods; microtubules are disassembled either generally (capping) or in the local region of the pseudopods (phagocytosis); and cells generally depleted of microtubules by colchicine show polarized phagocytosis via the microfilament-rich protuberance rather than uniform peripheral ingestion of particles via individual pseudopods. Cap formation can thus be viewed as occurring as an exaggeration of the same ultrastructural events that mediate phagocytosis. Similar changes in cell surface topography also accompany capping and phagocytosis. Thus, in nonfixed cells, Concanavalin A-receptor complexes aggregate into the region of the protuberance in colchicine-treated leukocytes (conventional capping) or into the region of pseudopod formation in phagocytizing leukocytes. In the latter case, the movement of lectin-receptor complexes occurs from membrane overlying peripheral microtubules into filament-rich pseudopods that exclude microtubules. These data provide evidence against a role for microtubules as "anchors" for lectin receptors. Rather, they indicate a preferential movement of cell surface Concanavalin A-receptor complexes towards areas of extensive (the protuberance) or localized (pseudopods) microfilament concentration. In conventional capping, Concanavalin A must be added to the colchicine-treated cells before fixation in order to demonstrate movement of receptors from a diffuse distribution into the protuberance. However, Convanavalin A receptors are enriched in the membrane associated with phagocytic particles as compared to the remaining membrane. This particle-induced redistribution of receptors is particularly prominent in colchicine-treated cells that phagocytize and are then fixed and Concanavalin A labeled; both lectin receptors and beads are concentrated over the protuberance. Thus, the final analogy between conventionally capped and phagocytic cells is that in both cases the properties of the plasma membrane in regions of microfilament concentration are modified by Concanavalin A itself (capping) or by the phagocytized particle, to limit locally the diffusion of Concanavalin A receptors.     

Neuronal Network Models of Phase Separation Between Limb CPGs of Digging Sand Crabs

Ordinary differential equations are used to model a peculiar motor behaviour in the anomuran decapod crustacean Emerita analoga. Little is known about the neural circuitry that permits E. analoga to control the phase relationships between movements of the fourth legs and pair of uropods as it digs into sand, so mathematical models might aid in identifying features of the neural structures involved. The geometric arrangement of segmental ganglia controlling the movements of each limb provides an intuitive framework for modelling. Specifically, due to the rhythmic nature of movement, the network controlling the fourth legs and uropods is viewed as three coupled identical oscillators, one dedicated to the control of each fourth leg and one for the pair of uropods, which always move in bilateral synchrony. Systems of Morris–Lecar equations describe the voltage and ion channel dynamics of neurons. Each central pattern generator for a limb is first modelled as a single neuron and then, more realistically as a multi-neuron oscillator. This process results in high-dimensional systems of equations that are difficult to analyse. In either case, reduction to phase equations by averaging yields a two-dimensional system of equations where variables describe only each oscillator’s phase along its limit cycle. The behaviour observed in the reduced equations approximates that of the original system. Results suggest that the phase response function in the two dimensional system, together with minimal input from asymmetric bilateral coupling parameters, is sufficient to account for the observed behaviour.     

Reversible particle movements associated with unstacking and restacking of chloroplast membranes in vitro

Freeze-fracture and freeze-etch techniques have been employed to study the supramolecular structure of isolated spinach chloroplast membranes and to monitor structural changes associated with in vitro unstacking and restacking of these membranes. High-resolution particle size histograms prepared from the four fracture faces of normal chloroplast membranes reveal the presence of four distinct categories of intramembranous particles that are nonrandomly distributed between grana and stroma membranes. The large surface particles show a one to one relationship with the EF-face particles. Since the distribution of these particles between grana and stroma membranes coincides with the distribution of photosystem II (PS II) activity, it is argued that they could be structural equivalents of PS II complexes. An interpretative model depicting the structural relationship between all categories of particles is presented. Experimental unstacking of chloroplast membranes in low-salt medium for at least 45 min leads to a reorganization of the lamellae and to a concomitant intermixing of the different categories of membrane particles by means of translational movements in the plane of the membrane. In vitro restacking of such experimentally unstacked chloroplast membranes can be achieved by adding 2-20 mM MgCl2 or 100-200 mM NaCl to the membrane suspension. Membranes allowed to restack for at least 1 h at room temperature demonstrate a resegregation of the EF-face particles into the newly formed stacked membrane regions to yield a pattern and a size distribution nearly indistinguishable from the normally stacked controls. Restacking occurs in two steps: a rapid adhesion of adjoining stromal membrane surfaces with little particle movement, and a slower diffusion of additional large intramembranous particles into the stacked regions where they become trapped. Chlorophyll a:chlorophyll b ratios of membrane fraction obtained from normal, unstacked, and restacked membranes show that the particle movements are paralleled by movements of pigment molecules. The directed and reversible movements of membrane particles in isolated chloroplasts are compared with those reported for particles of plasma membranes.     

Antibody-mediated internalization of B lymphocyte surface membrane immunoglobulin

The ultrastructural correlates of antibody-mediated internalization of membrane immunoglobulin determinants has been studied in the guinea pig using a horseradish peroxidase conjugate of rabbit anti-guinea pig immunoglobulin. We have observed that polar flow of ligand-induced micro-aggregated membrane immunoglobulin proceeds by local deformation of the plasma membrane into open-ended endocytic vesicles which in turn accumulate at the Golgi-associated pole prior to completion of endocytosis. ‘Capping’ does not appear to result from simple diffusion of the aggregates within the plane of the membrane. In addition, the question of whether anti-immunoglobulin can induce the formation of uropods on B lymphocytes in the guinea pig was examined. Formation of uropod-like structures on B lymphocytes appears to be factitious and induced by accumulation at the Golgi-associated cell pole of non-internalizable aggregates of membrane immunoglobulin linked to cell debris. These observations are interpreted as supporting the concept that spontaneous uropod formation by lymphocytes in the absence of anti-immunoglobulin reagents is a characteristic of antigen-reactive thymus-derived lymphocytes in the guinea pig.     

Effects of the actin-binding protein DNAase I on cytoplasmic streaming and ultrastructure of Amoeba proteus

Microinjection of DNAase I, which is known to form a specific complex with G-actin, induces characteristic changes in cytoplasmic streaming, locomotion and morphology of the contractile apparatus of A. proteus. Light microscopical studies show pronounced streaming originating from the uroid and/or the retracting pseudopods, which ceases 10--15 min after injection of DNAase I, at a time when ultrasctructural studies show that the actin filament system is very much reduced. These results suggest that a controlled reversible equilibrium between soluble and polymerized forms of actin is a necessary requirement for amoeboid movement. The topographic distribution of contractile filaments beneath the plasma membrane visualized by correlated light- and electron microscopy of DNAase I-injected cells establishes the importance of the membrane-bound filamentous layer for three major aspects of streaming: (1) Streaming originates by local contractions of a cell membrane-associated filament layer at the uroid and/or retracting pseudopods, creating a pressure flow. (2) This flow continues beneath the membrane, which is stabilized by filaments in the lateral regions between the posterior end, with a high hydrostatic pressure, and the anterior end, with a low hydrostatic pressure. (3) Pseudopods or extending areas are created by a local destabilization of the cell periphery caused by the separation of the filamentous layer from the plasma membrane.     

Observations by a novel method of surface changes during the syncytial blastoderm stage of the Drosophila embryo

Up to now the cleavage process during the syncytial blastoderm stage has proved hard to follow in living Drosophila embryos. However, this can be achieved by microinjection of TMRITC-BSA into the fluid between the vitelline membrane and the embryo surface. The superficial bulges are then visible with epifluorescence optics. Once formed the bulges go through cycles of flattening, expansion, division, and rounding up. A tendency was noted for more divisions to occur at angles closer to 90° than at more acute angles relative to the previous cleavage. However, the cleavage angles were frequently determined by the distribution of surface space around the bulges. As caps became more numerous they squeezed and pushed around each other while expanding. As a result of these movements the surface of the blastoderm becomes uniformly covered with nuclei by the time of cellularization.     

Dynamic Distribution of Chemoattractant Receptors in Living Cells During Chemotaxis and Persistent Stimulation

While the localization of chemoattractant receptors on randomly oriented cells has been previously studied by immunohistochemistry, the instantaneous distribution of receptors on living cells undergoing directed migration has not been determined. To do this, we replaced cAR1, the primary cAMP receptor of Dictyostelium, with a cAR1-green fluorescence protein fusion construct. We found that this chimeric protein is functionally indistinguishable from wild-type cAR1. By time-lapse imaging of single cells, we observed that the receptors remained evenly distributed on the cell surface and all of its projections during chemotaxis involving turns and reversals of polarity directed by repositioning of a chemoattractant-filled micropipet. Thus, cell polarization cannot result from a gradient-induced asymmetric distribution of chemoattractant receptors. Some newly extended pseudopods at migration fronts showed a transient drop in fluorescence signals, suggesting that the flow of receptors into these zones may slightly lag behind the protrusion process. Challenge with a uniform increase in chemoattractant, sufficient to cause a dramatic decrease in the affinity of surface binding sites and cell desensitization, also did not significantly alter the distribution profile. Hence, the induced reduction in binding activity and cellular sensitivity cannot be due to receptor relocalization. The chimeric receptors were able to “cap” rapidly during treatment with Con A, suggesting that they are mobile in the plane of the cell membrane. This capping was not influenced by pretreatment with chemoattractant.     

Surface functions during mitosis. III. Quantitative analysis of ligand- receptor movement into the cleavage furrow: diffusion vs. flow

The surface distribution of concanavalin A (Con A) bound to cell membrane receptors varies dramatically as a function of mitotic phase. The lectin is distributed diffusely on cells labeled and observed between mid-prophase and early anaphase, whereas cells observed in late anaphase or telophase demonstrate a marked accumulation of Con A- receptor complexes over the developing cleavage furrow (Berlin, Oliver, and Walter. 1978. Cell. 15:327-341). In this report, we first use a system based on video intensification fluorescence microscopy to describe the simultaneous changes in cell shape and in lectin-receptor complex topography during progression of single cells through the mitotic cycle. The video analysis establishes that fluorescein succinyl Con A (F-S Con A)-receptor complex redistribution begins coincident with the first appearance of the cleavage furrow and is essentially complete within 2-3 min. This remarkable redistribution of surface fluorescence occurs during only a modest change in cell shape from a sphere to a belted cylinder. It reflects the translocation of complexes and not the accumulation of excess labeled membrane in the cleavage furrow: first, bound fluorescent cholera toxin which faithfully outlines the plasma membrane is not accumulated in the cleavage furrow, and, second, electron microscopy of peroxidase-Con A labeled cells undergoing cleavage shows that there is a high linear density of lectin within the furrow while Con A is virtually eliminated from the poles. The rate of surface movement of F-S Con A was quantitated by photon counting during a repetitive series of laser-excited fluorescence scans across dividing cells. Results were analyzed in terms of two alternative models of movement: a flow model in which complexes moved unidirectionally at constant velocity, and a diffusion model in which complexes could diffuse freely but were trapped at the cleavage furrow. According to these models, the observed rates of accumulation were attainable at either an effective flow velocity of approximately 1 micron/min, or an effective diffusion coefficient of approximately 10(- 9) cm2/s. However, in separate experiments the lectin-receptor diffusion rate measured directly by the method of fluorescence recovery after photobleaching (FRAP) on metaphase cells was only approximately 10(-10) cm2/s. Most importantly, photobleaching experiments during the actual period of F-S Con A accumulation showed that lectin-receptor movement during cleavage occurs unidirectionally. These results rule out diffusion and make a process of oriented flow of ligand-receptor complexes the most likely mechanism for ligand-receptor accumulation in the cleavage furrow.     

Stage-specific mRNAs coding for subtypes of H2A and H2B histones in the sea urchin embryo

Phagocytosis, pinocytosis and the surface distribution of concanavalin A (ConA) have been analyzed during mitosis in several mammalian cell lines. Use of the bisbenzimidazole dye, Hoechst 33258, for chromosome staining after gentle fixation made possible the rapid identification and correlation of mitotic phase with surface properties.Phagocytosis of both opsonized and nonopsonized particles is markedly depressed in mitotic cells of the mouse macrophage cell line J774.1. The uptake of opsonized particles (IgG-coated erythrocytes) is Impaired from early prophase through early G1, whereas phagocytosis of non-opsonized particles (latex beads) is restored by telophase. Fluid pinocytosis, determined by the uptake of soluble horseradish peroxidase, is also inhibited during mitosis. Thus peroxidase-containing cytoplasmic vesicles were virtually absent from mid-prophase through telophase in both J774 and Chinese hamster ovary (CHO) cells.Adsorptive pinocytosis of ConA was determined from the different distributions of fluorescence in single cells incubated at 37°C with rhodamine-conjugated ConA (surface and cytoplasmic label), then fixed and further incubated with fluorescein-conjugated anti-ConA (surface only). The separate fluorescence of Hoechst, fluorescein and rhodamine could be optically isolated. In interphase J774 cells, ConA is rapidly internalized into cytoplasmic vesicles. In contrast, ConA is restricted to the plasma membrane from mid-prophase through telophase. In CHO, the depressed pattern of internalization is not fully established until metaphase.The surface distribution of ConA also varied dramatically as a function of mitotic phase. Between mid-prophase and early anaphase, the pattern of surface ConA-receptor complexes is diffuse. Once the cleavage furrow begins to develop, however, ConA moves into the region of the furrow. This was shown in J774, CHO and 3T3 mouse embryonic fibroblasts, and is probably universal. ConA movement into the membrane that overlies the microfilaments of the contractile ring is analogous to similar movements that occur in interphase cells during ConA cap formation and during the development of phagocytic pseudopods. The analogy emphasizes the common functional consequences of microfilament-membrane organization.It is evident that membrane processes which depend upon endocytosis-for example, certain hormone-induced signals-may be interrupted during mitosis. Inhibition of endocytosis thus may be a significant element in the control of cellular activities during mitosis and a strong influence on the properties of the emergent post-mitotic cell.     

Confined lateral diffusion of membrane receptors as studied by single particle tracking (nanovid microscopy). Effects of calcium-induced differentiation in cultured epithelial cells.

The movements of E-cadherin, epidermal growth factor receptor, and transferrin receptor in the plasma membrane of a cultured mouse keratinocyte cell line were studied using both single particle tracking (SPT; nanovid microscopy) and fluorescence photobleaching recovery (FPR). In the SPT technique, the receptor molecules are labeled with 40 nm-phi colloidal gold particles, and their movements are followed by video-enhanced differential interference contrast microscopy at a temporal resolution of 33 ms and at a nanometer-level spatial precision. The trajectories of the receptor molecules obtained by SPT were analyzed by developing a method that is based on the plot of the mean-square displacement against time. Four characteristic types of motion were observed: (a) stationary mode, in which the microscopic diffusion coefficient is less than 4.6 x 10(-12) cm2/s; (b) simple Brownian diffusion mode; (c) directed diffusion mode, in which unidirectional movements are superimposed on random motion; and (d) confined diffusion mode, in which particles undergoing Brownian diffusion (microscopic diffusion coefficient between 4.6 x 10(-12) and 1 x 10(-9) cm2/s) are confined within a limited area, probably by the membrane-associated cytoskeleton network. Comparison of these data obtained by SPT with those obtained by FPR suggests that the plasma membrane is compartmentalized into many small domains 300-600 nm in diameter (0.04-0.24 microns2 in area), in which receptor molecules are confined in the time scale of 3-30 s, and that the long-range diffusion observed by FPR can occur by successive movements of the receptors to adjacent compartments. Calcium-induced differentiation decreases the sum of the percentages of molecules in the directed diffusion and the stationary modes outside of the cell-cell contact regions on the cell surface (which is proposed to be the percentage of E-cadherin bound to the cytoskeleton/membrane-skeleton), from approximately 60% to 8% (low- and high-calcium mediums, respectively).     

Mechanism of formation, ultrastructure, and function of the cytoplasmic bridge system during morphogenesis in Volvox

The cytoplasmic bridge system that links all cells of a Volvox embryo and plays a crucial role in morphogenesis is shown to form as a result of localized incomplete cytokinesis; sometimes bridge formation occurs before other regions of the cell have begun to divide. Vesicles, believed to be derived from the cell interior, align along the presumptive cleavage furrow in the bridge-forming region. Apparently it is where these vesicles fail to fuse that bridges are formed. Conventional and high voltage transmission electron microscopy analyses confirm that bridges are regularly spaced; they possess a constant, highly ordered structure throughout cleavage and inversion. Concentric cortical striations (similar to those observed previously in related species) ring each bridge throughout its length and continue out under the plasmalemma of the cell body to abut the striations of neighboring bridges. These striations are closely associated with an electron-dense material that coats the inner face of the membrane throughout the bridge region and appears to be thickest near the equator of each bridge. In addition to the parallel longitudinal arrays of cortical microtubules that traverse the cells, we observed microtubules that angle into and through the bridges during cleavage; however, the latter are not seen once inversion movements have begun. During inversion, bridge bands undergo relocation relative to the cell bodies without any loss of integrity or change in bridge spacing. Observation of isolated cell clusters reveals that it is the sequential movement of individual cells with respect to a stationary bridge system, and not actual movement of the bridges, that gives rise to the observed relocation.     

Drosophila nonmuscle myosin II promotes the asymmetric segregation of cell fate determinants by cortical exclusion rather than active transport

Cell fate diversity can be achieved through the asymmetric segregation of cell fate determinants. In the Drosophila embryo, neuroblasts divide asymmetrically and in a stem cell fashion. The determinants Prospero and Numb localize in a basal crescent and are partitioned from neuroblasts to their daughters (GMCs). Here we show that nonmuscle myosin II regulates asymmetric cell division by an unexpected mechanism, excluding determinants from the apical cortex. Myosin II is activated by Rho kinase and restricted to the apical cortex by the tumor suppressor Lethal (2) giant larvae. During prophase and metaphase, myosin II prevents determinants from localizing apically. At anaphase and telophase, myosin II moves to the cleavage furrow and appears to "push" rather than carry determinants into the GMC. Therefore, the movement of myosin II to the contractile ring not only initiates cytokinesis but also completes the partitioning of cell fate determinants from the neuroblast to its daughter.     

A mathematical model for the dynamics of large membrane deformations of isolated fibroblasts

In this paper we develop and extend a previous model of cell deformations, initially proposed to describe the dynamical behaviour of round-shaped cells such as keratinocytes or leukocytes, in order to take into account cell pseudopodial dynamics with large amplitude membrane deformations such as those observed in fibroblasts. Beyond the simulation (from a quantitative, parametrized model) of the experimentally observed oscillatory cell deformations, a final goal of this work is to underline that a set of common assumptions regarding intracellular actin dynamics and associated cell membrane local motion allows us to describe a wide variety of cell morphologies and protrusive activity. The model proposed describes cell membrane deformations as a consequence of the endogenous cortical actin dynamics where the driving force for large-amplitude cell protrusion is provided by the coupling between F-actin polymerization and contractility of the cortical actomyosin network. Cell membrane movements then result of two competing forces acting on the membrane, namely an intracellular hydrostatic protrusive force counterbalanced by a retraction force exerted by the actin filaments of the cell cortex. Protrusion and retraction forces are moreover modulated by an additional membrane curvature stress. As a first approximation, we start by considering a heterogeneous but stationary distribution of actin along the cell periphery in order to evaluate the possible morphologies that an individual cell might adopt. Then non-stationary actin distributions are considered. The simulated dynamic behaviour of this cytomechanical model not only reproduces the small amplitude rotating waves of deformations of round-shaped cells such as keratinocytes [as proposed in the original model of Alt and Tranquillo (1995, J. Biol. Syst. 3, 905–916)] but is furthermore in very good agreement with the protrusive activity of cells such as fibroblasts, where large amplitude contracting/retracting pseudopods are more or less periodically extended in opposite directions. In addition, the biophysical and biochemical processes taken into account by the cytomechanical model are characterized by well-defined parameters which (for the majority) can be discussed with regard to experimental data obtained in various experimental situations.     

Compartmentalized structure of the plasma membrane for receptor movements as revealed by a nanometer-level motion analysis

Movements of transferrin and alpha 2-macroglobulin receptor molecules in the plasma membrane of cultured normal rat kidney (NRK) fibroblastic cells were investigated by video-enhanced contrast optical microscopy with 1.8 nm spatial precision and 33 ms temporal resolution by labeling the receptors with the ligand-coated nanometer-sized colloidal gold particles. For both receptor species, most of the movement trajectories are of the confined diffusion type, within domains of approximately 0.25 microns2 (500-700 nm in diagonal length). Movement within the domains is random with a diffusion coefficient approximately 10(-9) cm2/s, which is consistent with that expected for free Brownian diffusion of proteins in the plasma membrane. The receptor molecules move from one domain to one of the adjacent domains at an average frequency of 0.034 s-1 (the residence time within a domain approximately 29 s), indicating that the plasma membrane is compartmentalized for diffusion of membrane receptors and that long- range diffusion is the result of successive intercompartmental jumps. The macroscopic diffusion coefficients for these two receptor molecules calculated on the basis of the compartment size and the intercompartmental jump rate are approximately 2.4 x 10(-11) cm2/s, which is consistent with those determined by averaging the long-term movements of many particles. Partial destruction of the cytoskeleton decreased the confined diffusion mode, increased the simple diffusion mode, and induced the directed diffusion (transport) mode. These results suggest that the boundaries between compartments are made of dynamically fluctuating membrane skeletons (membrane-skeleton fence model).     

Location of sequences within rotavirus SA11 glycoprotein VP7 which direct it to the endoplasmic reticulum.

The Simian 11 rotavirus glycoprotein VP7 is directed to the endoplasmic reticulum (ER) of the cell and retained as an integral membrane protein. The gene coding for VP7 predicts two potential initiation codons, each of which precedes a hydrophobic region of amino acids (H1 and H2) with the characteristics of a signal peptide. Using the techniques of gene mutagenesis and expression, we have determined that either hydrophobic domain alone can direct VP7 to the ER. A protein lacking both hydrophobic regions was not transported to the ER. Some polypeptides were directed across the ER membrane and then into the secretory pathway of the cell. For a variant retaining only the H1 domain, secretion was cleavage dependent, since an amino acid change which prevented cleavage also stopped secretion. However, secretion of two other deletion mutants lacking H1 and expressing truncated H2 domains was unaffected by this mutation, suggesting that these proteins were secreted without cleavage of their NH2-terminal hydrophobic regions or secreted after cleavage at a site(s) not predicted by current knowledge.     

Chemotaxis in Dictyostelium: how to walk straight using parallel pathways

Dictyostelium is one of the most successful and best-studied organisms for research into the mechanisms that drive chemotaxis. In this review, we discuss recent progress in the field. Phosphatidylinositol 3-kinase (PI 3-kinase), previously thought by some to be essential for chemotaxis, has now been proven to be dispensable. However, other pathways are emerging which might connect signalling to migration. In particular, phospholipase A2 homologues appear to play an important role. Other areas of current interest include the fundamental processes by which cells move - pseudopods have been found to be generated in many different ways. Similarly, chemotaxis may be mediated by multiple checks on the number of pseudopods, rather than by simple generation of new pseudopods on demand. Finally, we review several advances in the theory of how cells convert shallow, noisy chemical gradients into overt movement.     

Quantitative analysis of actin patch movement in yeast

To investigate the mechanism of cortical actin patch movement in yeast, we implement a method for computer tracking the motion of the patches. Digital images from fluorescence microscope movies of living cells are fed into an image-processing program, which generates two-dimensional patch coordinates in the plane of focus for each movie frame via an algorithm based on detection of rapid intensity variations. The patch coordinates in neighboring frames are connected by a minimum-distance algorithm. The method is used to analyze control cells and cells treated with the actin-depolymerizing agent latrunculin. The motion of the patches in both cases, as analyzed by mean-square patch displacements, is found to be a random walk on average, with a much lower diffusion coefficient for the latrunculin-treated cells. The mean-squared patch travel distances for all of the latrunculin-treated cells are lower than those for all of the control cells. The patches move independently of one another. We develop a quantitative criterion for the presence of directed motion, and show that numerous patches in the control cells display directed motion to a very high degree of certainty. A small number of patches in the latrunculin-treated cells display directed motion.     

Studies on the cellular basis of morphogenesis in the sea urchin embryo. Directed movements of primary mesenchyme cells in normal and vegetalized larvae

A time-lapse study has been made of the movements of the primary mesenchyme cells in the developing sea urchin larva. It shows that these cells move by pseudopod formation and contraction, and that a transition takes place--within a few hours--from a more or less random cluster, in the early mesenchyme blastula, to a well-organized, coherent pattern on the ectoderm of the gastrula. This organization is achieved by a striking random exploration of the wall of the larva by the pseudopods, followed by their contraction. The final pattern of the mesenchyme reflects those regions of the wall where the contacts between pseudopods and wall are most stable. The mechanism is thus one of selective fixation rather than of selective conduction. The pseudopodal contacts are seen to be continually made and broken, even when the final pattern is formed. The pseudopods of several cells may fuse to form a common pseudopod, these cells then migrating together. This is particularly evident in vegetalized larvae, but is also typical of the ventral side. Despite considerable variations in the way in which the final pattern is achieved, several main phases can be distinguished. The first is a radial displacement of the cells from the vegetal plate onto the presumptive ectoderm, followed by a phase of dispersion. The cells then gradually accumulate at a characteristic level, and form a ring. During this process, and when the ring is formed, the cells tend to accumulate in two clusters along the ring. The pseudopods of the cells in these clusters join into a cable, the end of which is highly branched; it explores the ectoderm, and extends the cell clusters to form branches from the ring. In vegetalized larvae, the pattern of distribution is simplified, but the same principles apply. It is suggested that the variations in the way in which the pattern is achieved are, in all probability, merely a reflexion of the lack of precision in the time sequence of changes in adhesive properties of the primary mesenchyme and blastocoel wall.     

The Effects of Rhythmicity and Amplitude on Transfer of Motor Learning

We perform rhythmic and discrete arm movements on a daily basis, yet the motor control literature is not conclusive regarding the mechanisms controlling these movements; does a single mechanism generate both movement types, or are they controlled by separate mechanisms? A recent study reported partial asymmetric transfer of learning from discrete movements to rhythmic movements. Other studies have shown transfer of learning between large-amplitude to small-amplitude movements. The goal of this study is to explore which aspect is important for learning to be transferred from one type of movement to another: rhythmicity, amplitude or both. We propose two hypotheses: (1) Rhythmic and discrete movements are generated by different mechanisms; therefore we expect to see a partial or no transfer of learning between the two types of movements; (2) Within each movement type (rhythmic/discrete), there will be asymmetric transition of learning from larger movements to smaller ones. We used a learning-transfer paradigm, in which 70 participants performed flexion/extension movements with their forearm, and switched between types of movement, which differed in amplitude and/or rhythmicity. We found partial transfer of learning between discrete and rhythmic movements, and an asymmetric transfer of learning from larger movements to smaller movements (within the same type of movement). Our findings suggest that there are two different mechanisms underlying the generation of rhythmic and discrete arm movements, and that practicing on larger movements helps perform smaller movements; the latter finding might have implications for rehabilitation.