Cellular Tensegrity Research Articles

The development of sporoderm, tapetum and Ubisch bodies in Dianthus deltoides (Caryophyllaceae): Self-assembly in action

The purpose of this work was to study in detail with TEM the successive stages of pollen wall and tapetum development in Dianthus deltoides and to clarify the mechanisms underlying the developmental processes. At the young tetrad stage a thin glycocalyx layer is poorly structured, being a mixture of glycoproteins, secreted by Golgi vesicles through the process of exocytosis. At the middle tetrad stage the plasma membrane acquires a deeply invaginated profile as a result of self-assembly cellular tensegrity, and its pinnacles determine the sites of the future spines. Then, lipoid sporopollenin (SP) precursors and monomers are added into the periplasmic space. The concentrations of all substances in the periplasmic space are under genome control. As a consequence of self-assembly hydrophilic–hydrophobic interactions, “islets” of osmiophilic lipoid substances occur in the glycocalyx mixture, first as spherical units, then as plates, with further lateral growth and the appearance of the hexagonal reticulate pattern of the future tectum. At maturity, the reticulate pattern is not evident. At the late tetrad stage spines are initiated as outgrowths of the glycocalyx in the form of cone-like liquid-crystal “skeletons”, the latter accumulate SP. Columellae are initiated very late in ontogeny, at the stage when the tetrads separate into monads, on the base of the glycocalyx rod-like units — the cylindrical micelles. The first endexine lamella with a central white line appears at the young free microspore stage on the base of laminate micelles. The second endexine layer is granulate and appears on the base of spherical micelles. Pollen grains are polyporate with aperturate membranes, consisting of lamellate endexine-1 and an underlying dilation – oncus – comprising granulate endexine-2. The pore membranes bear large cupola-like tectal spines. The tapetum secretes a peri-tapetal membrane with Ubisch bodies, first with clusters of globules, then exine-like spines on the top of clusters. These Ubisch bodies mimic the pattern of the exine. The entire sequence of developmental events can be explained as the development of a self-assembling system under initial genomic regulation.

Force Control of a Cellular Tensegrity Structure with Model Uncertainties and Partial State Measurability

AbstractMechanical force is known to influence the behavior of biological cells. To study how external mechanical forces may affect cellular response and function necessitates the development of sophisticated force‐control techniques for accurate application of dynamical forces on living cells. In this paper, we develop a robust force control algorithm that enables accurate application of an external force on a dynamics model of biological cells. Such force control is achieved by utilizing a six‐strut cellular tensegrity model constructed based on the structural approach. Leveraging on the more comprehensive description of the nonlinearity and dynamic coupling of internal structural elements provided by this tensegrity model, we synthesize a novel robust control law that can effectively deal with model uncertainties and partial state measurability. Numerical simulations (involving forces of different dynamics profiles) are conducted to demonstrate the effectiveness of the proposal force control algorithm.

2D-DIGE Proteomic Analysis of Mesenchymal Stem Cell Cultured on the Elasticity-tunable Hydrogels

The present study focuses on mechanotransduction in mesenchymal stem cells (MSCs) in response to matrix elasticity. By using photocurable gelatinous gels with tunable stiffness, proteomic profiles of MSCs cultured on tissue culture plastic, soft (3 kPa) and stiff (52 kPa) matrices were deciphered using 2-dimensional differential in-gel analysis (2D-DIGE). The DIGE data, tied to immunofluorescence, indicated abundance and organization changes in the cytoskeletonal proteins as well as differential regulation of important signaling-related proteins, stress-responsing proteins and also proteins involved in collagen synthesis. The major CSK proteins including actin, tubulin and vimentin of the cells cultured on the gels were remarkably changed their expressions. Significant down-regulation of α-tubulin and β-actin can be observed on gel samples in comparison to the rigid tissue culture plates. The expression abundance of vimentin appeared to be highest in the MSCs cultured on hard gels. These results suggested that the substrate stiffness significantly affects expression balances in cytoskeletal proteins of MSCs with some implications to cellular tensegrity.

Biochemical Characterization of the Cell-Biomaterial Interface by Quantitative Proteomics

Surface topography and texture of cell culture substrata can affect the differentiation and growth of adherent cells. The biochemical basis of the transduction of the physical and mechanical signals to cellular responses is not well understood. The lack of a systematic characterization of cell-biomaterial interaction is the major bottleneck. This study demonstrated the use of a novel subcellular fractionation method combined with quantitative MS-based proteomics to enable the robust and high-throughput analysis of proteins at the adherence interface of Madin-Darby canine kidney cells. This method revealed the enrichment of extracellular matrix proteins and membrane and stress fibers proteins at the adherence surface, whereas it shows depletion of extracellular matrix belonging to the cytoplasmic, nucleus, and lateral and apical membranes. The asymmetric distribution of proteins between apical and adherence sides was also profiled. Apart from classical proteins with clear involvement in cell-material interactions, proteins previously not known to be involved in cell attachment were also discovered.

From Cellular Mechanotransduction to Biologically Inspired Engineering

This article is based on a lecture I presented as the recipient of the 2009 Pritzker Distinguished Lecturer Award at the Biomedical Engineering Society annual meeting in October 2009. Here, I review more than thirty years of research from my laboratory, beginning with studies designed to test the theory that cells use tensegrity (tensional integrity) architecture to stabilize their shape and sense mechanical signals, which I believed to be critical for control of cell function and tissue development. Although I was trained as a cell biologist, I found that the tools I had at my disposal were insufficient to experimentally test these theories, and thus I ventured into engineering to find critical solutions. This path has been extremely fruitful as it has led to confirmation of the critical role that physical forces play in developmental control, as well as how cells sense and respond to mechanical signals at the molecular level through a process known as cellular mechanotransduction. Many of the predictions of the cellular tensegrity model relating to cell mechanical behaviors have been shown to be valid, and this vision of cell structure led to discovery of the central role that transmembrane adhesion receptors, such as integrins, and the cytoskeleton play in mechanosensing and mechanochemical conversion. In addition, these fundamental studies have led to significant unexpected technology fallout, including development of micromagnetic actuators for non-invasive control of cellular signaling, microfluidic systems as therapeutic extracorporeal devices for sepsis therapy, and new DNA-based nanobiotechnology approaches that permit construction of artificial tensegrities that mimic properties of living materials for applications in tissue engineering and regenerative medicine.

Sporoderm development in Trevesia burckii (Araliaceae). I. Tetrad period: Further evidence for the participation of self-assembly processes

The developmental events in the periplasmic space and cytoplasm of microspores, and in the tapetum of Trevesia burckii during the tetrad period have been traced in detail during microspore ontogeny from the sporogenous cell stage to the late tetrad stage and the initiation of the foot layer. The data obtained provide support to two of our previously proposed hypotheses: (1) the glycocalyx (scaffolding primexine matrix) is a colloidal system; (2) the involvement of processes of self-assembly of a number of colloidal micellar systems to the exine development. The main structures of the reticulate ectexine up to the establishment of the foot layer are columellae and tectum which evidently form on a base of spherical and cylindrical transitive micelle mesophases after sporopollenin accumulation. The importance of the callose envelope surrounding microspores for exine development is discussed in the light of recent findings. Two possible pathways of pattern determination are possible. One suggests the role of the plasma membrane in pattern imprinting and the corresponding necessity of the transfer of 2-D information to 3-D. Our current supposition is that the other, self-assembly physical phenomena, cellular tensegrity, also participate in the process of the establishment of a wide-spread reticulate exine pattern which appears as the result of an interplay of the microspore cytoskeletal prestress and the resistance from the ECM (exocellular matrix = glycocalyx) adhesive sites. More and more information is appearing that provides evidence for the importance of mechanical forces and physico-chemical regularities in the living world.

Effects of cytoskeletal prestress on cell rheological behavior

Normal tissue development requires that cells alter their mechanical behavior in different microenvironments to carry out their diverse functions. During cell spreading, migration, invasion and mitosis, cells exhibit a high degree of deformability, exhibiting almost a fluid-like behavior, whereas within quiescent differentiated tissues, cells must behave like an elastic solid to maintain their structural integrity in the face of an applied mechanical stress. A growing body of experimental evidence suggests that rheological properties of adherent cells depend on pre-existing tensional stress (“prestress”) borne by the cytoskeleton. This prestress results from the action of tensional forces borne by actin microfilaments, transmitted over intermediate filaments and resisted by both extracellular matrix adhesions and internal microtubules. Observations that the prestress influences mechanical properties of the cell are intimately related to the cellular tensegrity model. This model depicts the cytoskeleton as an interconnected network of cables that carry pre-existing tension that is balanced by compression-bearing struts and by anchoring forces of the substrate. This paper offers a brief survey of the basic concept of cellular tensegrity model, comparison of model predictions with experimental data obtained from rheological measurements on living cells, and comparison with other models that have been used in studies of rheology of cells.

Tensegrity and type 1 dermal dendrocytes in acromegaly

Skin is enlarged and doughy in acromegaly. Alterations have been reported in the structure of the extracellular matrix (ECM) and in the mechanical properties of the dermis. It is recognized that internal tensions in the dermis give rise to active cell-ECM and cell-cell mechanical interactions. This mechanobiological aspect can lead to functional and conformational changes in the cells. Such alterations in connective tissue cells have not been explored so far in acromegaly. The aim of the study was to assess structural changes in the dermal ECM, and in the number and shape of factor XIIIa-positive type I dermal dendrocytes (DDs) in acromegaly. A total of 32 patients with acromegaly (three untreated, 17 with active disease under somatostatin analogues and 12 cured) were examined. Both the maximum and the most recent IGF-1 (insulin-like growth factor) dosages recorded in the patients' files were retrieved. Skin biopsies were taken from their forearms. Histochemistry, immunohistochemistry, morphometry and electron microscopy were used. Collagen-bundle coarsening and acidic glycosaminoglycan deposits were found irrespective of the current endocrine status. Dermal dendrocytes were often markedly reduced in numbers, but those present were plump with few dendrites. A negative correlation was found between DD numbers and the maximum IGF-1 dosages recorded in the patients' files. A similar although weaker trend was found with the most recent IGF-1 dosages. Collagen, glycosaminoglycans and DDs appeared affected in acromegaly, even when the hormonal status was controlled after surgery and/or somatostatin analogue treatment. The persisting conformational change of DDs suggests a modification in cellular tensegrity, perhaps initiated by hormonal effects and probably maintained by alterations in the ECM interactions.

Microtubules may harden or soften cells, depending of the extent of cell distension

Experimental data show that disruption of microtubules causes cells to either become stiffer or softer. Current understanding of these behaviors is based on several different mechanisms, each of which can account for only stiffening or softening. In this study we offer a model that can explain both these features. The model is based on the cellular tensegrity idea. Key premises of the model are that cell shape stability is secured through pre-existing mechanical stress (prestress) borne by the actin cytoskeletal network, and that this prestress is partly balanced by cytoskeletal microtubules and partly by the extracellular matrix. Thus, disturbance of this balance would affect cell deformability. The model predicts that disruption of microtubules causes an increase or a decrease in cell stiffness, depending on the extent to which microtubules participate in balancing the prestress which, in turn, depends on the extent of cell spreading. In highly spread cells microtubules have a minor and negative contribution to cell stiffness, whereas in less spread cells their contribution is positive and substantial. Since in their natural habitat cells seldom exhibit highly spread forms, the above results suggest that the contribution of microtubules to cell deformability cannot be overlooked.

Cell spreading controls balance of prestress by microtubules and extracellular matrix.

The controversy surrounds the cellular tensegrity model. Some suggest that microtubules (MTs) must bear a significant portion of cell contractile stress (prestress) if tensegrity is a useful model. Previously we have shown that for highly spread airway smooth muscle cells (areas>2500 microm2) MTs balance a significant but small potion (average 14%) of the prestress. To further explore if controlling the degree of cell spreading could modulate the portion of the prestress balanced by MTs, we utilized a recent method by which tractions are quantified in cells that are constrained within micropatterned adhesive islands of defined sizes on the surface of flexible polyacrylamide gels containing fluorescent microbeads. The prediction is that if MTs balance a portion of the contractile stress, then, upon their disruption, the portion of the stress balanced by MTs would shift to the substrate, causing an increase in traction and strain energy. We first activated the cells maximally with histamine and then disrupted the MTs with colchicine. Histamine resulted in an increase in intracellular calcium whereas ensuing colchicine addition in the presence of histamine did not change intracellular calcium concentration, suggesting there was no additional net increase in contractile stress inside the cell. We found that following disruption of MTs the increase in traction and strain energy varied with the degree of cell spreading: as the cell projected areas increased from 500 micrometer 2 to about 1800 micrometer 2, the percent increase in tractions decreased from 80% to about a few percent and the percent increase in strain energy decreased from 200% to almost zero percent, indicating the portion of the prestress balanced by MTs decreased as the cells increased spreading. These findings demonstrate that complementary role of the extracellular matrix and the MTs in balancing the prestress is controlled by the degree of cell spreading.

Weightlessness acts on human breast cancer cell line MCF-7

Because cells are sensitive to mechanical forces, weightlessness might act on stress-dependent cell changes. Human breast cancer cells MCF-7, flown in space in a Photon capsule, were fixed after 1.5, 22 and 48 h in orbit. Cells subjected to weightlessness were compared to 1g in-flight and ground controls. Post-flight, fluorescent labeling was performed to visualize cell proliferation (Ki-67), three cytoskeleton components and chromatin structure. Confocal microscopy and image analysis were used to quantify cycling cells and mitosis, modifications of the cytokeratin network and chromatin structure. Several main phenomena were observed in weightlessness: • The perinuclear cytokeratin network and chromatin structure were looser. • More cells were cycling and mitosis was prolonged. Finally, cell proliferation was reduced as a consequence of a cell-cycle blockade. • Microtubules were altered in many cells. The results reported in the first point are in agreement with basic predictions of cellular tensegrity. The prolongation of mitosis can be explained by an alteration of microtubules. We discuss here the different mechanisms involved in weightlessness alteration of microtubules: i) alteration of their self-organization by reaction-diffusion processes, and a mathematical model is proposed, ii) activation or desactivation of microtubules stabilizing proteins, acting on both microtubule and microfilament networks in cell cortex.

Tensegrity I. Cell structure and hierarchical systems biology.

In 1993, a Commentary in this journal described how a simple mechanical model of cell structure based on tensegrity architecture can help to explain how cell shape, movement and cytoskeletal mechanics are controlled, as well as how cells sense and respond to mechanical forces (J. Cell Sci. 104, 613-627). The cellular tensegrity model can now be revisited and placed in context of new advances in our understanding of cell structure, biological networks and mechanoregulation that have been made over the past decade. Recent work provides strong evidence to support the use of tensegrity by cells, and mathematical formulations of the model predict many aspects of cell behavior. In addition, development of the tensegrity theory and its translation into mathematical terms are beginning to allow us to define the relationship between mechanics and biochemistry at the molecular level and to attack the larger problem of biological complexity. Part I of this two-part article covers the evidence for cellular tensegrity at the molecular level and describes how this building system may provide a structural basis for the hierarchical organization of living systems--from molecule to organism. Part II, which focuses on how these structural networks influence information processing networks, appears in the next issue.

A model for mechanotransduction in bone cells: the load-bearing mechanosomes.

The skeleton's response to mechanical force, or load, has significance to space travel, the treatment of osteoporosis, and orthodontic appliances. How bone senses and processes load remains largely unknown. The cellular basis of mechanotransduction, however, likely involves the integration of diffusion-controlled signaling pathways with a solid-state scaffold linking the cell membrane to the genes. Here, we integrate various concepts from models of connective membrane skeleton proteins, cellular tensegrity, and nuclear matrix architectural transcription factors, to describe how a load-induced deformation of bone activates a change in the skeletal genetic program. We propose that mechanical information is relayed from the bone to the gene in part by a succession of deformations, changes in conformations, and translocations. The load-induced deformation of bone is converted into the deformation of the sensor cell membrane. This, in turn, drives conformational changes in membrane proteins of which some are linked to a solid-state signaling scaffold that releases protein complexes capable of carrying mechanical information, "mechanosomes", into the nucleus. These mechanosomes translate this information into changes in the geometry of the 5' regulatory region of target gene DNA altering gene activity; bending bone ultimately bends genes. We identify specific candidate proteins fitting the profile of load-signaling mechanosomes.

A Cellular Tensegrity Model to Analyse the Structural Viscoelasticity of the Cytoskeleton

This study describes the viscoelastic properties of a refined cellular-tensegrity model composed of six rigid bars connected to a continuous network of 24 viscoelastic pre-stretched cables (Voigt bodies) in order to analyse the role of the cytoskeleton spatial rearrangement on the viscoelastic response of living adherent cells. This structural contribution was determined from the relationships between the global viscoelastic properties of the tensegrity model, i.e., normalized viscosity modulus (η *), normalized elasticity modulus (E *), and the physical properties of the constitutive elements, i.e., their normalized length (L *) and normalized initial internal tension (T *). We used a numerical method to simulate the deformation of the structure in response to different types of loading, while varying by several orders of magnitude L * and T *. The numerical results obtained reveal that η * remains almost independent of changes in T * (η *∝T *+0.1), whereas E * increases with approximately the square root of the internal tension T * (from E *∝T *+0.3 to E *∝T *+0.7). Moreover, structural viscosity η * and elasticity E * are both inversely proportional to the square of the size of the structure (η *∝L *−2 and E *∝L *−2). These structural properties appear consistent with cytoskeleton (CSK) mechanical properties measured experimentally by various methods which are specific to the CSK micromanipulation in living adherent cells. Present results suggest, for the first time, that the effect of structural rearrangement of CSK elements on global CSK behavior is characterized by a faster cellular mechanical response relatively to the CSK element response, which thus contributes to the solidification process observed in adherent cells. In extending to the viscoelastic properties the analysis of the mechanical response of the cellular 30-element tensegrity model, the present study contributes to the understanding of recent results on the cellular-dynamic response and allows to reunify the scattered data reported for the viscoelastic properties of living adherent cells.

Can melatonin regulate the expression of prohormone convertase 1 and 2 genes via monomeric and dimeric forms of RZR/ROR nuclear receptor, and can melatonin influence the processes of embryogenesis or carcinogenesis by disturbing the proportion of cAMP and cGMP concentrations? Theoretic model of controlled apoptosis

The presented model of controlled apoptosis has been based on the assumption that correct information exchange between an organism as a whole, and each of its cells is conditioned by mutual proportions of cAMP and cGMP concentrations (C cAMP, C cGMP), according to the formula C cAMP× C cGMP= ‘a’ (constant). The regulation of balance of these ‘second messengers’ in a cell and an extracellular space would depend on the mutual proportions of concentrations of Melatonin and monomers of Melanin. These indoloderived compounds could be the activators of the transcription factors i.e. RZR and NFkappa-B, regulating the expression of Prohormone Convertase (PC) gen and Nitric Oxide Synthase (NOS) gen, respectively. Additionally, maternal Melatonin and Nitric Oxide (NO), being able to pass through trophoblast or placenta freely, would play decisive role in the synchronization of embryogenesis and intrauterine development of the fetus. In case of an embryo or a fetus, the result of C cAMPand C cGMPmultiplication, different from the proper constant ‘a’-value, would mean occurrence of disorders in the structure and functioning of the cellular tensegrity system and, in consequence, disturbances in the intercellular information exchange. It would lead to deviation in cellular metabolism, oriented cell movement, uncontrolled apoptosis, and as a consequence, would lead to the development of fetal defects. In case of a child or an adult, a sudden occurrence and prolongation of such disturbances in C cAMP–C cGMPproportions would induce a process of apoptosis of normal cells and an initiation of a cancerogenesis. On the other hand, the recovery of equilibrium in the information exchange system would initiate apoptosis of neoplastic cells, and simultaneously, proliferation of connective tissue cells. According to the presented hypothesis, a decrease in C cAMPand destabilization of the C cAMP–C cGMPbalance in an embryo or a fetus would result from relatively excessive amounts of maternal Melatonin (monomers) in fetal circulation, while a decrease of C cAMPand destabilization of the C cAMP–C cGMPbalance in a child or an adult would be a consequence of relatively insufficient amounts of Melatonin (dimers) in an organism. It seems possible, that determination of both C cAMPand C cGMPwould enable an early detection of high risk of developmental defects occurrence in an embryo or a fetus and neoplastic processes in a child or an adult. This method might also be considerably useful in monitoring a safe substitutional hormonotherapy. Copyright 2001 Harcourt Publishers Ltd INTRODUCTION

Opposing views on tensegrity as a structural framework for understanding cell mechanics.

Donald E. Ingber: Important new theories in science often ignite heated debates. If they do not, they are probably of little significance. Thus a strong argument in support of the importance of the tensegrity model of cell and tissue architecture first proposed almost 20 years ago ([23][1], [24][2

Direct observations of the mechanical behaviors of the cytoskeleton in living fibroblasts.

Cytoskeletal proteins tagged with green fluorescent protein were used to directly visualize the mechanical role of the cytoskeleton in determining cell shape. Rat embryo (REF 52) fibroblasts were deformed using glass needles either uncoated for purely physical manipulations, or coated with laminin to induce attachment to the cell surface. Cells responded to uncoated probes in accordance with a three-layer model in which a highly elastic nucleus is surrounded by cytoplasmic microtubules that behave as a jelly-like viscoelastic fluid. The third, outermost cortical layer is an elastic shell under sustained tension. Adhesive, laminin-coated needles caused focal recruitment of actin filaments to the contacted surface region and increased the cortical layer stiffness. This direct visualization of actin recruitment confirms a widely postulated model for mechanical connections between extracellular matrix proteins and the actin cytoskeleton. Cells tethered to laminin-treated needles strongly resisted elongation by actively contracting. Whether using uncoated probes to apply simple deformations or laminin-coated probes to induce surface-to-cytoskeleton interaction we observed that experimentally applied forces produced exclusively local responses by both the actin and microtubule cytoskeleton. This local accomodation and dissipation of force is inconsistent with the proposal that cellular tensegrity determines cell shape.

Stiffening response of a cellular tensegrity model.

Living cells exhibit, as most biological tissues, a stiffening (strain-hardening) response which reflects the nonlinearity of the stress-strain relationship. Tensegrity structures have been proposed as a comprehensive model of such a cell's mechanical response. Based on a theoretical model of a 30-element tensegrity structure, we propose a quantitative analysis of its nonlinear mechanical behavior under static conditions and large deformations. This study provides theoretical foundation to the passage from large-scale tensegrity models to microscale living cells, as well as the comparison between results obtained in biological specimens of different sizes. We found two non-dimensional parameters (L*-normalized element length and T*-normalized elastic tension) which govern the mechanical response of the structure for three types of loading tested (extension, compression and shear). The linear strain-hardening is uniquely observed for extension but differed for the two other types of loading tested. The stiffening response of the theoretical model was compared and discussed with the living cells stiffening response observed by different methods (shear flow experiments, micromanipulation and magnetocytometry).

Traction fibre: toward a "tensegral" model of the spindle.

Most current hypotheses of mitotic mechanisms are based on the "PAC-MAN" paradigm in which chromosome movement is generated and powered by disassembly of kinetochore microtubules (k-MTs) by the kinetochore. Recent experiments demonstrate that this model cannot explain force generation for anaphase chromosome movement [Pickett-Heaps et al., 1996: Protoplasma 192:1-10]. Another such experiment is described here: a UV-microbeam cut several kinetochore fibres (k-fibres) in newt epithelial cells at metaphase and the half-spindle immediately shortened: in several cells, the remaining intact spindle fibres bowed outwards as they came under increased compression. Thus, severing of k-MTs can lead to increased tension between chromosomes and poles. This observation cannot be explained by models in which force is produced by motor molecules at the kinetochore actively disassembling k-MTs. Rather, we argue that tensile forces act along the whole k-fibre, which, therefore, can be considered as a classic "traction fibre." We suggest that anaphase polewards force is generated by MTs interacting with the spindle matrix and when k-MTs are severed, polewards force continues to act on the remaining kMT-stub; spindle MTs act as rigid struts concurrently resisting and being controlled by these forces. We suggest that the principles of "cellular tensegrity" [Ingber, 1993: J. Cell Sci. 104:613-627] derived from the behaviour and organization of the interphase cell apply to the spindle. In an evolutionary context, this argument further suggests that the spindle might originally have evolved as the mechanism by which a single tensegral unit (cytoplast) is divided into two cytoplasts; use of the spindle for segregating chromosomes might represent a secondary, more recent development of this primary function. If valid, this concept has implications for the way the spindle functions and for the spindle's relationship to cytokinesis.

Traction fibre: Toward a “tensegral” model of the spindle

Most current hypotheses of mitotic mechanisms are based on the “PAC-MAN” paradigm in which chromosome movement is generated and powered by disassembly of kinetochore microtubules (k-MTs) by the kinetochore. Recent experiments demonstrate that this model cannot explain force generation for anaphase chromosome movement [Pickett-Heaps et al., 1996: Protoplasma 192:1–10]. Another such experiment is described here: a UV-microbeam cut several kinetochore fibres (k-fibres) in newt epithelial cells at metaphase and the half-spindle immediately shortened; in several cells, the remaining intact spindle fibres bowed outwards as they came under increased compression. Thus, severing of k-MTs can lead to increased tension between chromosomes and poles. This observation cannot be explained by models in which force is produced by motor molecules at the kinetochore actively disassembling k-MTs. Rather, we argue that tensile forces act along the whole k-fibre, which, therefore, can be considered as a classic “transaction fibre.” We suggest that anaphase polewards force is generated by MTs interacting with the spindle matrix and when k-MTs are severed, polewards force continues to act on the remaining kMT-stub; spindle MTs act as rigid struts concurrently resisting and being controlled by these forces. We suggest that the principles of “cellular tensegrity” [Ingber, 1993: J. Cell Sci. 104:613–627] derived from the behaviour and organization of the interphase cell apply to the spindle. In an evolutionary context, this argument further suggests that the spindle might originally have evolved as the mechanism by which a single tensegral unit (cytoplast) is divided into two cytoplasts; use of the spindle for segregating chromosomes might represent a secondary, more recent development of this primary function. If valid, this concept has implications for the way the spindle functions and for the spindle's relationship to cytokinesis. Cell Motil. Cytoskeleton 37:1–6, 1997. © 1997 Wiley-Liss, Inc.