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Biomedical subjects

A K Harris

Publications and source records attributed to A K Harris.

At least 19 recordsLinked to original sources
PubMed1990

Effects of electric fields on fibroblast contractility and cytoskeleton.

We used silicone rubber substrata and fluorescent staining of cytoskeletal components to study the mechanisms by which electrical voltage gradients cause reorientation of embryonic chick fibroblasts in tissue culture. No evidence was found for a direct stimulation of cell contractility, either parallel or perpendicular to the voltage gradient. Instead, there was a gradual weakening in cell contractility in the axis parallel to this gradient, accompanied by progressive retraction of lamellae oriented along this axis, apparently due to selective weakening of cell-substratum adhesions. The cells then elongated perpendicular to the electric field, and strengthened their contractility in that axis. Fluorescence microscopy showed that cytoplasmic actin stress fibers and microtubules oriented perpendicular to the imposed voltage gradient. Many more cases were observed in which cell morphology had reoriented, but the actin fibers had not, as compared to the converse (cytoskeleton oriented, but no morphology). This disparity further supports the interpretation that the redirection of cell contractility is a consequence of morphological reorientation, rather than its cause. We also studied the effects of reversing the polarity of the electric fields at constant intervals (of as long as 1 minute). Fibroblasts failed to orient in response to such alternating fields, even after long exposure, but these same cells did reorient in response to pulsed currents in a consistent direction separated by "rest periods" (with no current). This combination of results is more consistent with an electrophoretic mechanism than with one depending on voltage-induced changes in membrane permeabilities.

Actins↗
PubMed1989

Simulation testing of mechanisms for inducing the formation of the contractile ring in cytokinesis.

There is persuasive evidence that the role of the mitotic apparatus (MA) in cytokinesis is to control the location of the cleavage furrow. The geometric aspects of this interaction between the MA and the cortex are complex and, thus, computer simulation can be a useful means for testing hypotheses about the induction process. White and Borisy (1983. J. Theor. Biol. 101:289-316) used computer simulations to show that long-range signals from the asters, varying inversely as various powers of distance, produce summed effects that are minima at the equator of spherical cells. Their results have seemed to support the "polar relaxation" class of hypotheses, in which the effect of the asters is to weaken cortical contractility so that contraction becomes maximized at the equator because it is least inhibited there. However, the experimental studies of Rappaport and Rappaport (1988. J. Exp. Zool. 247:92-98) indicate that the asters actually strengthen cortical contractility. In this paper, we use computer simulation to determine how signals from the MA will need to vary in effect as functions of distance to cause cortical contractility to become maximized where the furrows are to be induced. Although we confirm that inverse power inhibitory signals could induce equatorial furrows in spherical cells, we also find that this ability is destroyed by flattening, constricting, or distorting cells into cylinders, geometries for which Rappaport's experiments show furrows form (1986. Int. Rev. Cytol. 105:245-281). We then show that stimulatory signals of the right kind would induce furrows at the locations observed, in spherical cells as well as cells distorted by experimental manipulation. These signals must be constant out to a threshold distance but decrease abruptly beyond that distance. We also show that this ability depends on having the "drop-off" threshold occur at just the right distance relative to the dimensions of the cell and separation of the asters.

Cell Cycle↗
PubMed1988

Locomotion of sponges and its physical mechanism.

Active locomotion by individual marine and freshwater sponges across glass, plastic and rubber substrata has been studied in relation to the behavior of the sponges' component cells. Sequential tracing of sponge outlines on aquarium walls shows that sponges can crawl up to 160 microns/hr (4 mm/day). Time-lapse cinemicrography and scanning electron microscopy reveal that moving sponges possess distinctive leading edges composed of motile cells. Sponge locomotion was found to be mechanically similar to the spreading of cell sheets in tissue culture both with respect to exertion of traction (which causes the wrinkling of rubber substrata) and with respect to the patterns of adhesive contacts formed with the substratum (as observed by interference reflection microscopy). Other similarities include the orientation of sponge locomotion along grooves and the preferential extension onto more adhesive substrata. Neither the patterns of wrinkling produced in rubber substrata nor the distributions of adhesive contacts seen by interference reflection microscopy show evidence of periodic, propagating waves of surface contractions, such as would be expected if the sponges' mechanism of locomotion were by peristalsis or locomotory waves. Our observations suggest that the displacement of sponges is achieved by the cumulative crawling locomotion of the cells that compose the sponge's lower surface. This mode of organismal locomotion suggests new explanations for the plasticity of sponge morphology, seems not to have been reported from other metazoans, and has significant ecological implications.

Animals↗
PubMed1988

Changes in fibroblast contractility, morphology, and adhesion in response to a phorbol ester tumor promoter.

We have studied the effects of the phorbol ester tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the contractility, locomotion, morphology, and adhesion of two mammalian fibroblastic cell lines. Using the silicone rubber substratum technique, we have found that the first observable response to the tumor promoter is a rapid weakening of cell contractility (8-15 min). This is followed by gradual morphological changes, characterized by a hyperextension of the cells' leading lamellae, which stretch out to an unlimited degree, and occasionally even detach from the cell bodies. Treated cells also become able to crawl onto hydrophobic substrata which are insufficiently adhesive to support the spreading of untreated fibroblasts. We suggest that both the hyperextension and the ability to spread on nonadhesive surfaces can be explained as consequences of the reduced contractility, and that this reduced contractility may also help to explain the increased invasiveness and loss of anchorage dependence by transformed cells.

Actins↗
PubMed1987

Cell motility and the problem of anatomical homeostasis.

The locomotion of tissue cells need not be invasive or disrupt normal tissue geometry, as occurs in cancer. The normal relationship between anatomical structure and cell locomotion is exactly the reverse, with motility serving to create and maintain the structures of the body. This relationship is most extreme in sponges, where time-lapse films show that the cells move about continually in patterns that restructure these animals' simple anatomy. The cells choose their position according to their differentiated cell type, which is the opposite of what is usually assumed to occur in development and has important implications for the functional significance of histotypic cell sorting. A particular type of cellular force that seems to be important in morphogenesis is the traction that all motile tissue cells exert. This traction can be studied by culturing cells on very thin sheets of silicone rubber, so that the locations and variations in the cellular forces are made visible by the wrinkles they produce in the rubber substratum. One finding has been that the traction forces exerted by untransformed fibroblasts are very much stronger than is needed for their own locomotion, but are well adapted for the function of rearranging and aligning collagen fibres to form structures like ligaments, tendons and muscles. These forces are found to be greatly weakened by neoplastic transformation, however, suggesting that malignant invasiveness results from some sort of deflection of cell traction forces from their proper morphogenetic functions, so as to produce uncontrolled invasion. To explain how motile cells create and maintain structures, as well as how their locomotion sometimes becomes perverted into the form of cancerous invasiveness, what seems to be needed is an extension of the concept of homeostasis to apply to the control of geometric relations between cells. This task may not be easy; one obstacle is the widespread belief that asymptotic stability implies the minimization of free energy. Instead, I suggest that stability results from balance of opposing forces within tissues, and that genes control which anatomical shapes will exist by determining the rules by which the relative strengths of these forces vary as functions of shape: to control shape, one must control the way forces vary with shape.

Animals↗
PubMed1985

Morphogenetic rearrangement of injected collagen in developing chicken limb buds.

A fundamental question concerning the development of the extracellular matrix is what factors control the arrangement of collagen fibrils within a tissue and at the same time allow for the great diversity of geometric forms exhibited by collagen. In this report, we test the possibility that physical forces within the embryo serve to organize collagen fibers into regular patterns. In particular, we test the prediction that patterns of stress having this morphogenetic function are generated by cell traction, the contractile force exerted by cells to propel themselves. To study the effects of these mechanical forces on the extracellular matrix, type I collagen was fluorescently labeled and injected into developing chicken wing buds. When the injected limbs were allowed to develop and then examined histologically, the exogenous collagen was found incorporated within normal connective tissues of the wing. The labeled collagen became arranged according to its site of injection, forming parts of tendons, perichondria, cartilages, perineuria, and blood vessels. Since the injected collagen formed a gel within minutes of its injection, the subsequent incorporation of this performed collagen within organized structures cannot be explained in terms of molecular self-assembly or other mechanisms occurring during collagen deposition. These results demonstrate that, within developing tissues, patterns of forces exist that are capable of physically rearranging collagen and determining its long-range order.

Animals↗
PubMed1984

Tissue culture cells on deformable substrata: biomechanical implications.

A method has been developed for the study of the forces which individual cells exert during their locomotion. Polydimethyl-siloxane (silicone fluid) was crosslinked on its surface by brief flaming to form a thin layer of silicone rubber. Tissue culture cells of many types were then plated out onto these rubber substrata and the propulsive forces these cells exert as they adhere and spread became visible as wrinkles and other distortions in the rubber. From time-lapse films of these distortions, it appears that the component cells of the body move by exerting shearing forces through their plasma membranes. How these forces are exerted and how this technique for observing them could be made more quantitative are discussed.

Animals↗
PubMed1984

Generation of spatially periodic patterns by a mechanical instability: a mechanical alternative to the Turing model.

We have studied the generation of spatial patterns created by mechanical (rather than chemical) instabilities. When dissociated fibroblasts are suspended in a gel of reprecipitated collagen, and the contraction of the gel as a whole is physically restrained by attachment of its margin to a glass fibre meshwork, then the effect of the fibroblasts' traction is to break up the cell-matrix mixture into a series of clumps or aggregations of cells and compressed matrix. These aggregations are interconnected by linear tracts of collagen fibres aligned under the tensile stress exerted by fibroblast traction. The patterns generated by this mechanical instability vary depending upon cell population density and other factors. Over a certain range of cell concentrations, this mechanical instability yields geometric patterns which resemble but are usually much less regular than the patterns which develop normally in the dermis of developing bird skin. We propose that an equivalent mechanical instability, occurring during the embryonic development of this skin, could be the cause not only of the clumping of dermal fibroblasts to form the feather papillae, but also of the alignment of collagen fibres into the characteristic polygonal network of fibre bundles - which interconnect these papillae and which presage the subsequent pattern of the dermal muscles serving to control feather movements. More generally, we suggest that this type of mechanical instability can serve the morphogenetic functions for which Turing's chemical instability and other reaction-diffusion systems have been proposed. Mechanical instabilities can create physical structures directly, in one step, in contrast to the two or more steps which would be required if positional information first had to be specified by chemical gradients and then only secondarily implemented in physical form. In addition, physical forces can act more quickly and at much longer range than can diffusing chemicals and can generate a greater range of possible geometries than is possible using gradients of scalar properties. In cases (such as chondrogenesis) where cell differentiation is influenced by the local population density of cells and extracellular matrix, the physical patterns of force and distortion within this extracellular matrix should even be able to accomplish the spatial control of differentiation, usually attributed to diffusible 'morphogens'.

Animals↗
PubMed1983

Increased contractile strength and tightened adhesions to the substratum result from reverse transformation of CHO cells by dibutyryl cyclic adenosine monophosphate.

We have examined the changes in cellular contractility and adhesive morphology that accompany the reverse transformation of CHO cells caused by dibutyryl cyclic adenosine monophosphate. The ability of these cells to distort elastic silicone rubber substrata was found to increase markedly over the 5 h period following addition of cyclic AMP and testosterone to the medium, and to relax again following hormone removal. Parallel changes were observed in the interference reflexion image of the adhesion of these cells to glass substrata; broad grey-appearing 'close' contacts characteristic of the transformed state were gradually replaced by numerous small black-appearing 'focal contacts'. These changes in adhesive morphology required considerably longer (20-25 h) to be completed or to revert, however.

Animals↗
PubMed1983

Mechanical aspects of mesenchymal morphogenesis.

Many embryonic cells generate substantial contractile forces as they spread and crawl. These forces mechanically deform each cell's local environment, and the resulting distortions can alter subsequent cell movements by convection and the mechanisms of contact guidance and haptotaxis. Here we develop a model for the cumulative effects of these cell-generated forces and show how they can lead to the formation of regular large-scale patterns in cell populations. This model leads to several predictions concerning the effects of cellular and matrix properties on the resulting patterns. We apply the model to two widely studied morphogenetic processes: (a) patterns of skin-organ primordia, especially feather germ formation, and (b) the condensation of cartilagenous skeletal rudiments in the developing vertebrate limb.

Animals↗
PubMed1981

Fibroblast traction as a mechanism for collagen morphogenesis.

To make visible the traction forces exerted by individuals cells, we have previously developed a method of culturing them on thin distortable sheets of silicone rubber. We have now used this method to compare the forces exerted by various differentiated cell types and have examined the effects of cellular traction on re-precipitated collagen-matrices. We find that the strength of cellular traction differs greatly between cell types and this traction is paradoxically weakest in the most mobile and invasive cells (leukocytes and nerve growth cones). Untransformed fibroblasts exert forces very much larger than those actually needed for locomotion. This strong traction distorts collagen gels dramatically, creating patterns similar to tendons and organ capsules. We propose that this morphogenetic rearrangement of extracellular matrices is the primary function of fibroblast traction and explains its excessive strength.

Animals↗