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Behavior of the centriolar duplexes during early mitosis was investigated. In general, both duplexes are capable of migrating about the cell as a unit with little change in their center to center spacing prior to separation to form the spindle poles. This duplex separation may occur at any point within the mid-prophase-prometaphase period. If it is delayed to prometaphase, transitory monopolar spindles were observed.
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Protoplasts of Avena sativa rotate in an alternating electric field provided that at least two cells are located close to each other. An optimum frequency range (20 to 30 kHz) exists where rotation of all cells exposed to the field is observed. Below and above this frequency range, rotation of some cells is only occasionally observed. The angular velocity of rotation depends on the square of the electric field strength. At field strengths above the value leading to electrical breakdown of the cell membrane, rotation is no longer observed due to deterioration of the cells. The absolute value of the angular velocity of rotation at a given field strength depends on the arrangement of the cells in the electric field. A maximum value is obtained if the angle between the field direction and the line connecting the two cells is 45 degrees. With increasing distance between the two cells the rotation speed decreases. Furthermore, if two cells of different radii are positioned close to each other the cell with the smaller radius will rotate with a higher speed than the larger one. Rotation of cells in an alternating electric field is described theoretically by interaction between induced dipoles in adjacent cells. The optimum frequency range for rotation is related to the relaxation of the polarization process in the cell. The quadratic dependence of the angular velocity of rotation on the field strength results from the fact that the torque is the product of the external field and the induced dipole moment which is itself proportional to the external field. The theoretical and experimental results may be relevant for cyclosis (rotational streaming of cytoplasm) in living cells.
Diverse cell types, including Amoebae, leukocytes, embryonic cells and tumour cells move about on solid surfaces to accomplish such activities as feeding, bacterial destruction, embryological development and metastasis. Theories of the mechanism of this movement are reviewed and a model is proposed which invokes the existence of specific, laterally mobile, transmembranous structures in the cell membrane, which are reversibly adhesive for both the contractile apparatus of the cell internally, and the substratum externally. By this model, the movement of all these cell types can be explained.
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We separated osteoclasts from bone and observed the effect of several known and potential mediators of the control of bone resorption on their cytoplasmic motility. We already found that calcitonin (CT), a hormone that inhibits bone resorption, regularly causes complete inhibition of cytoplasmic motility, specific for osteoclasts, through a trypsin-sensitive membrane receptor [1]. We report here that prostaglandin I2 (PGI2) and dibutyryl cyclic AMP induce an identical change in osteoclastic behavior. We found that theophylline, which inhibits intracellular cyclic AMP degradation, and which itself had no effect on osteoclastic motility, potentiated the cytoplasmic inhibition caused by CT, PGI2, and cyclic AMP. This suggests that PGI2 and CT cause cytoplasmic quiescence by increasing the intracellular level of cyclic AMP, a view compatible with the known ability of CT to increase cyclic AMP in bone [2]. Parathyroid hormone (PTH), PGE2, and 1,25 dihydroxycholecalciferol (1,25 (OH)2D3), hormones known to stimulate osteoclasts, did not stimulate the activity of either active or quiescent isolated osteoclasts. The undoubted ability of these hormones to stimulate osteoclastic activity in vivo may therefore be mediated through a primary hormonal interaction with another cell type.
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This brief review summarizes gravity-induced changes in bioelectric parameters and evaluates their contribution to our understanding of the sensing of gravity, and the transduction and transmission of the gravity stimulus in plants. During the last few decades, information has accumulated demonstrating gravity-induced changes in surface potentials, membrane voltages, endogenous electric currents and ion fluxes. These changes point to the plasma membrane as the site of perception and transduction of the gravity signal. To date, it is reasonable to assume that gravity affects the state of ion channels (in particular, Ca2+ channels) and the activity of ion pumps (in particular, the electrogenic H(+)-ATPase) in the plasma membrane leading to intracellular and apoplasmic changes in ion activities and in membrane voltages. The flow of H+ and Ca2+ currents is probably the means by which information about gravity is amplified and transmitted from sensing to responding cells. No data are available so far about the effect of microgravity on bioelectric parameters. However, it would be interesting to learn if plants become hypersensitive to gravity during a prolonged stay in microgravity. If so, such plants might fire action potentials after return to earth, because more Ca2+ channels than usual may be activated by 1 g in microgravity-adapted plants.
The plasmodium of the true slime mold Physarum polycephalum is a large amoeboid organism that displays "smart" behavior such as chemotaxis and the ability to solve mazes and geometrical puzzles. These amoeboid behaviors are based on the dynamics of the viscoelastic protoplasm and its biochemical rhythms. By incorporating both these aspects, we constructed a mathematical model for the dynamics of the organism as a first step towards understanding the relation between protoplasmic movement and its unusual abilities. We tested the validity of the model by comparing it with physiological observation. Our model reproduces fundamental characteristics of the spatio-temporal pattern of the rhythmic movement: (1) the antiphase oscillation between frontal tip and rear when the front is freely extending; (2) the asynchronous oscillation pattern when the front is not freely extending; and (3) the formation of protoplasmic mounds over a longer time scale. Both our model and physiological observation suggest that cell stiffness plays a primary role in plasmodial behaviors, in contrast to the conventional theory of coupled oscillator systems.
Cytokinesis in higher-plant cells involves the formation of a cell plate in the interzone between the separating chromatids. The process is directed by the phragmoplast, an array of microtubules, actin filaments, and membranous elements. To determine if the role of actin in cytokinesis is dependent on myosin, we treated stamen hair cells of Tradescantia virginiana L. with 2,3-butanedione monoxime (BDM), an inhibitor of myosin ATPase and ML-7, a specific inhibitor of myosin light-chain kinase. Treatment with BDM resulted in a tilted cytokinetic apparatus during early initiation and a wavy cell plate with curved phragmoplasts during late lateral expansion. Treatment with ML-7 also resulted in inefficient late lateral expansion of the cell plate, with effects ranging from slower expansion to complete inhibition. Taken together, these results implicate myosin in the control of cell plate expansion and alignment.
Holospora obtusa is a Gram-negative bacterium inhabiting the macronucleus of the ciliate Paramecium caudatum. Experimental infection with H. obtusa was carried out under nocodazole treatment. Nocodazole has been shown to cause disassembly of the cytoplasmic microtubules radiating from the cytopharynx and postoral fibers in P. caudatum. Treatment with this drug did not prevent the ingestion of both prey bacteria and H. obtusa, but it reduced the phagosome number and affected cyclosis. In situ hybridization revealed infectious forms of this endobiont very close to the macronucleus, but never inside it. These results indicate that disassembly of microtubules does not impair transportation of the infectious forms of H. obtusa in the cytoplasm, but that it completely blocks the invasion of the nucleus by the bacteria.