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PubMed1992

Conditions for assembly of tubulin-based structures in unfertilized sea urchin eggs. Spirals, monasters and cytasters.

Cytasters were induced in the unfertilized eggs of the sea urchin Lytechinus pictus by two different methods: (1) treatment with hexylene glycol or taxol, which are known to lower the tubulin critical concentration in vitro, but do not activate the nuclear cycle, and (2) by raising the cytoplasmic pH from that of unfertilized cytoplasm to that of fertilized, which activated the nuclear cycle and initiated tubulin polymerization. In the unactivated eggs, with increasing concentrations of the inducing agents, temperature and duration of treatment, microtubule structures that formed showed a progression from loose microtubule networks and spiral arrays to tightly organized cytasters. In eggs incubated in sea water containing 2.5 or 10 mM ammonium acetate titrated with HCl or NaOH in steps from pH 5.0 to pH 9.0, the nuclear cycle and tubulin polymerization were initiated at about pH 7.0. The degree of development attainable after three hours was dependent on the pH, with spirals forming at the threshold level of pH 7.0, monasters at pH 7.5, and at pH 8.5 cells formed cytasters, multipolar spindles and even completed multipolar divisions. In unactivated eggs, tubulin was made available by lowering of the tubulin critical concentration; in activated eggs it was made available from some previously unavailable store. The evidence suggests that the amount of assembly-competent tubulin available, regardless of how it is made available, determines the type of structure that is formed, with microtubule bundles and spirals at the lower concentrations and functional cytasters at the upper. We also describe some details of cytaster formation, including the role of microtubule interactions and the movement of dense granules to the aster centers in hexylene glycol-treated eggs.

Alkaloids↗
PubMed1986

Cell motility.

Approximately 20 years have passed since the first isolation of tubulin, dynein and non-muscle actomyosin. During this period actomyosin, microtubule-dynein or microtubules plus other ATPases (kinesin) have been implicated in almost all cellular motile phenomena. A possible exception is that polymerization of actin in a preferred direction could be responsible for extending the leading edge of a fibroblast. We now tend to look for an explanation of a motile process in terms of the cross-bridge cycle or some variation on this general mechanism. It will be of considerable interest to determine whether the new system for organelle movement fits this concept. In spite of great progress in identifying the basis of motility, fundamental problems remain unsolved. The nature of the conformation change of actomyosin in the cross-bridge cycle is still unknown and the solution of this problem may require the determination of the three-dimensional structure of the actin-myosin-nucleotide complex. We are still at the stage of classifying proteins that bind to actin and microtubules. The control of the dynamic state of actin and the microtubules in the cell is still poorly understood. Recent studies give hope that it will be possible to understand the mechanism of mitosis, but the biochemical study of this problem has only just begun.

Actomyosin↗
PubMed1977

Some clues as to the formation of protrusions by Fundulus deep cells.

One of the ways in which Fundulus deep cells move in vivo is by putting out long, fingerlike protrusions. This involves a change in the shape of the cell as a whole, with cytoplasmic flow, and is not just a local phenomenon. Moreover, particles on the cell surface move toward a protrusion as it is forming, suggesting surface flow. The role of surface flow is discussed both on a grown level and in respect to molecular fluidity. Long, stable protrusions can be pulled from cells by the application of negative pressure at a constant rate and these behave in a similar way to those formed during cell locomotion. Such long protrusions must be structured. The importance of contractile properties of the cytoplasm in the formation of protrusions was studied by treating cells with media that modify cellular contractility.

Animals↗
PubMed1982

Migration of human neutrophils in hydrated collagen lattices.

The migration of human neutrophilic granulocytes in hydrated collagen lattices was studied by a combination of cinemicroscopy, and scanning and transmission electron microscopy. The basic pattern of cell migration in collagen was similar to that observed previously for these cells on inert material surfaces; i.e., a cycle of cell extension and cytoplasmic flow into the leading extension. In general, however, neutrophils in collagen were less spread than on glass or plastic surfaces. Thin lamellipodia were absent and the leading extension of the cells was often an elaborately folded pseudopodium. In addition, neutrophils migrating in collagen were never observed to have retraction fibres at the tail end of the cells, although a uropod was usually seen. In the region of the uropod, extensive blebbing of the cells often occurred, and when this happened, forward movement of the cells ceased. At the ultrastructural level, both the leading pseudopodium and the blebs at the tail of the cell were found to contain a dense cytoskeletal network from which cell organelles were excluded. Finally, the cells were found to be coated with an extensive glycocalyx, and individual collagen fibres were sometimes observed within the glycocalyx.

Cell Movement↗
PubMed1983

Studies of nuclear and cytoplasmic behaviour during the five mitotic cycles that precede gastrulation in Drosophila embryogenesis.

Using differential interference contrast optics, combined with cinematography, we have studied the morphological changes that the living, syncytial embryo undergoes from stage 10 through 14 of Drosophila embryogenesis, that is just prior to and during formation of the cellular blastoderm. We have supplemented these studies with data collected from fixed, stained, whole embryos. The following information has been obtained. The average duration of nuclear cycles 10, 11, 12 and 13 is about 9, 10, 12 and 21 min, respectively (25 degrees C). In these four cycles, the duration of that portion of the mitotic period that lacks a discrete nuclear envelope is 3, 3, 3 and 5 min, respectively. The length of nuclear cycle 14 varies in a position-specific manner throughout the embryo, the shortest cycles being of 65 min duration. During nuclear cycles 10 through 13, it is commonly observed in living embryos that the syncytial blastoderm nuclei enter (and leave) mitosis in one of two waves that originate nearly simultaneously from the opposite anterior and posterior poles of the embryo, and terminate in its midregion. From our preparations of quick-frozen embryos, we estimate that these mitotic waves take on average about half a minute to travel over the embryonic surface from pole to equator. The yolk nuclei, which remain in the core of the embryo when the rest of the nuclei migrate to the periphery, divide in synchrony with the migrating nuclei at nuclear cycles 8 and 9, and just after the now peripherally located nuclei at nuclear cycle 10. After cycle 10, these yolk nuclei cease dividing and become polyploid. The syncytial embryo has at least three distinct levels of cytoskeletal organization: structured domains of cytoplasm are organized around each blastoderm nucleus; radially directed tracks orient colchicine-sensitive saltatory transport throughout the peripheral cytoplasm; and a long-range organization of the core of the embryo makes possible coherent movements of the large inner yolk mass in concert with each nuclear cycle. This highly organized cytoplasm may be involved in providing positional information for the important process of nuclear determination that is known to occur during these stages.

Animals↗
PubMed1985

Organelle motility in rat pituitary clonal cells. I. Dynamic movements of intracellular organelles.

Intracellular organelle motion within clonal pituitary tumor cells (GH3) was observed directly with a contrast enhancement, computer-video microscope system. All particles except nuclei moved in a complex fashion. Two types of particles predominated; one large and round, the other small and elongated. We classified the movements of these particles as saltation, oscillation and slow translocation. Saltation was directional movement with velocity of the order of 1 micron/sec. Oscillation was local motion occurring within 1 micron that showed no specific direction. Its velocity was similar to that of saltation. Large particles, in particular, showed the 3rd type of movement, slow translocation. The velocity appeared to be one order slower than that of saltation. We also examined the cells with fluorescent, dark-field and electron microscopies. We concluded that the large round particles were lysosomes and the small elongated ones mitochondria. The microtubule depolymerizer, vinblastine and the microfilament depolymerizer, cytochalasin D, completely inhibited all the types of organelle movement. The mechanism and significance of these organelle movements are discussed.

Animals↗
PubMed1991

Cell behavior and actomyosin organization in Dictyostelium during substrate exploration.

The behavior of individual Dictyostelium amebae was quantitatively analyzed with the computer-assisted "Dynamic Morphology System" (Soll, Voss, Varnum-Finney and Wessels, (1988) J. Cell. Biochem., 37: 177-192.). The same amoebae were then fixed and analyzed for filamentous (F-) actin and myosin (myosin-II, or "conventional" myosin) by fluorescence microscopy using the "agar-overlay method" (Yumura, and Fukui (1985) Nature, 314: 194-196.). This procedure provides a novel description of the behavior and morphometric changes preceding the static analysis of cytoskeletal organization in the same cell. It is demonstrated that when translocating cells make contact with an etched-smooth glass interface, 14% cross the interface, 20% either reverse direction or migrate along the interface, and the remaining 45% stay at the site. Cells contacting the interface from the smooth or etched side show equivalent behavioral responses. Upon contact with the interface, they project numerous lamellipodia and pseudopodia. While the lamellipodial projections exhibit cycles of spreading and retraction, the pseudopodia show lateral scanning motion, analogous to "substrate exploration" in fibroblasts (Albrecht-Buehler (1976) J. Cell Biol., 69: 275-286.). F-actin is localized in the lamellipodia and pseudopodia of amoebae contacting the interface. There is also discernable cortical F-actin, while conventional myosin appears to be excluded from the cortex and dispersed throughout the cytoplasm. The myosin displays a transient filamentous lattice at the base of newly forming lamellipodia. The ultrastructural study suggests that the new lamellipodia are formed on the dorsal surface and subsequently make contact with the substrate, indicating the dorsoventral sequence of polarity of the motile/sensory cellular organs. The present study demonstrates substrate exploration in Dictyostelium amoebae, and suggests its coupling to dynamic reorganization of the actomyosin cytoskeleton. The possible role of single-headed small myosin(s) (myosin-I, or mini-myosin) is discussed.

Actomyosin↗
PubMed1981

Microspore and microgametophyte development in relation to biological activity of environmental pollutants.

The pattern of synthesis of nucleic acids and proteins in the development of the microspore and later during pollen germination and tube growth is discussed. In the pollen grain at the time of anthesis all the proteins that are required for germination and early tube growth are either already present, or if new proteins are synthesized, the messenger RNAs for their synthesis already exist in the ungerminated pollen grain. In addition, similar proteins are synthesized on new mRNAs during germination and pollen tube growth as are synthesized on premade mRNAs. The genetic program during at least the latter part of pollen maturation prior to anthesis is thus the same as that during pollen germination and tube growth. Accordingly, one cannot treat mature pollen with mutagens and expect to be able to score the pollen tubes for mutant proteins. Treatment with mutagenic compounds would have to be during pollen maturation in the anther, before the transcription for the proteins required during germination and pollen tube growth has occurred. Available evidence indicates that this is very early in pollen development, possibly soon after meiosis. The value of pollen tubes to monitor for chemicals that affect the intracellular motility systems of organisms is also discussed.

Cytoplasmic Streaming↗
PubMed2004

Calcium dynamics in dendritic spines and spine motility.

A dendritic spine is an intracellular compartment in synapses of central neurons. The role of the fast twitching of spines, brought about by a transient rise of internal calcium concentration above that of the parent dendrite, has been hitherto unclear. We propose an explanation of the cause and effect of the twitching and its role in the functioning of the spine as a fast calcium compartment. Our molecular model postulates that rapid spine motility is due to the concerted contraction of calcium-binding proteins. The contraction induces a stream of cytoplasmic fluid in the direction of the dendritic shaft, thus speeding up the time course of spine calcium dynamics, relative to pure diffusion. Simulations indicate that chemical reaction rate theory at the molecular level can explain spine motility. They reveal two time periods in calcium dynamics, as measured recently by other researchers. It appears that rapid motility in dendritic spines increases the efficiency of calcium conduction to the dendrite and speeds up the emptying of the spine. This could play a major role in the induction of synaptic plasticity. A prediction of the model is that alteration of spine motility will modify the time course of calcium in the dendritic spine and could be tested experimentally.

Animals↗