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Self-Organisation of Biological Morphogenesis: General Approaches and Topo-Geometrical Models
In modern science, a most adequate conceptual framework for treating the behaviour of complex dynamic systems is given by the theory of self-organization (e.g., Prigogine, 1980). The developing organisms may be definitely attributed to self-organizing entities by a number of criteria and, above all, by their capacity for spontaneous breaks of the symmetry order. We define those breaks of macroscopical symmetry as spontaneous which do not imply any definite macroscopical causes (dissymmetrizators), let they be located outside or inside the embryo. As is well established by descriptive and experimental embryology, such symmetry breaks are taking place not only at the level of a visible morphology, but also within the phase space of the developmental potencies. The latter means that embryonic development is always associated with a progressive narrowing and specification of the morphogenetical potencies initially delocalized throughout embryonic space.
Both for an experienced and for a naive observer the development of a living sample, be it plant or animal, looks, first of all, as a regular succession of complicated changes in the shapes and mutual arrangement of its parts; such a succession is usually defined as a morphogenesis while its components as morphogenetic processes. Invaginations, evaginations and the bending of epithelial layers, condensations of freely moving mesenchymal cells, as well as the changes in shapes and overall proportions of the large masses of almost immobile plant cells may serve as the examples of morphogenetic processes. As was shown by the molecular biology within several last decades, all of these processes are based upon a highly regulated motile activity of the molecular and supramolecular components of the living cells. In the first approximation, all of these processes may be considered as mechanical, what means that they are associated with the production of mechanical forces and changes in space positions of the material constituents.
Regular patterns of mechanical stresses are perfectly expressed on the macromorphological level in the embryos of all taxonomic groups studied in this respect. Stress patterns are characterized by the topological invariability retained during prolonged time periods and drastically changing in between. After explanting small pieces of embryonic tissues, they are restored within several dozens minutes. Disturbance of stress patterns in developing embryos irreversibly breaks the long-range order of subsequent development. Morphogenetically important stress patterns are established by three geometrically different modes of cell alignment: parallel, perpendicular, and oblique. The first of them creates prolonged files of actively elongated cells. The second is responsible for segregation of an epithelial layer to the domains of columnar and flattened cells. The model of this process, demonstrating its scaling capacities, is described. The third mode which follows the previous one is responsible for making the curvatures. It is associated with formation of “cell fans,” the universal devices for shapes formation due to slow relaxation of the stored elastic energy.
Species-specific morphology in thecate hydroids is considered as a function of 2 fundamental morphogenetic characteristics: parameters of growth pulsations and the relation between the migratory activities of the endo- and ectodermal cells of the growing tips. Comparative, experimental and modelling data are presented suggesting that increases in the values of these parameters lead to gradual transformation of the narrow tubular rudiments of primitive thecates to the more transversely extended and later bilaterally symmetrical morphologies of advanced forms. There is a corresponding change in the mode of branching, from stolonal through alternate to opposite, with densely packed hydranths and hydrothecae. The relations between the traditional systematic approach to this group and the present ontogenetically based interpretation are discussed.
A deep (although at the first glance naïve) question which may be addressed to embryonic development is why during this process quite definite and accurately reproduced successions of precise and complicated shapes are taking place, or why, in several cases, the result of development is highly precise in spite of an extensive variability of intermediate stages. This problem can be attacked in two different ways. One of them, up to now just slightly employed, is to formulate robust macroscopic generative laws from which the observed successions of shapes could be derived. Another one, which dominates in modern embryology, regards the development as a succession of highly precise 'micropatterns', each of them arising due to the action of specific factors, having, as a rule, nothing in common with each other. We argue that the latter view contradicts a great bulk of firmly established data and gives no satisfactory answers to the main problems of development. Therefore we intend to follow the first way. By doing this, we regard developing embryos as self-organized systems transpierced by feedbacks among which we pay special attention to those linked with mechanical stresses (MS). We formulate a hypothesis of so-called MS hyper-restoration as a common basis for the developmentally important feedback loops. We present a number of examples confirming this hypothesis and use it for reconstructing prolonged chains of developmental events. Finally, we discuss the application of the same set of assumptions to the first steps of egg development and to the internal differentiation of embryonic cells.
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