XII.7 Epigenetic processes play a significant role in the individual development of a multicellular organism
The development of a multicellular individual (ontogenesis) constitutes a set of extremely complicated and simultaneously precisely spatially and temporally coordinated steps.Gradual implementation of these steps leads to the formation of a large, highly organized unit – the body of the organism – from a single cell.While regulation of the development of a unicellular organism entails only a limited number of processes of a nongenetic nature, in addition to genetic processes, i.e. temporally phased expression of the individual genes, control of the development of a multicellular organism entails epigenetic processes to a substantial degree, in addition to genetic processes.An epigenetic developmental process is a process whose primary cause lies in genetic processes, but which proceeds and is controlled to a substantial degree independently of these processes.These are most frequently various physiological interactions of cells in the tissue.Cell growth, cell differentiation and mutual movement of cells, tissues and the emerging organs are usually controlled by signals of a physical, chemical or mechanical nature, obtained from the other tissues and organs.In some cases, processes occurring at the level of expression of genetic information participate to various degrees in this type of control.Then, for example, the gradient of a certain substance, called a morphogen, can start up transcription of a particular gene or a group of genes (Gottlieb 1998).However, in other cases, epigenetic regulation of developmental processes can occur quite autonomously of the processes in the nucleus.For example, the morphogen gradient can regulate the activity of an allosteric enzyme and the activity of this enzyme can determine the phenotype of the particular cells directly or through a signal cascade.
Epigenetic regulation mechanisms are active, for example, in controlling the shape of the long bones in vertebrates.The shape of these bones is determined to a substantial degree by the location and order in time of the formation of growth cartilage, where the formation of this cartilage, i.e. its position and temporal sequence of differentiation in the individual parts of the future bones, is also affected by mechanical pressures formed as a consequence of movement of the embryo (Carter, Mikic, & Padian 1998)A case of control of the internal architecture of the trabeculae of hollow bones has been recorded. They are frequently differentiated only in the post-embryonic stage, once again as a consequence of the action of pressure loading of the particular bones (Ho & Saunders 1982). The creation of the entire body architecture in plants in dependence on shading of the individual branches and parts thereof falls in the same category.
Signals controlling the development of the organism can originate either directly in the tissues of the developing organism or in the external environment.Here, the external environment refers to both the abiotic environment and the internal environment of the mother organism where development of the embryo occurs in a great many cases.The participation of signals from the external environment can mean that two embryos with identical genome, formed, for example, by some form of parthenogenesis, developing in different environments or in the bodies of two genetically different mother individuals can develop into two individuals with different phenotypes (Schmid & Dolt 1994).
The importance of epigenetic processes for the development of a multicellular organism can be demonstrated on the following example “from life”.Let’s imagine that the members of an advanced extraterrestrial civilization land on the surface of the Earth.In order to satisfy their tax payers, they must, of course, send back the most interesting living creatures that the encounter on the surface of the Earth (giraffe, fiddler crab, ash tree and Mr. George Brown from West Reading) to their zoological-botanical garden.Transport of living organisms doesn’t come into question because Mr. Brown doesn’t like to travel and Mrs. Brown might notice his absence after some time.In addition, transport would last too long and tax payers are frequently impatient.Thus, our extraterrestrials can only send information on the structures of the individual organisms and reconstruct the individual organisms on their home planet according to this information.They obtain the information by putting the giraffe in a three-dimensional scanner, which records the positions of the individual atoms one after another with a resolution of tenths of a nanometer and then they send the obtained information in the form of a linear sequence of ones and zeros to the director of the zoological garden.He would download the information into his computer, would press the appropriate buttons with his horrible ugly green tentacle and the output equipment, a sort of three-dimensional printer for the A000 format (10m x 10m x 10m) would create an exact copy of the giraffe, once again with a resolution of tenths of a nanometer.It is apparent that the amount of information required for this description of a giraffe, i.e. without using some form of compression of 1044 bytes, would be enormous, incomparably greater than the amount of genetic information in the genome of the giraffe, i.e. the information required and, for the members of an advanced civilization, certainly sufficient for the creation of a giraffe by natural means.If the haploid genome of a giraffe were to have a size similar to that of human beings, 3.109 nucleotides, and if we assume that approximately the same information is stored in epigenetic form (which I consider to be overestimated, but my colleague Anton Markoš will surly appreciate my good will), it would be sufficient to transmit information of the order of 1010 bytes.The entire process of transmission of a giraffe by brute force, i.e. point by point, would thus be about as “economical” as attempting to transmit the picture of a fractal in high resolution (Fig. I.2) in a similar way, i.e. to transmit the relevant picture point by point, rather than to transmit a mathematical formula a quarter of a line long, enabling the construction of the particular fractal in any resolution at all. Nature chose an incomparably more economical route for transfer of information; it does not transfer information on the structure of the organism, but rather on the starting of genetic and especially epigenetic processes, through which the particular structure is spontaneously formed.One more example for illustration.The extraterrestrials tasted meat loaf made of giraffe on the Earth.If they want the director of the zoological gardens to taste this delicacy, they have two possibilities:a) put a slice of meat loaf in the scanner, b) send the director the recipe.Question:Which of these possibilities is more economical from the point of view of the amount of information that must be sent?
In conclusion, a comment on the terminology.Epigenetic processes are very frequently mentioned in the contemporary professional literature primarily in connection with epigenetic heredity.This term denotes the heredity of phenotype traits that is not mediated by information encoded in the nucleotide sequence (see II.8).A very extensive, although certainly not the only and possibly even not the most important mechanism of epigenetic heredity, includes methylation of nucleotides in the DNA and active, long-term retention of the methylated state of the relevant nucleotides in the given cell line by methylation of the newly synthesized chain of replicated DNA at sites where the old chain bears the methylation label.The relevant cell line can, of course, include the germinal cell line, so that the epigenetic information can be passed on from one generation to the next (Roemer et al. 1997).As methylation of the regulation area frequently controls the expression of the relevant genes, transfer of the methylation label is also manifested in transfer of the relevant phenotype properties from one generation to the next.Although the epigenetic heredity of phenotype properties of the individual lines of somatic cells is certainly also active in the embryogenesis of multicellular organisms, the phenomenon of epigenetic heredity is not directly related to the phenomenon of epigenetic developmental processes; these are two different concepts.
Fig. XII.8 Mechanism of formation of innervation of the skeletal muscle. In the first phases of the innervation process (I), the nerve fibres of the individual nerve cells grow between the muscle fibres and form a number of synapses on the surface of the muscle cells in their vicinity (II). In the next phase, the neurons send signals, on the basis of which the muscle cells shrink (III). The synapses on the muscle cells that shrink “spontaneously” from the viewpoint of a particular neuron, in actual fact gradually disappear because of the predominance of signals coming from another neuron. Finally, the system arrives at a state where each muscle fibre has a large number of synapses on its surface, but they are all formed by a single nerve cell (IV).