Cell structures and their significance for ameboid movement

A pattern of epidermal cell migration during wound healing

Epidermal repair during wound healing is under investigation at both the light and electron microscopic levels. Suction-induced subepidermal blisters have been employed to produce two complementary model wound healing systems. These two model systems are: (a) intact subepidermal blisters, and (b) opened subepidermal blisters (the blister roof was removed immediately after induction, leaving an open wound). From these studies a pattern of movement for epidermal cells in wound healing is proposed. This pattern of movement is the same for both model systems. Epidermal cells appear to move by rolling or sliding over one another. Fine fibers oriented in the cortical cytoplasm may play an important role in the movement of these epidermal cells. Also instrumental in mediating this movement are intercellular junctions (desmosomes) and a firm attachment to a substrate through hemidesmosomes. In the intact subepidermal blisters hemidesmosomal attachment is made to a continuous and homogeneous substrate, the retained basal lamina. In the opened subepidermal blisters contact of epidermal cells is made to a discontinuous substrate composed of sporadic areas of fibrin and underlying mesenchymal cells.

Distribution of microfilament bundles during rotation of the nucleus in 3T3 cells treated with monensin

Cytoskeletal aspects of monensin-treated 3T3 cells with rotating nuclei were studied by immunofluorescence. The pattern of intermediate filaments and microtubules appeared unchanged when compared with control cells having a stationary nucleus. In contrast, the actin microfilament bundles appeared to have a consistent distribution in cells with rotating nuclei. Typically, we did not find long microfilament bundles that traverse the length of the cytoplasm of cells that were fixed at the time of nuclear rotation. Instead, there was a local distribution of short microfilament bundles situated ventrally to the nucleus and oriented at various angles to one another and to the predominant distribution of microfilament bundles in the cell. The observations suggest that the actin cytoskeleton is reorganized locally before or during rotation of the nucleus.

Early development of the granule cell in the cerebellum of the chick embryo

The sequence of differentiation of the cerebellar granule cell in chick embryos from the eighth to the 15th days of incubation has been studied in Golgi-stained celloidin sections. In the germinal-cell phase, the presumptive granule cell sends out one or two horizontal processes which may originate either in the body of the cell or in the extension which attaches it to the pial surface. Thus the germinal cell may be converted into either a monopolar or a bipolar presumptive granular cell. Bipolar cells may have two processes of the same length (symmetrical cells) or of unequal length (asymmetrical cells). In the symmetrical as well as asymmetrical bipolar cells the leading process is formed, by means of which the perikaryon emigrates until it situates itself definitely in the internal granular layer. Thus, symmetrical and asymmetrical bipolar cells give rise to a granule cell with parallel fibers of equal or different lengths. The monopolar element may originate a second process or may remain in the monopolar phase until it reaches the internal granular layer. Once there, it completes the formation of the parallel fibers.

A novel 36,000-dalton actin-binding protein purified from microfilaments in Physarum plasmodia which aggregates actin filaments and blocks actin-myosin interaction

In the plasmodia of Physarum polycephalum, which show a cyclic contraction-relaxation rhythm of the gel layer, huge aggregates of entangled actin microfilaments are formed at about the onset of the relaxation (R. Nagai, Y. Yoshimoto, and N. Kamiya. 1978. J. Cell Sci. 33:205-225). By treating the plasmodia with Triton X-100, we prepared a demembranated cytoskeleton consisting of entangled actin filaments and found that the actin filaments hardly interact with rabbit skeletal myosin. From the cytoskeleton we purified a novel actin-binding protein which binds stoichiometrically to actin and makes actin filaments curled and aggregated. It also inhibits the ATPase activity as well as the superprecipitation of reconstituted rabbit skeletal muscle actomyosin. This protein has a polypeptide molecular weight of 36,000 and binds 7 mol of actin/mol 36,000 polypeptide.

Laser light-scattering analysis of protoplasmic streaming in the slime mold Physarum polycephalum

Laser light scattering is shown to be an effective means of obtaining a rapid, objective assessment of dynamic changes in the intact plasmodium of the myxomycete Physarum polycephalum during bidirectional (shuttle) streaming. The motion of material in a 100 mum diameter region of a plasmodial vein was studied by following changes in the autocorrelation function of the fluctuations in the scattered light intensity. The autocorrelation function was recorded at 10 s intervals and analyzed to follow changes in the flow velocity of protoplasm associated with shuttle streaming. Rhythmic velocity changes and a "beating" pattern of velocity maxima were readily observed. In an attempt to locate the site of underlying structural changes in the vein responsible for the changing pattern of flow, the average scattered intensity was separated into components derived from moving and stationary scatterers. Periodic variations in the light intensity due to stationary scatterers are related to the streaming cycle and indicate the occurrence of important structural changes in the vein walls. Two possible interpretations of the data are offered; one involving gross dynamic changes in vein structure, the other involving the formation, contraction, or breakdown of fibrillar material in the vein wall during the streaming cycle.

Transformation of cytoplasmic actin. Importance for the organization of the contractile gel reticulum and the contraction--relasation cycle of cytoplasmic actomyosin

(1) Within the low viscous flowing endoplasm of Physarum polycephalum a considerable amount of actin is in the non-filamentous state. This can be demonstrated by applying poly-L-lysin to surface spreads of native protoplasm. (2) It has been shown that in protoplasmic drops the endoplasm-ectoplasm transformation is accompanied by an actin polymerization from the non-filamentous state to F-actin. (3) The actual state of the labile G-F-actin equilibrium determines the varying consistency (viscosity) of the cytoplasm. (4) Increasing viscosity can be interpreted as being brought about by a) shifting of the G-F-actin equilibrium to the filamentous side, and (b) increased myosin-mediated binding sites between actin filaments. (5) Polymerization and depolymerization processes are involved in the rhythmically occurring contraction-relaxation cycle of cytoplasmic actomyosin in Physarum. (6) Cytoplasmic actin and myosin represent the architectural proteins of the contractile gel reticulum in eukaryotic cells. (7) The importance of the regulation of actin polymerization as a basic control mechanism of the eukaryotic cell is discussed.

[A contribution to the physiology of movement of gregarines: elements and modus of cellular movement (author's transl)]

1. Cytochalasin B (= CCB, Phomin) in several concentrations inhibits the visible movement of gregarines. The fine structure of the cell is simultaneously changed, fibrillar bundles are desorganized. These fibrils are regarded as myonemes. 2. These myonemes are directed peripherally of the cell longitudinal and transverse to its axis. The longitudinal myonemes are organized in separated strings stretching along the top of the epicyte folds, between the plasmalemm and the secondary membranous layer. Fibrils under this layer serve as skeleton and as thus antagonist. The skeleton fibrils remain unaltered after CCB inhibition. They are located in a parallel direction to the myonemes. Their number corresponds together in one epicyte fold. The transverse myonemata surround the central plasma nearby the basal lamella, which cannot be found in some of the regarded species. 3. The co-work of skeleton-fibrils, stiff pellicle and myonemes allows to describe the modus of all known types of movement. Only change of coordination yields the multitude of these kinds of movement.

Cycling aggregation patterns of cytoplasmic F-actin coordinated with oscillating tension force generation

Isometric contracting protoplasmic veins of Physarum polycephalum show cycling patterns of cytoplasmic F-actin, dependent on their oscillating contraction behaviour (minute rhythms). The process of fibrillogenesis represents a parallel arrangement of F-actin chains ("plasma filaments, microfilaments") during the isometric contraction phase. A part of the results of the present work corroborates previous results on stretch-activated veins which showed that the fibrillar form of F-actin reflects the isometric contracted state. During isometric relaxation phase, a disaggregation of the fibrillar pattern takes place and is accompanied by a deparallelisation of F-actin chains. Therefore, the isometric relaxed state of cytoplasmic actomyosin is non-fibrillar in nature. Thus, the morphologically detectable fibrillar form of cytoplasmic actomyosin, according to physiological interpretation, is solely representative of the isometric contracted state. The question whether assembly-disassembly processes, e.g. G equilibrium F-actin-transformation, play a role in the contraction-relaxation cycle is discussed.

Microfilaments in human epithelial cancer cells

The occurence, distribution, and ultrastructural morphology of microfilaments in malignant epithelial cells of invasive squamous cell carcinoma of human oral cavity were studied by electron microscopy. The findings are compared with those in malignant oral epithelial cells of carcinoma-in-situ. In the malignant cells of invasive carcinoma, microfilaments 50-70 A in diameter are prominent in the cortical cytoplasm of the lateral and basal cell surfaces, adjacent and parallel to the plasma membrane, and extending into cell processes and microvillous extensions. Additional microfilaments are found to run from the peripheral cytoplasm to the perinuclear region. The microfilaments are aggregated into bundles aligned parallel to the long axis of the cell and display foci of increased electron density. They also tend to be aggregated into complex polygonal arrays. These microfilaments are similar in organization, concentration and ultrastructural architecture to those of various other nonmuscle cells, where they are thought to be capable of contraction and associated with cell motility. The presence of a microfilament system believed to be associated with contractile and motile cell processes may be an important characteristic of malignant cells of invasive tumors. The lack of abundant organized microfilaments in malignant cells in the absence of tumor invasion, and the presence of a prominent microfilament system in cells of invasive tumors, suggest that the microfilaments are related to the invasive properties of malignant tumor cells.

The distribution, ultrastructure, and chemistry of microfilaments in cultured chick embryo fibroblasts

The distribution, ultrastructure, and chemistry of microfilaments in cultured chick embryo fibroblasts were studied by thin sectioning of flat-embedded untreated and glycerol-extracted cells, histochemical and immunological electron microscopic procedures, and the negative staining of cells cultured on electron microscopic grids. In these cultured cells, the microfilaments are arranged into thick bundles that are disposed longitudinally and in looser arrangements in the fusiform-shaped cells. In the latter case, they are concentrated along the margins of the flattened cell, on the dorsal surface, and particularly at the ends of the cell and its ventral surface, where contact is made with the plastic dish or with other cells. Extracellular filaments, presumably originating from within the cell, are found at these points of contact. The microfilaments are composed in part of an actin-like protein. These filaments are between 70 and 90 A in diameter, they are stable in 50% glycerol, they have an endogenous ATPase (myosin-like?) associated with them, they bind rabbit muscle heavy meromyosin, and they specifically bind antibody directed against isolated actin-like protein. In the cultured chick embryo fibroblasts, the microfilaments are essential for the establishment and maintenance of form, and they are probably critical elements for adhesion and motility. The microfilaments might also serve as stabilizers of intramembranous particle fluidity.

The sensitivity of developing cardiac myofibrils to cytochalasin-B (electron microscopy-polarized light-Z-bands-heartbeat)

Developing cardiac muscle cells of 11- to 13-somite chick embryos are sensitive to cytochalasin-B. In cultured chick embryos, ranging in development from 11 to 13 somites, hearts stop beating in the presence of this agent. Both polarized light and electron microscopic examination show that cytochalasin-B disrupts existing myofibrils and inhibits the formation of new ones. Discrete Z-bands are not present in treated heart cells and thick, presumably myosin, filaments are found in disarray. These effects are reversible; after cytochalasin-B is removed from the medium, heartbeat recovers and myofibrils with discrete Z-bands reappear. Fibrillar sensitivity appears to be a function of age since fibrils in hearts of embryos having from 22 to 28 pairs of somites are more resistant.

[Kinematics and ultrastructure of plasmic factor regions in the egg of Wachtliella persicariae L. (Diptera) : I. The behaviour of ooplasmic partial systems in the normal egg]

The experimental results ofGEYER-DUSZYNSKA (1959), speaking in favour of three ooplasmic factors localized in the pole plasm, in the basophilic oosome material contained therein, as well as in the periplasm of the posterior egg pole ofWachtliella persicariae, suggested to investigate for further factor regions with other technical means. Since ooplasmic factor regions may be indicated by initial regions of morphogenetic development, kinematics were used for in vivo analysis of early embryonic development by means of time-lapse motion pictures. Electron microscopic investigations added to the micro-morphological aspects of plasmic systems within the egg for a better understanding of nature and effectivity of ooplasmic factors.Cleavage nuclei do not move exclusively by means of their spindle activity during anaphase movement. The nuclear envelope of cleavage energides consists of either two unit membranes with pores or of many tube-like as well as membranous elements. The appearence of a complex multi-layered nuclear envelope coincides with the moving phase of energides, an observation which is discussed in relation to the possibility of active nuclear movement. During late preblastoderm the entoplasm contains horse-shoe-shaped and multilobed vitellophagues with dense karyoplasm. With the blastoderm formed, the nuclei may become pycnotic, their membranes fragmenting at the same time. These fragments probably are piled up to form annulated membranes.The pole plasm does not show specific structures apart from the oosome material, contained therein. It is free of yolk material and nearly exclusively consists of ground plasm. The basophilic oosome material within the pole plasm is not surrounded by any membranes. It consists of numerous ribosome-like units and is restricted to the plasm of the future pole cells. The micro structure of the oosome material is preserved at least till the germ band has reached its maximal length.The cell membranes develop by invagination of the oolemma which penetrates into the egg interior. While pole cells and blastoderm cells become tied off, the ground plasm possibly participates in the growing-in process of the cell membranes by developing fibrous differentiations at the terminal extensions of oolemma folds.There is no clear cut limitation between periplasm and entoplasm. The periplasm which is without yolk material, appears rich in ground plasm and does not contain specific ultra structures. During the process of cleavage external ooplasmic regions of the egg are shifted in rhythmical pulsation parallel to its longer axis by a maximum of about 6 % of the entire egg length. Topographic statements of certain areas concerning any anlage therefore are bound to suffer from an adequate lack of exactness. Since comparable shifting processes within the egg plasm probably are common in insects other thanWachtliella, they should be considered as a certain source of error.At 60+-3 % of egg length as measured from its posterior pole, there exists a cleavage centre, an initial region of intravitelline cleavage and of repeated mitotic waves. Adjacent to the middle of the egg follows an initial region of germ band formation (differentiation centre). By their electron microscopic appearance, both developmental centres are not characterized by specific ultra structures. The factor region at the posterior pole exclusively represents an initial region of cell wall formation during superficial cleavage.Other than any experimental marking procedure the technique of time-lapse motion pictures permits to evaluate quantitatively and without artificial interfering the shifting of presumptive segment material during morphogenetic movements of the germ band. The embryonic material of the blastoderm at the egg equator is used for building up the first abdominal segment. The prothoracal and mesothoracal material at about 60% of the egg length stays in site when the germ band becomes extended lengthwise. Closely behind the differentiation centre there is a region of maximal extension as well as of shortening of the germ band. No proliferous growth of segments (segment formation zone) has been found.

Structural aspects of saltatory particle movement

A variety of cells possess particles which show movements statistically different from Brownian movements. They are characterized by discontinuous jumps of 2-30 micro at velocities of 0.5-5 micro/sec or more. A detailed analysis of these saltatory, jumplike movements makes it most likely that they are caused by transmission of force to the particles by a fiber system in the cell outside of the particle itself. Work with isolated droplets of cytoplasm from algae demonstrates a set of fibers involved in both cytoplasmic streaming and saltatory motion, suggesting that both phenomena are related to the same force-generating set of fibers. Analysis of a variety of systems in which streaming and/or saltatory movement occurs reveals two types of fiber systems spatially correlated with the movement, microtubules and 50 A microfilaments. The fibers in Nitella (alga) are of the microfilament type. In other systems (melanocyte processes, mitotic apparatus, nerve axons) microtubules occur. A suggestion is made, based on work on cilia, that a microtubule-microfilament complex may be present in those cases in which only microtubules appear to be present, with the microfilament closely associated with or buried in the microtubule wall. If so, then microfilaments, structurally similar to smooth muscle filaments, may be a force-generating element in a very wide variety of saltatory and streaming phenomena.

Fine structure of protein-storing plastids in bean root tips

The fine structure of leucoplasts in root tip cells of Phaseolus vulgaris L. has been studied in material fixed in glutaraldehyde followed by osmium tetroxide and poststained in uranyl acetate and lead citrate. Plastid development has been followed from the young stages in and near the meristematic region, through an ameboid stage, to the larger forms with more abundant storage products in the outermost cells. The plastids contain a dense stroma penetrated by tubules and cisternae arising from the inner membrane of the plastid envelope. Also located in the stroma are lamellae, ribosome-like particles, phytoferritin granules, and fine fibrils in less dense regions. In some elongate plastids microfilaments run lengthwise in the stroma near the surface. The same plastids store both starch and protein, but in a strikingly different manner. The starch is deposited in the stroma, while the protein always is accumulated within membrane-bounded sacs. These sacs arise as outgrowths from a complex of interconnected tubules which in turn appears to originate by coalescence and proliferation of tubules and cisternae arising from the inner plastid membrane. This "tubular complex" bears a strong resemblance to the prolamellar body of etiolated chloroplasts, but is smaller and ordinarily less regularly organized, and is apparently light-insensitive. Crystallization of the protein commonly occurs in the sacs and occasionally takes place within the tubules of the complex as well. The fine structure of the leucoplasts is discussed in relation to that of etiolated chloroplasts. Suggestions are made concerning the function of the tubular complex, role of the ameboid plastid forms, and manner of accumulation of the storage protein in the plastids.

[On the plasmatic filaments in assimilate conducting cells, their development and fine structure]

Taking into account the literature on the so-called sieve-tube slime ("mictoplasm", "slime strands") and regarding its fine structure more in detail the term plasmatic filament ("Plasmafilament") is proposed and will be used in this paper to characterize the individual exceedingly fine subunit of the plasmatic network (or slime) in sieve elements. Up to now plasmatic filaments have mostly been erroneously called "fibrils". The dimension of a fibrill has now been defined anew and differentiated from its subunit "plasmatic filament".In the first part of these investigations some aspects of the development of plasmatic filaments and their spreading over the total lumen of Dioscorea sieve elements will be reported.Previous to the first appearance of filaments the later sieve element abounds in plasmatic components, the groundplasm being extremely rich in ribosomes (Fig. 1). The difference between young sieve elements and the neighbouring parenchyma cells is nearly imperceptible apart from a slight variation in ribosome density. Plastids are very useful in distinguishing these two cell types from each other. The development of osmiophilic inclusions that characterize sieve-element plastids in Dioscorea has already been initiated in these very young cells.The earliest stages in the formation of plasmatic filaments that up to now have been revealed in Dioscorea show masses of filaments, some short and granular in appearance (Fig. 2: *), some already elngated and filamentous (Fig. 2: F). After expanding over the entire cell those filaments still look like having their origin directly in groundplasm (Fig. 5). Elements of the ER-system and many ribosomes cross the plasmatic filaments during all developmental stages of their network, which is at no time surrounded by any membrane.In sieve elements of Dioscorea, Primula, Cuscuta and Cucumis our investigations furthermore yielded some detail on the filament substructure. A cross-sectioned plasmatic filament is composed of an osmiophilic outer ring with a light centre (Fig. 11) corresponding in a longitudinal view to two deeply contrasted outer layers and an inner one without any contrast (Fig. 8). An individual filament has an overall diameter of 120-150 Å and an up to now indeterminable length that exceeds at least several microns.The real nature of these fine structures will be discussed in relation to similar structures and their meaning in plant and animal cells.