XII.7.1 Some epigenetic processes create highly organized structures through intra-individual selection and ongoing testing for functionality of the emerging systems

During individual development, the complicated structures of the bodies of multicellular organisms are gradually formed by epigenetic processes, within which the individual organs in the body are connected together to form a highly integrated functional unit.Even a very minor deviation from the ideal structure could have drastic consequences for the functionality and thus viability of the organism.Because of the complexity of living systems and the unpredictability of external effects, which could affect the course of development, it is not possible to prevent the occurrence of deviations from the ideal state.Consequently, it is important during epigenetic processes controlling the development of multicellular organisms by regularly testing the functionality of the individual emerging organs.

In the formation of the bloodstream, initially an undifferentiated network of simple blood capillaries is formed, with a density corresponding only to the level of anoxia of the particular tissue section (Klasbrun & Soker 1993).Differentiation of part of these capillaries into far larger and anatomically more complicated veins occurs in direct dependence on the pressure and amount of blood flowing through the particular capillary.This ensures that, even if the individual shape of the circulatory system differs in all the individuals in the population in dependence on differences in their genotype and in the variable effects of the environment on the progress of embryogenesis, the blood system will be functional in all cases and will effectively distribute blood in the individual tissues (Gerhart & Kirschner 1997).

In many cases, certain structures are first formed at random and in great excess and only those that are functional remain preserved, while the others disappear.Thus a certain analogue of natural selection occurs at the cellular and organ level in embryogenesis.In the creation of innervation of the skeletal muscle, a large number of nerve projections, derived from a large population of neurons, grow into the muscle.These projections branch out in the muscle and form an enormous number of nerve-muscle synapses with individual muscle fibres.The same neurons form connections with a large number of muscle fibres and a single muscle fibre is innervated by a great many neurons.However, after a relatively short time, most of the projections and most of the nerve-muscle synapses disappear so that, finally, each neuron forms many synapses with only one muscle fibre and each muscle fibre remains innervated by only a single neuron (Fig. XII.8).If we artificially prevent transfer of the nerve impulse in a particular muscle, for example in that we expose it for a long time to the action of blockers for acetylcholine receptors, most of the synapses are preserved and each muscle fibre remains innervated by the projections derived from a great many neurons (Balice-Gordon & Lichtman 1994).The mechanism of innervation of muscle fibres based on the principle of natural selection of synapses is apparently the only conceivable mechanism through which it is possible, in complicated organisms with a large number of cells, to ensure that each muscle fibre is finally innervated by one and only one neuron.

The functioning of innervation of the skeletal muscle can be tested in a natural way during embryogenesis.The exact mechanism of testing the functionality is not yet known; however, it will most probably be based on gradual transfer of a signal through the individual neurons and simultaneous disappearance of those synapses that are located on muscle fibres to which signals arrive from other neurons.However, in some cases, it is more difficult to test the functionality of the individual structures.For example, sight perceptions in mammals are evaluated in the lateral sight nuclei of the thalamus.Similar to the skeletal muscles, also here the axons derived from cells innervating the retina of both eyes finally form a dense network of synapses with a great many nerve cells.It is only during further progress of embryogenesis that most of the synapses disappear and the network of nerve connections becomes less dense.The final state is such that neighboring parts of the brain are interconnected with neighboring parts of retina of a particular eye.At first sight, it might seem that that elimination of superfluous connections could be controlled so that similar signals come to the thalamus from neighboring parts of the retina (as a reaction to individual optical signals) and the correlation or lack of correlation of the signals arriving at neighboring cells of the thalamus could determine whether the nerve synapses with the relevant cells remain preserved or disappear. However, in mammals, a fundamental problem is associated with the development of the embryo within the body of the mother and the related lack of optical stimuli.In order for it to be possible to test the functionality of the whole system even under these conditions, optical stimuli arriving from the individual areas of the retina can be only simulated.It has actually been observed that, during embryogenesis, when the relevant parts of the thalamus are being formed, locally delimited waves of activity are spontaneously generated on the retina, which alternately affect the individual areas of the retina, and can thus replace the missing optical stimuli (Shatz 1996).

The formation of structures based on the principle of random formation and testing of the functionality provides the developmental processes with great plasticity and allows them to effectively adapt to various randomly occurring situations.If, for example, we remove one eye from the embryo, the entire area of the optical nucleus of the thalamus remains innervated by axons derived from the remaining eye.If, on the other hand, we implant a third eye in the embryo, the brain forms the relevant region of cells for it that are capable of processing the relevant optical signals and the third eye will be functional in adulthood (Reh & Constantine-Paton 1985).The system understandably reacts in a similar way, not only to external, for example, experimental interventions, but also to internal defects, i.e. to those that are caused, for example, by the existence of a certain new mutation {11880}.Plasticity of epigenetic processes thus ensures that, even with a drastic intervention into developmental processes, more or less functional structures are mostly formed.s

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.