The organ of form: towards a theory of biological shape

J. Social Biol. Struct . 1987 10, 73-83

The organ of form : towards a theory of biological shape Francisco J . Varela CREA, Ecole Polytechnique, Paris, France and Samy Frenk Rolf Institute, Boulder, Colorado, USA Cells and the extracellular matrix (ECM) immediately surrounding them engage in reciprocal determinations. But the ECM is also a global structure because it is continuous throughout the body . We argue that this local-global articulation is a central element in the determination of an animal's form, and we show how it participates in all the other dimensions of animal life . Specific experimental implications and further consequences of this view are discussed .

What is shape? Living organisms have shape . This is so obvious a statement that we take it for granted and become oblivious of the fact that there is no adequate theory of living form and shape in contemporary biology . In this paper, when we employ a theory of shape we shall be concerned with not only the principles which determine the spatial patterns of bodies, but also how such patterns participate in all the dimensions of animal life such as movement, cognition, disease, and communication . Thus, we are concerned with form not only in terms of geometric relations and proportions, as developed in the tradition of D'Arcy Thompson (1961), but beyond that with shape as an integral component in the dynamics of a living system . In this paper, we shall present an outline for such a theory of biological shape . Our approach is based on current biological research . However, it is not a mere aggregate of current facts, but rather a conceptual scaffolding from a very specific vantage point . It defines a living system as a totality, and how the phenomena proper to life unfold from this peculiar organization . This framework has been presented extensively elsewhere (Maturana & Varela, 1980 ; Varela, 1979) . Rather than recapitulating on these ideas here, we shall use them in the context of shape ; the reader will thus understand these ideas through examples of their usage . The knife's distinctions As a first approximation we can define a living form as a collection of spatial distinctions in an organism . A distinction is the act of defining what constitutes the components of 0140-1750/87/010073+11 $03 .00/0

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a given unity . The shape of a table, for instance, is such a collection of spatial relations between the components (table top and legs) of the unity table . Hence, a discussion of shape must start by making explicit the distinctions we make in an organism as a composite unity and their spatial relationships . Traditionally, biological shape has been the province of anatomy (literally : separating the parts) . Anatomical studies began in earnest with Vesalius and his monumental De Fabrica Corporis Humana in 1543 . Since then, his observations have been refined substantially to constitute a data base which most scientists would consider `stable' or achieved, in same basic sense, although a few minor details are continuously being added . This, of course, applies to human anatomy . Animal anatomy is a more open field because of the immense diversity of species . The spirit of Vesalius' work is present almost unchanged in the human anatomy a medical student must learn today . What are the fundamental distinctions implicit in this venerable tradition? They can easily be described: the parts of an organism which can be distinguished (and related in space) are those which result from the actions of a knife . Vesalius started his own studies in a cultural context where hunting and butchery were widespread . Evidently, he drew from that context and, more importantly, from the distinguishing instruments used then . The knife has now been refined to become a scalpel but the principle remains the same . The knife edge separates that which falls on both sides as the distinguished components . It separates bone from muscle and muscle from viscera . Thus, we end up with the separation between soft parts, muscles, and skeleton which seems so familiar to our Western minds . What we have said so far concerns what most biologists would call classical human anatomy . Modern biology has developed additional tools for dissections and instruments whose implicit distinctions are radically different . These instruments penetrate into the cellular and molecular level and belong to microscopic anatomy and cellular and molecular biology . The most important of these new tools used to make distinctions is the microscope . It revealed in the eighteenth century a fundamentally different distinction relative to bodies : cells . The microscope, and tools for molecular separation developed later, can distinguish units bounded by membranes which are fundamental components of every living organism . What might not be so apparent is the fact that delimiting cells reveals, by contradistinction, what is not bounded by cells in the body . This aggretate of non-cellular substance is the so-called connective tissue (see Fig . 1) . It includes the space under a covering epithelium, the gaps between muscle bundles, the spacing between viscera, as well as ligaments and fascia . The remarkable thing about this non-cellular component is that it is a continuum . To make this point apparent, let us consider a cross-section through the neck of a human body (Fig . 2) . Let us move through the tissue from the outside to the inside . At the outer surface we find the skin which appears as a layer surrounding the entire cross-section . Immediately under it we find connective tissue, first in the form of a basal membrane under the epithelial cells and then as a subcutanous layer . Notice that, although there are some cellular elements in this connective tissue such as fibroblasts and blood cells, typically this is a non-cellular matrix of fibrous and viscous material . We shall return to the constitution of the connective tissue below . Moving a bit deeper into the cross-section, we come into contact with muscle bundles where cellular elements clearly predominate, although we are able to see connective tissue in the form of fascia which surround the muscle . The degree of condensation of the connective tissue associated

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Fig. 1. A microscopic view of the network of elements within a small section of the loose connective tissue in the guinea-pig stained with Bizzozero’s method; x 800. A: bundles of collagen bundles. B. C: fibroblasts. F: elastic fibers. V: blood vessel. Taken jirom Ramdn y Cajal & Tell0 y Muiioz (1950; Fig. 193)

with muscular elements varies from extremely lax to a thick packet as is found in a tendon. Moving even deeper inside, we encounter bone, which is also in continuity with the rest of connective tissue. It differs from it by deposition of mineral elements, especially calcium, and the arrangement of precise geometrical patterns produced by a sparse but active population of cells. In short, from this briefjourney through a cross-section of a neck, we see that the usual anatomical descriptions implicit in the knife’s actions produce separations in connective tissue which amount to arbitrary distinctions of degrees of density rather than qualitatively different constituents of the neck’s shape. A more adequate distinction is between cellular aggregates (epithelium, muscle bundles, and so on) and the surrounding space matrix (ECM). which is filled with an extracellular Let us now extend this point of view of the continuity of the space between cellular elements beyond the two dimensions of the cross-sections described above to the entire three dimensions of the body. Thus, let us consider the shoulder beyond the forearm, then the trunk, until we encompass the entire body. It is perhaps easiest to evoke what we mean by yet another thought-experiment. Imagine we take the dead body of an animal, say a cat, and we drop the entire thing in a detergent which dissolves only cellular elements, leaving the ECM untouched. We leave it in the detergent long enough to extract every piece of the cellular components, and then we pull the cat-minus-cells out of the detergent bath. What we would see is still a cat’s shape, only in negative as it were, where only the space around the cells remains visible. The cat’s shape is a continuum:

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Fig. 2. A cross-section through the neck of a human at the level of the trachea, as seen from above. Taken from Spateholz (1902;jg. 307). Fettpolster = layer of fat; Muskeln des Nackens = neck muscles

there is no clear transition between the basement membrane of the skin, the muscle’s fascia, the bones, or the connective tissue between the viscera. Our basic intention here is to argue why the continuity (or global interconnectedness) of the ECM should be brought into the foreground as an essential key to the understanding of biological form. In fact, we believe it constitutes an organ of form. The sections that follow unfold the arguments to support the adequacy of this designation and its consequences. The biology of the extracellular

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Studies concerning the ECM have been carried out mostly in the last 20 years, and they are still at a stage where they are not that familiar to biologists and non-biologists alike, although this is changing rapidly. Over the last 20 years, techniques from biochemistry, ultrastructure, and immunohistochemistry, have revealed the fundamental universality of the ECM components (Hay, 1981a).

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As a first approximation, the ECM is a matrix of fibrous materials secreted by cells of various kinds and bound together in intricate tangles. The most conspicuous, and first to be described, of these fibrous components is collagen, an ubiquitous protein which can exist in various degrees of aggregation. Next to collagen in abundance are the polysaccharides and a combination of polysaccharides and proteins, or glycoproteins. There is also a rich variety of mucopolysaccharides, including hyluronate, chondroitin sulfate, collectively called glycosaminoglycans (GAG) (Fig. 3). By and large, these biochemical characterizations have remained separate from cellular biology until recently. Interest in this area has increased because of the steady accumulation of observations pointing to the precise and extensive relationship between the ECM and the surfaces of all the cells in the body. According to these observations, there are multiple ways in which collagen, glycoproteins, and GAG can be arranged to form highly specific links to receptors located on cells’ membranes. Thus, the ECM is in a position to exert specific and dramatic changes on the cellular dynamics, just as much as, say, a hormone or a neurotransmitter (Fig. 4). Test

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Fig. 3. Collagen is a protein which can take many different forms depending on the conditions. Tropocollagen (TC) is the building block, which polymerizes in the various ways shown in the diagram. Taken from Hay (1981a)

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In brief, the intimate milieu of every cell in our bodies is not a bland and homogeneous soup of nutrients. Instead, this intimate milieu has a precise achitecture provided by all of the intricacies of the ECM components, with an ongoing dynamic exchange with the cell surface.

Morphocycles and the organ of shape

Having introduced the basic questions about shape and the key qualities of the ECM, we can now turn to the central idea we wish to introduce here. It consists of considering simultaneously the local and global qualities of the ECM. The link between global and local is given by the cyclic (or self-referential) nature of the interactions between cells and their surrounding space containing the ECM. Let us clarify this. At every location, the ECM is produced by cellular elements of that particular region. But the local ECM can also influence cell dynamics, thus constituting a cycle of reciprocal interaction between cellular and non-cellular constituents. But this local reciprocity is not the entire story, for whatever local action occurs is necessarily conditioned by the



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continuity of each local ECM with the adjacent ECM, and, through them, the entire body . As in the notion of a field and its corresponding particles, there is in living shape a dynamic complementarity : the entire global shape of the body affects the local conditions for ECM/cell relationship, but at the same time the local dynamics conditions how the entire body is actually built . We call this reciprocal determination between cellular elements of a multicellular animal and the continuous extracellular matrix a morphocycle . Thus, a morphocycle is a process, that is an ongoing bootstrapping, whereby a shape is produced by the body's cells . But this shape in turn conditions (through the continuity of the ECM) what the cells do . Or, in other words, a morphocycle is the process whereby a local action between the ECM and cell surfaces produces the global effect of shape and is in turn constrained by it . In this kind of dynamics of mutual and complementary reciprocity, it is tempting to take one side of the process as dominant (Goguen & Varela, 1979) . However, it is clear that at any given time, a body is the result of a very prolonged history of uninterrupted morphocycles, and what is due to cells and what is due to shape is inseparable . In fact, even if we retrace the steps of a body's shape back in time, the problem is not solved, for even the zygote did not exist in a vacuum, but already inside another shape . From what we have said, it seems appropriate to refer to the whole continuous ECM as an organ of shape, since it is through it that existence and form become inseparable .

Cases Let us consider some examples which illustrate the above ideas in action . Development morphology of organs The way in which a specific (i .e . local) kind of ECM can condition the differentiation of cells, and further, be an integral part of specifying the characteristic morphology of an organ, is a recent and much debated possiblility (Hay, 1981 b ; Lewis, 1984) . For instance, for many years researchers have tried to induce normal differentiation of mammary glands in vitro with the aid of inducing hormones . Such attempts met with little or no success . However, when mammary cells are cultivated in the presence of the ECM of the mammary gland, adequate differentiation takes place and produces functional mammary glands . Furthermore, this differentiaton is possible with just the mammary stroma (i .e . the isolated fibrous components of the local ECM) and in the absence of any inducing hormone . If the flexibility of the ECM is inhibited by various means (such as accessibility to oxygen), the capacity for differentiation is correspondingly lost (Shannon & Pitelka, 1981) . Thus, the ECM is capable of acting back onto cells by mechanisms which involve genetic repression and derepression, giving rise to changes in the cells they enclose which, in turn, produce an ECM peculiar to their configuration . Also, the role of fibroblasts, the cell class found sparsely in the ECM, is beginning to be clearer . They have been found to have remarkable traction properties through their secretion of a collageneous matrix, capable of dictating much of the structure of the skeleton, location of the muscles, routes of nerves, and patterning of the skin (Chevallier & Kinney, 1982 ; Lewis et al., 1981) . Thus, morphocycles are centrally involved in the differentiation of the function and shape of organs . Cancer and connective tissue One of the most devastating aspects of cancer is the capacity of tumors to spread from their primary site to other organs . Part of the difficulty of understanding these processes,



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which is a key for the preventive treatment of cancer, is the diversity of cells in the primary tumor and the way in which metastatic tumors are selected accordingly . Various different locations in the body select different cellular classes upon which to start the growth of a malignant tumor . Predictably, this selection occurs with the concourse of the cells of the target organs . Further, such selection also occurs with the participation of the specific kind of ECM because it mediates between the invading malignant cells and the future site of tumor growth (Fidler & Hart, 1982 ; Auerbach & Auerbach, 1981) . Thus, a malignant tumor might not grow in the lymph nodes of the neck, but it does so actively in the nodes under the armpit . Thus, one key to the mechanism of metastasis is the presence of ongoing morphocycles at each location . Do muscles act by pulling on the tendons? The standard textbook interpretation of how a muscle acts is that it pulls on the tendon in which it terminates . The traction produced by the muscle's shortening is directly transmitted through the tendon to the bone, which is thus mechanically displaced . We may ask, however, what is the evidence for this accepted view? It it were true, we would expect some kind of mechanical continuity between muscle cells and the surrounding collagen . From the ultrastructural point of view, such continuity is not all that clear . Muscle fibers are surrounded, but not directly linked, to their surrounding ECM . This raises the possibility of an alternative interpretation of muscle action, one that puts further emphasis on the continuity and integrity of the organ of form . In fact, when a muscle contracts, it not only shortens but also thickens . The diameter of the fibers is correspondingly increased, which causes the connective sheath to be pulled perpendicular to the line of sarcomere shortening . If there is a strong continuity in the connective sheath, the increase in the diameter will also result in a pull on the tendon and bone . Recent experiments show, in fact, that weakening the continuity of the connective tissue around the muscle belly also weakens its capacity for action (Kirkwood, Maturana & Varela, unpublished data) . It is possible, of course, that both mechanisms act in unison . But it is only if we think about the organ of form as a continuum that the second, and perhaps predominant mode of action of muscle action is properly understood . The cases mentioned in this section range from the very detailed to the suggestive and from the molecular to the macroscopic . They are intended as a showcase of how the present perspective can be projected into specific problems and contribute fresh new alternatives . Natural history of the organ of shape From the point of view presented here, the organ of shape is the specific structure which makes possible the spatial co-existence of cells in an aggregate which operates as a unity, as a whole organism . Thus, shape is synonymous with the very existence of a metazoan or multicellular animal .t Furthermore, the biochemistry of the ECM is surprisingly universal throughout the entire range of vertebrate and perhaps also invertebrate life (Hay, 1981) . This universality is also present in other fundamental living dynamics such as the genetic code, membrane transport, or metabolic pathways . Like these, the mutual effects between ECM and cell dynamics tend to be very conservative mechanisms throughout evolution, as fundamental building blocks which are rarely, if ever, subject to modification . t Although this discussion does not in principle exclude plant shapes, there is little comparative material on vegetal ECM . What follows, therefore, applies to all kingdoms with the possible exception of plants .

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This is a very interesting fact when considered in the light of the universal nature of multicellularity. Contrary to traditional views, cellular aggregates constituting an organism with a distinct shape exists not only amongst the macroscopic creatures, vertebrates and invertebrates. Multicellularity is present in all of the five kingdoms: monera (i.e. bacterialike), protysta (i.e. protozoa-like), fungi, plants, and animals (Margulis & Schwartz, 1982). In all of these kingdoms, one can find individuals which are multicellular, although in the case of vertebrates this is an obligatory feature. In the first three kingdoms, in contrast, many members lead a life as independent, free-living, single cells (Fig. 5). There is evidence for the presence of multicellular animals dating back to the Edicarian period at the beginning of the Phanerozoic eon some 3 billion years ago. This is the period to which the oldest known living fossils have been traced (Cloud &

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Fig. 6 . The history of reciprocal coupling between two autopoietic units, symbolized here by a circular arrow, can have two possible outcomes: containment or juxtaposition . In one case, one has a history of symbiosis ; in the other, the secretion of a common space, i .e . a shape Glaessner, 1982) . Since multicellularity is as old as life is, shape, in the sense understood here, is almost as old as life . Bodies and shapes did not begin with fishes or lizards . Life must have arisen by the constitution of minimal autopoietic units, self-producing units capable of generating their own boundaries (Maturana & Varela, 1980 ; Varela, 1979) . But once populations of such autonomous units arose, the possibility of being factors of reciprocal selective histories arose at the same time for these units . In such histories of recurrent interactions between two primitive cells, there are two possible logical outcomes : either their boundaries dissolve by one becoming contained in the other, or else their boundaries do not dissolve but become juxtaposed in the same space to each other (see Fig . 6) . The first possibility is a case of symbiosis, where one kind of cell becomes a permanent host to another . This seems to have been precisely the path taken in the history of modern, eucaryotic cells (Margulis, 1981) . However, the logical dual nature of this symbiosis is that cells become strongly bound by the specification of a common space produced by the joint dynamics of the participating cells . This is tantamount to saying that the participating cells secrete their own surrounding space : an extracellular matrix which delimits precisely what is and what is not part of it . To state this in yet another way, crossing the boundaries (as in the origin of eucaryotes) could be described as endo-symbiosis . Preserving the cellular boundaries while sharing a mutually specified space could be described as exo-symbiosis, which becomes another word for shape . Endo- and exo-symbiosis have been present from the very beginning of the natural history of life, since these were options open to the very first populations of autopoietic systems . Further, the dual option of endo- and exo-symbiosis can operate not only between cells, but also between multicellular organisms themselves . A lichen and a parasite are examples of this principle applied at a higher level of recursion . Conclusion The origin of shape and its morphocyles are token names for an entire context and research program in which to understand biological shape, its material substrate, its



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natural history, and the way in which it can participate in various aspects of an animal's life . This proposed perspective consists basically of bringing into alignment a number of data from current research with a specific perspective on living systems . The intuition behind our framework is that space is a constitutive element in the dynamics of living organisms just as much as the solidity of their molecular constituents . We are only just beginning to realize the importance of this mutual partnership between cellular dynamics and specified/specifying space . In the light of the present perspective, the understanding of biological phenomena has hopefully been enriched and unified . Beyond such an aesthetic reward, the present hypothesis does lead to interesting new questions which can be addressed experimentally, such as those outlined in the section `Cases', above . It is also interesting to consider the usefulness of this perspective as a foundation for the whole array of disciplines and techniques collectively known as `bodywork', where shape and posture are seen as being inseparable from consciousness itself and the wholeness of human experience .

Acknowledgements F .V . is Foudation de France Professor of Coquitive Science . The financial support of the Prince Trust Fund is gratefully acknowledged .

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