Guiding principles in cell locomotion and cell aggregation

Moving through a changing world: Single cell migration in 2D vs. 3D

Migration of single adherent cells is frequently observed in the developing and adult organism and has been the subject of many studies. Yet, while elegant work has elucidated molecular and mechanical cues affecting motion dynamics on a flat surface, it remains less clear how cells migrate in a 3D setting. In this review, we explore the changing parameters encountered by cells navigating through a 3D microenvironment compared to cells crawling on top of a 2D surface, and how these differences alter subcellular structures required for propulsion. We further discuss how such changes at the micro-scale impact motion pattern at the macro-scale.

Extra-embryonic tissue spreading directs early embryo morphogenesis in killifish

The spreading of mesenchymal-like cell layers is critical for embryo morphogenesis and tissue repair, yet we know little of this process in vivo. Here we take advantage of unique developmental features of the non-conventional annual killifish embryo to study the principles underlying tissue spreading in a simple cellular environment, devoid of patterning signals and major morphogenetic cell movements. Using in vivo experimentation and physical modelling we reveal that the extra-embryonic epithelial enveloping cell layer, thought mainly to provide protection to the embryo, directs cell migration and the spreading of embryonic tissue during early development. This function relies on the ability of embryonic cells to couple their autonomous random motility to non-autonomous signals arising from the expansion of the extra-embryonic epithelium, mediated by cell membrane adhesion and tension. Thus, we present a mechanism of extra-embryonic control of embryo morphogenesis that couples the mechanical properties of adjacent tissues in the early killifish embryo.

Cell migration: from tissue culture to embryos

Cell migration is a fundamental process that occurs during embryo development. Classic studies using in vitro culture systems have been instrumental in dissecting the principles of cell motility and highlighting how cells make use of topographical features of the substrate, cell-cell contacts, and chemical and physical environmental signals to direct their locomotion. Here, we review the guidance principles of in vitro cell locomotion and examine how they control directed cell migration in vivo during development. We focus on developmental examples in which individual guidance mechanisms have been clearly dissected, and for which the interactions among guidance cues have been explored. We also discuss how the migratory behaviours elicited by guidance mechanisms generate the stereotypical patterns of migration that shape tissues in the developing embryo.

The role of engineering approaches in analysing cancer invasion and metastasis

In the past decade, novel materials, probes and tools have enabled fundamental and applied cancer researchers to take a fresh look at the complex problem of tumour invasion and metastasis. These new tools, which include imaging modalities, controlled but complex in vitro culture conditions, and the ability to model and predict complex processes in vivo, represent an integration of traditional with novel engineering approaches; and their potential effect on quantitatively understanding tumour progression and invasion looks promising.

Topological singularities and symmetry breaking in development

The review presents a topological interpretation of some morphogenetic events through the use of well-known mathematical concepts and theorems. Spatial organization of the biological fields is analyzable in topological terms. Topological singularities inevitably emerging in biological morphogenesis are retained and transformed during pattern formation. It is the topological language that can provide strict and adequate description of various phenomena in developmental and evolutionary transformations. The relationship between local and global orders in metazoan development, i.e., between local morphogenetic processes and integral developmental patterns, is established. A topological inevitability of some developmental events through the use of classical topological concepts is discussed. This methodology reveals a topological imperative as a certain set of topological rules that constrains and directs embryogenesis. A breaking of spatial symmetry of preexisting pattern plays a critical role in biological morphogenesis in development and evolution.

Does buckling instability of the pseudopodium limit how well an amoeba can climb?

The maximum force that a crawling cell can exert on a substrate is a quantity of interest in cell biomechanics. One way of quantifying this force is to allow the cell to crawl against a measurable and adjustable restraining force until the cell is no longer able to move in a direction opposite to the applied force. Fukui et al. (2000) reported on an experiment where amoeboid cells were imaged while they crawled against an artificial gravity field created by a centrifuge. An unexpected observation was that the net applied force on the amoeba did not seem to be the primary factor that limited its ability to climb. Instead, it appeared that the amoeba stalled when it was no longer able to support a pseudopodium against the applied gravity field. The high g-load bend the pseudopodium thereby preventing its attachment to the target point directly ahead of the cell. In this paper we further refine this idea by identifying the bending of the pseudopodium with the onset of elastic instability of a beam under its own weight. It is shown that the principal features of the experiment may be understood through this model and an estimate for the limiting g-load in reasonable accord with the experimental measurements is recovered.

In ovo time-lapse analysis after dorsal neural tube ablation shows rerouting of chick hindbrain neural crest

Previous analyses of single neural crest cell trajectories have suggested important roles for interactions between neural crest cells and the environment, and amongst neural crest cells. To test the relative contribution of intrinsic versus extrinsic information in guiding cells to their appropriate sites, we ablated subpopulations of premigratory chick hindbrain neural crest and followed the remaining neural crest cells over time using a new in ovo imaging technique. Neural crest cell migratory behaviors are dramatically different in ablated compared with unoperated embryos. Deviations from normal migration appear either shortly after cells emerge from the neural tube or en route to the branchial arches, areas where cell-cell interactions typically occur between neural crest cells in normal embryos. Unlike the persistent, directed trajectories in normal embryos, neural crest cells frequently change direction and move somewhat chaotically after ablation. In addition, the migration of neural crest cells in collective chains, commonly observed in normal embryos, was severely disrupted. Hindbrain neural crest cells have the capacity to reroute their migratory pathways and thus compensate for missing neural crest cells after ablation of neighboring populations. Because the alterations in neural crest cell migration are most dramatic in regions that would normally foster cell-cell interactions, the trajectories reported here argue that cell-cell interactions have a key role in the shaping of the neural crest migration.

Neural crest cell dynamics revealed by time-lapse video microscopy of whole embryo chick explant cultures

DiI-labeled cranial neural crest cells were followed in whole embryo chick explant cultures using time-lapse confocal microscopy. Neural crest cells emerged along the dorsal midline of all rhombomeres. There was a small amount of mixing of neural crest cells between adjoining rhombomeres as cells emerged from the dorsal midline; this mixing persisted during their migration out of the neural tube. Neural crest cell-free zones lateral to rhombomere 3 (r3) and r5 resulted from neural crest cells migrating in either rostral or caudal directions to join other neural crest cells exiting adjacent to r2, r4, or r6. Neural crest cells migrated in a wide variety of individual cell behaviors, ranging from rapid unidirectional motion to stationary and even backward movement (toward the neural tube). Neural crest cells also migrated collectively, extending filipodia to form chain-like cell arrangements. In the midbrain and r1 region, many chains stretched from the dorsal midline to just beyond the lateral extent of the neural tube. In the r7 region, cells linked together and stretched laterally from the neural tube to other neural crest cells migrating into the third branchial arch. The unpredictable cell trajectories, the mixing of neural crest cells between adjoining rhombomeres, and the diversity in cell migration behavior within any particular region imply that no single mechanism guides migration. The regional differences in cell migration characteristics suggests that influential factors may vary spatially along the rostrocaudal axis in the head.

Influence of culture substrata on the differentiation of advanced passage glial cells in cultures from aged mouse cerebral hemispheres

We have previously reported that glial cells derived from aged mouse cerebral hemispheres (MACH) in primary cultures and after several passages consist of protoplasmic astrocytes (Type 1), differentiated stellate astrocytes (Type 2), a few oligodendrocytes, and also glial precursors. In this study, we examined the influence of culture substrata: plastic, poly-L-lysine, laminin or collagen on the differentiation of MACH glial cells of advanced passages (P18-19) using glutamine synthetase (GS) and cyclic nucleotide phosphohydrolase (CNP) activity as biochemical markers for astrocytes and oligodendrocytes, respectively. Cultures were also examined morphologically using light microscopy. In general, GS activity was increased in cultures grown on the three chemical substrata versus plastic alone with the most striking effect being the 2-fold increase observed in those cells grown in laminin. No differences were noted in CNP activity. Morphologically, proliferation of protoplasmic (Type 1) astrocytes was enhanced by culture day 2 on polylysine substratum and stellate differentiated (Type 2) astrocytes were noted on collagen. The striking feature in cultures grown on laminin was the presence of astrocytes with markedly long processes. Thus, morphological astrocyte differentiation appears to correspond to the increased GS activity. We propose that the extracellular matrix components such as collagen and laminin may play an important role in promoting glial precursors to differentiate into astrocytes.

Role of the dermal tracts in the pigment pattern of the frog

The pigment pattern of the ventral skin of the frog Rana esculenta is compared in skin fragments grown for 24 hr with or without antiserum directed to fibronectin (anti-FN). Melanocyte-stimulating hormone (MSH) was added to the medium during the last hour in culture in order to enhance visibility of melanophores in the ventral region of the frog skin. Comparison of these two treatments provides information regarding the precise localization of melanophores in the dermal tracts and their involvement in the pigment pattern of the ventral frog skin. In this regard, the whitish pigment pattern of skin fragments is compared to the tiny black spots found on anti-FN treated skin fragments and the abundant blotchy spots found on skin cultured alone. The distribution of melanophores in the dermal tracts observed in vertical semithin sections is found to be related to the three different levels of the dermal tracts. This report demonstrates the importance of fibronectin as a substrate for the melanophore migration, the importance of the tract level for the melanophore localization both involved in the pigment pattern of the ventral skin.

Cell behavior during early development in the South American annual fishes of the genus Cynolebias

Living embryos of three species of South American annual fishes, Cynolebias constanciae, C. nigripinnis, and C. whitei, were observed from fertilization through the 10-somite stage. A description of normal stages of development applicable to all three species of Cynolebias is presented. Cleavage (stages 1-10) is meroblastic and produces a typical teleost blastoderm. Following cleavage (stages 11-13) blastomeres segregate into two populations, viz., 1) a population of deep blastomeres that will disperse as single motile cells, and 2) a hemispherical shell of outer blastomeres that flattens to form an enveloping cell layer (EVL). When epiboly of the EVL and the yolk syncytial layer (YSL) commences (stage 14), deep blastomeres clump together as a consolidation mass and then migrate outward as single cells on the YSL. When epiboly is concluded (stage 19), deep blastomeres have completely dispersed. If diapause does not intervene, the dispersed phase lasts only a few days. Subsequently, the dispersed cells come together to form a definitive aggregate (stage 27). Embryogenesis within the reaggregated mass of previously dispersed cells produces a typical teleost embryo. Early development in Cynolebias resembles that of other South American annual fishes, such as Austrofundulus, in that a phase of deep blastomere dispersion and reaggregation spatially and temporally separates epiboly from embryogenesis. Several features of development markedly differ from Austrofundulus. There are far fewer (250 vs. 2,500) deep blastomeres. Deep cells of Cynolebias are flattened rhomboids with filipodial extensions in contrast to the amoeboid cells of Austrofundulus. Blastomeres of dispersion and reaggregation stages in Cynolebias send out numerous cell surface extensions onto the YSL and in contact with one another, and often line up in rows as do some African annual fishes, e.g., Nothobranchius. During Dispersion II (stage 21), Reaggregation I (stage 22), and Reaggregation II (stage 23), deep cells move in an oriented pattern with respective mean velocities of 3.48 +/- 0.91, 1.28 +/- 0.46, and 1.31 +/- 0.31 microns/minute. Cells move toward a granular mass of unknown composition, located at the YSL-yolk interface in the lower hemisphere of the egg. This mass appears to coincide with the site of cell reaggregation.

A head signal influences apical migration of interstitial cells in Hydra vulgaris

Although interstitial cells of hydra can migrate either apically or basally along the body column, there is a distinct bias toward apical cell accumulation. This apical bias could be produced by a local vectorial property of the tissue or it may be controlled by a more global property, such as a signal from the apical head region. The migration behavior of BrdU-labeled interstitial cells was examined in several types of grafts to distinguish between these two general types of migration control. Grafting BrdU-labeled midgastric region tissue into a host in either the normal or the reverse orientation had no effect on the apical bias, indicating that a local vectorial cue was probably not guiding cells apically. In grafts with heads or with feet at both ends of the body column, there was no directional bias in migration if the labeled tissue was equidistant from both ends. In the two-headed grafts, if the labeled tissue was closer to one end, there was a bias in the direction of the closer head. The results suggest that a graded signal emanating from the head creates the apical bias and may attract cells via chemotaxis. The apical bias is enhanced in decapitated animals regenerating a head, indicating that the attracting signal is present and is possibly stronger in regenerating heads. The signal for cell migration may be involved in a patterning process underlying head regeneration.

Is chemotaxis a factor in the migration of precardiac mesoderm in the chick?

The chick heart is formed from bilateral patches of presumptive cardiac mesoderm cells which migrate over the endoderm and fuse in the midline. We have tested the possibility that this migration is controlled, at least in part, by a chemotactic substance exuded by the anterior end of the endoderm. We have used chick/quail combinations to follow naturally marked cells during the course of their migration. Chimaeric embryos were formed by fusing together parts of chick and quail embryos of stage 5-6. Each embryo possessed two pairs of precardiac regions, the quail pair lying immediately anterior to that of the chick. These chimaeras were then explanted in embryo culture. In the event of chemotaxis, cells from the posterior end of the quail precardiac mesoderm might be expected to invade the chick area. Samples of explants and chimaeras were examined at intervals from 2 to 24 h, but in no case were cells found to have changed their direction of migration as a result of the proximity of anterior endoderm. It is concluded that this work does not provide evidence for a chemotactic attraction by the anterior end of the endoderm.

Physical influences on neural crest cell migration in avian embryos: contact guidance and spatial restriction

Several ideas on how neural crest (NC) cell migration in bird embryos might be dependent on the physical qualities of the internal embryonic environment were studied. Contact guidance has been suggested to direct NC cells ventrally in the trunk, but this has been subject to doubt (see Newgreen and Erickson, 1986, Int. Rev. Cytol. 103, 118-119). On reexamination, in situ extracellular matrix (ECM) and cell processes on the medial face of the somites were found appropriately oriented for this function. In addition, tissue culture models of oriented ECM could induce orientation of NC cells which mimicked that observed in the embryo. It is concluded that in this situation, oriented structures contribute to directed migration of NC cells in vivo, but the mechanism of contact guidance (i.e., steric or adhesive guidance) could not be ascertained. Contact guidance, in the form of steric guidance, has also been suggested as limiting ventrad NC cell movement at the midbrain level due to an insurmountable ridge on the side of the midbrain. The presence of this ridge was confirmed but it is unlikely to be responsible for prevention of ventrad migration, because, although it subsides very rapidly, the cells still refuse to move ventrad, and because models of this ridge in vitro proved to be no obstacle to NC cells. NC cell migration is also described as being limited by gross space between other organs or tissues. In vitro, NC cells could penetrate Nucleopore filters with pore diameters of 0.86 micron or greater. Observation of cell-free spaces in embryos showed that these were almost all much larger than the minimum pore size established experimentally. It is therefore concluded that in general the dimensions of gross tissue spaces probably do not set important limits for NC cell migration, but that the dimensions of transiently distensible microspaces between ECM fibrils may be a critical physical parameter.

Factors influencing neuronal growth in primary cultures derived from 3-day-old chick embryos

We compared neuronal growth patterns in primary cultures prepared by dissociating 3-day-old chick embryos, either whole embryo (E3WE) or head only (E3H) and plating the dispersed cells onto Petri dishes coated with either poly-L-lysine, collagen or laminin. The culture medium was Dulbecco's Modified Eagle's Medium (DMEM), supplemented with either 5 or 10% fetal bovine calf serum (FCS). As we have previously described, in E3WE cultures on poly-L-lysine the neuronal primary growth patterns were aggregation with neuritic fasciculation, presence of growth cones with microspikes and very few flat cells. In contrast with cultures grown on poly-L-lysine, in cultures grown on collagen or laminin the distinct growth pattern was extensive networks of isolated and differentiated neurons lying on acquired monolayers of flat cells. When 5% FCS was used, as compared to 10% FCS, neuronal aggregates were fewer and smaller on poly-L-lysine; on collagen or laminin a tendency to aggregate was observed. Several differences were observed in the E3H cultures when compared to E3WE: (a) aggregates were less numerous with the prevailing pattern being a web-like, self-contained aggregate; (b) aggregates connected with other aggregates or flat cells were rare and the aggregate adhesivity was minimized; (c) neurons on collagen or laminin formed networks with the exception of a few, small aggregates displaying no fasciculation; (d) flat cells did not form a monolayer but islets which hosted the neuronal meshy networks. We attribute these differences in the growth patterns between the various types of cultures to be the combined result of a variety of environmental signals, derived from the provided substrata, the serum and the nonneuronal cell factors and cell surface, all primarily regulating neuronal adhesivity.

Gross and ultrastructural development of the saccule of the toadfish Opsanus tau

The development of the saccule of the inner ear in the toadfish was studied using light and scanning electron microscopy. Development was studied from the early embryo (2-3 days postfertilization), when the otocyst first forms, to the early-aged juvenile when the development of the inner ear approximates that of the adult (4 weeks postfertilization). The ultrastructural features examined included the morphological sequence of ciliary bundle growth, the development of orientation patterns of the ciliary bundles, and the relation of the ultrastructural development to overall gross development. Gross development may be divided into four distinct morphological stages. Stage I encompasses the time from initial formation of the otocyst until the start of stage II, which is the stage when the pars inferior begins migrating ventrally. In stage III the pars inferior continues to elongate ventrally. Stage IV starts when the pars inferior elongates in a rostral and caudal direction. The ear attains its adult shape in stage IV. The differentiation of the sensory cells begins during stage I. During the early part of stage I, a small cilium is found on the apical surface of each cell throughout the otocyst. In the middle and late periods of stage I, a few microvillous buds add to the surface of the cells that already have a kinocilium. These early ciliary bundles are clustered on the rostral-ventral and caudal walls of the otocyst. There is no clear patterning to the orientation of these ciliary bundles. In stage II the ventral stretching of the labyrinth wall causes a spreading of the clustered bundles along the ventral and medial walls of the pars inferior. The orientation of the ciliary bundles has no distinct pattern. In stage III the orientations of the ciliary bundles appear adultlike, although there are so few ciliary bundles that it is difficult to make a definite determination. During stage IV, hair cells with an adultlike horizontal and vertical orientation pattern are found on the rostral and caudal sections of the saccular macula, respectively. The transition region lying between these areas has ciliary bundles with various orientations.

Effect of citric acid conditioning on fibroblast cell density in periodontal wounds

The present experiment was undertaken to study the repopulation of curetted root surfaces by fibroblast-like cells in experimental periodontal wounds. 6 beagle dogs were used. After reflecting mucoperiosteal flaps, fenestration wounds were made through the buccal cortical plate exposing roots of maxillary canines in 6 beagle dogs. Exposed root surfaces were curetted to remove cementum and periodontal ligament. On 1 side, the exposed root surface was conditioned with citric acid (pH 1) for 3 min while the contralateral root was treated with distilled water. Millipore filters were placed over the wounds to prevent flap connective tissues from contacting the exposed root surface. Histometric analysis was made 10 days after wounding. Fibroblast-like cells were seen migrating into the fenestration space and were primarily aligned along the root surface. Cell densities measured at the borders and central part of the wound showed significantly lower values (p less than 0.01 and p less than 0.05) in acid-conditioned wounds compared to controls. No significant difference in cell density was noted between the borders and central part of the wound in the acid-conditioned group. In the controls, the cell density at the borders was significantly greater than at the center, suggesting active cell migration from the borders. The present findings suggest that citric acid conditioning of root dentin may result in a low cell density during the early stages of healing in experimental periodontal wounds.

Optic fibers follow aberrant pathways from rotated eyes in Xenopus laevis

The rotated eye paradigm has been a major experimental test of the neuronal specificity model for the development of ordered retinotectal connections in amphibians. In most studies, however, no optic fiber pathways were traced from rotated eyes and correlated with visuotectal projections. As an initial approach to this question, optic fibers from eyes rotated at different embryonic stages were traced with 3H-proline autoradiography. Three experimental series were prepared: in situ eye rotations, isochronic transplants of eyes rotated between embryos at the same stage, and heterochronic transplants of eyes rotated between embryos at different stages. Single or multiple optic fiber pathways developing from rotated eyes are identified by their sites of entry and trajectory in the brain. These include a normal chiasmatic (CH) pathway, and three aberrant pathways, identified as trigeminal (TR), diencephalic (DI), and oculomotor (OC). The latter three enter the brain ipsilaterally, some crossing contralaterally via commissural pathways. Depending on stage and type of operation, TR pathways develop in 50-100% of the animals, while CH pathways are more common after rotation at stage 21/22. The surgical procedure affects the initial trajectory of fibers from the retina, perturbs guidance cues in the surrounding orbit, and determines the patterns of optic pathways that develop. In most cases, optic fibers follow motor (oculomotor) or sensory (trigeminal) nerves, usually the first fibers encountered near the orbit by axonal pioneers exiting the retina. Evidently, optic fibers exhibit no pathway selectivity; any axon serves as a guidance cue. Tecta are innervated in about 50% of the cases, usually by fibers following abnormal trajectories from CH and OC pathways. The results suggest that the development of ordered visuotectal projections from rotated eyes is a complex process that may be independent of the trajectory of fiber arrival. Unless pathways and visuotectal maps are directly compared in each animal, however, the question remains open because we still do not know which anomalous pathways, if any, correlate with ordered projections.

Orientation of gingival fibroblasts in simulated periodontal spaces in vitro. SEM observations

The present study examined the orientation of gingival fibroblasts in simulated periodontal spaces in vitro. Extracted human teeth were root planed followed by root resection and root canal instrumentation. The middle and cervical thirds of each root were cut transversely to create 600-micron thick sections. Cortical bovine bone was cut, sectioned and contoured to create bone rings 600 micron thick with an internal diameter large enough to accommodate a root slice leaving a circumferential space varying from approximately 0.1 to 1.0 mm. Root slices and bone rings were incubated in a solution of collagenase and hyaluronidase to remove all remaining soft tissue and partially demineralized in EDTA (18%) for 30 minutes. Human gingival fibroblasts (HGF) were plated to confluency in tissue culture dishes. The dentin slices were then gently placed over the HGF monolayer along with bone rings around them to create simulated periodontal spaces. Control root slices were placed without bone rings around them. Cultures were maintained under standard tissue culture conditions. Representative specimens were obtained after 2, 3 and 4 weeks of culture and processed for scanning electron microscopy (SEM). At 2 weeks, the HGF had formed sheets of cells attached to the periphery of the root slices at one end and to the inner surface of bone rings at the other end. The orientation of cell sheets varied from being perpendicular to the periphery of the slice to oblique. At 3 and 4 weeks, the density and size of cell sheets increased and the orientation was maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

Matrix-driven translocation of cells and nonliving particles

Cells of metazoan organisms produce and react to complex macromolecular microenvironments known as extracellular matrices. Assembly in vitro of native, compositionally nonuniform collagen-fibronectin matrices caused translocation of certain types of cells or polystyrene-latex beads from regions lacking fibronectin into regions containing it. The translocation process was not due to diffusion, convection, or electrostatic distribution effects, but may depend on nonequilibrium phenomena at the interface of contiguous collagen matrices formed in the presence and absence of fibronectin or particles. Extracellular matrix formation alone was sufficient to drive translocation by a biophysical process that may play a role in cellular migration during embryogenesis, as well as in other types of tissue reorganization such as inflammation, wound healing, and tumor invasion.

An indirect immunofluorescence study of the distribution of fibronectin during the formation of the cushion tissue mesenchyme in the embryonic heart

Indirect immunofluorescence studies have localized fibronectin (FN) within the trunco-conal ridges of the chick embryo heart during the formation of the cushion tissue mesenchyme. Prior to cell migration into the endocardial pads, fluorescence for FN is demonstrated almost entirely in association with the basal surfaces of endocardium and myocardium. Scattered spots and thin dotted-strands of fluorescent material can be demonstrated in the cardiac jelly. Cushion tissue (CT) cells migrating into the cardiac jelly have patches of fluorescent material associated with their surfaces. Filopodial processes always show intense fluorescence. The close association between the fluorescence and the surface of the CT cells suggests that FN may be implicated in the interaction of these cells with the matrical components of the cardiac jelly and, therefore, in the process of cell migration into the endocardial pads. The intensity and amount of FN staining decreased concomitantly with the progressive accumulation of cells in the cushion areas. After the completion of CT cell migration only reduced amounts of faint fluorescence remained in the endocardial pad areas. The possible significance of the changes observed in the distribution of FN during the formation of the cushion tissue mesenchyme is discussed.

Neurite growth cone-substratum adherence increases in vitro

During experiments characterizing the turning response of dorsal root ganglion neurites toward NGF, it was observed that growth cone-substratum adherance increased with time in culture. The experiments reported here indicate that the observed increase in growth cone-substratum adherance is significant and can be detected with both collagen and poly-L-lysine substrates. The increased adherance is apparently due to a substance(s) produced and released by the ganglia which binds to the substrate, increasing adherance. Flow chamber studies indicate that the substrate-bound substance(s) may be necessary for neurite growth onto artificial tissue culture substrata.

Shape, motility and locomotor responses of neutrophil granulocytes

Relationships among neutrophil shape, motility and locomotion were analysed. Spherical cells were always non-motile. Two types of motile cells could be distinguished 1) motile polarized neutrophils that were capable of locomotion under conditions of limited adhesion and 2) motile non-polar cells that did not locomote under the same conditions. Polarity appears to be a secondary phenomenon related to locomotion or motility. It can also be reversed, indicating that it is not a characteristic based on fixed cellular structures. The fact that a neutrophil is polarized on the substratum does not provide conclusive evidence of cell motility or locomotion. Neutrophils were found to exhibit orthokinetic but not klinokinetic responses. The speed was mainly or exclusively determined by changes in the proportion of migrating cells. Changes in the speed of the migrating subset play a less important role. Locomotion depends on the activity of the motile apparatus and on adhesion to the substratum. By measuring motility in suspension and adhesion to the substratum the proportion of migrating cells can be predicted. The results show that adhesion and changes in motility play a variable role in chemokinesis. Chemokinetic factors may change only adhesion (example: HSA) or adhesion and the activity of the motile apparatus. The effects of chemokinetic factors on motility and adhesion may be synergistic or antagonistic. The separate assessment of adhesion to the substratum and of motility of floating cells may also be useful for the analysis of biological activities of chemokinetic factors, mechanisms regulating chemokinesis and defects of locomotion.

Axonal interactions with connective tissue and glial substrata during optic nerve regeneration in Xenopus larvae and adults

Axonal elongation through connective tissue and glial environments was compared following resection of the optic nerves in Xenopus tadpoles and frogs. During initial stages of fiber outgrowth, axons encountered connective-tissue matrices of varying degrees of complexity in the ablation gaps. Many of the neuritic sprouts were randomly directed after leaving the retinal stump, and a neuroma-like swelling ultimately formed at the cut edge. Although a large number of axons managed to traverse the lesion and associate with the cranial stump, many other fibers were less appropriately directed, especially in the frog where a greater infiltration of dense collagen occurred between the separated segments of the optic nerve. Axons often deviated from their cranially oriented pattern of outgrowth after entering the lesion and invaded surrounding extraocular muscles; others advanced along neighboring blood vessels and cranial nerve branches. In more extreme circumstances, fibers were completely misdirected at the cut end of the retinal stump and ultimately extended adjacent to the retinal segment back toward the eye. A more organized pattern of axonal elongation was observed in the presence of the glial substratum of the central stump, and growth cones appeared to associate preferentially with astrocyte endfeet in both tadpoles and frogs. These observations show that axons in the regenerating optic nerve of the amphibian can interact with a variety of cells and tissues and that the general direction of their outgrowth, at least in more peripheral regions of the visual pathway, appears to be dependent upon the orientation and, possibly, molecular properties of the terrain which they contact. In general, the basic environmental factors which either foster or impede axonal elongation in this regenerating system appear analogous to those influencing fiber outgrowth during regeneration in the peripheral nervous system of various species.

Effect of root surface alterations on periodontal healing. I. Surgical denudation

The present study was undertaken to evaluate the effect of root surface denudation on periodontal healing. Twelve teeth, distributed in four squirrel monkeys, were extracted and reimplanted after surgically denuding the coronal root surface of connective tissue fibers and cementum by root planing. The reimplantation schedule provided three teeth for histologic analysis at 1, 3, 7 and 21 days after reimplantation. One day after reimplantation a zone of fibrin enmeshing erythrocytes and inflammatory cells was interposed between th root surface and the remaining periodontal fibers attached to the alveolar bone. Epithelium migrated rapidly along the denuded root, had reached the alveolar crest at 3 days, and was within the ligament space at 7 days. At 21 days, the epithelium was at the apical limit of root instrumentation, which corresponded to the level of attached connective tissue fibers on the root surface. No evidence of new connective tissue attachment was observed on the denuded root surface. It was concluded that the absence of fibers on the root surface resulted in apical migration of the epithelium, and precluded formation of new connective tissue attachment.

Primordial germ cells of Xenopus embryos: the role of fibronectin in their adhesion during migration

Primordial germ cells (PGCs) of Xenopus laevis are highly migratory. The last section of their migratory pathway is through the dorsal mesentery of the tadpole gut. This in vivo pathway is rich in fibronectin, a glycoprotein that promotes cell adhesion and migration in vitro. Isolated PGCs are associated with cells from the mesentery and with fibronectin. Treatment with trypsin removes both the mesentery cells and the fibronectin. The PGCs do not appear to resynthesize detectable fibronectin in vitro. In contrast, cultured adult mesentery epithelial cells synthesize large amounts of fibronectin and lay it down in subcellular fibrils that align with intracellular microfilament bundles. PGCs plated on cultured mesentery cell layers adhere to them, elongate and align with the microfilament bundles of the mesentery cells. PGCs adherent to mesentery cell layers are closely associated with fibronectin; moreover, F(ab)2 fragments of anti-Xenopus fibronectin IgG inhibit the adhesion and spreading of PGCs on the mesentery. These results indicate that PGCs can adhere to mesentery cells via fibronectin produced by the latter cells and suggest that fibronectin may be involved in the migration of PGCs in vivo.

Myoblasts are aligned with collagen fibrils in regenerating frog tadpole tails

Myoblasts in the regenerating frog tadpole tail differentiate from mesenchymal cells that lie next to the basement membrane of the epidermis of the tail. As these cells elongate and form myotubes, they orientate uniformly in the longitudinal axis of the tail. The collagen fibrils of the basement membrane adjacent to the myogenic cells are also orientated in the tail axis just prior to and during the time when the myogenic cells are elongating. This has been demonstrated by transmission electron microscopy of thin sections, by differential interference contrast microscopy of isolated basement membranes, and by scanning electron microscopy of the inner surface of the basement membrane. Since elongating myoblasts are in contact with the longitudinally orientated fibrils, the latter could provide directional cues to the elongating myoblasts. This proposition is supported by the finding that isolated basement membranes readily orientate cells that are cultured upon their inner surfaces.