1. Mutations in somatic cells, which lead to disturbances in the normal
mechanisms controlling the division of cells.
2. Inducing under the action of physical and chemical mutagenic
factors. To the physical factors different ionizing irradiations can be reffered
— X-ray, γ-ray, UV-light, α- and β-particles, fast neutrons, etc., and to the
chemical ones — some analogues of purine and pyrimidine bases,
alkylating agents, acridines, etc.
3. Transformations with the participation of oncogenic viruses. The
virus theory gained currency since 1912 after the discovery of the first
oncogenic virus of Rous, causing sarcomas in the connective tissue in
birds, the virus SV-40 known for the formation of polyomes in monkeys,
etc. But this theory was not proved because of the unclear mechanisms
of integrating DNA and RNA-containing oncoviruses into the cell genome
and the numerous observed cases of virus presence in them without
causing tumour formations, as well as the unreceptiveness of many cells
to them.
Without ignoring the influence of the factors of surroundings, there is
reason to consider the transforming normal cells of higher organisms
into their neoplasmic analogues to be a process of returning from limited
to unlimited division, which is a norm in unicellular organisms. Thus, the
cells in multicellular organisms “remind” of their phylogenesis, which can
be accepted as a proof in favour of the concept about the origin of the
higher multicellular organisms from unicellular ancestors (Nicolov, 1997,
b).
Noteworthy is the assumption of Bungo Wada (1979) that there exist
genes controlling the appearance and retention of division spindle. For
convenience sake he has marked them by SFI (the spindle formation
inducer gene) and SFR (the spindle repressor gene). Under the
influence of SFI-gene the cells form a division spindle and divide up to
the termination of its activity. If this gene is defective or inhibited under
the influence of SFR-gene, then the cells cannot form a division spindle
and as a result they will not divide, and under certain conditions can turn
into differentiated state. Though hypothetical, the expressed opinion
deserves attention, since it directs the investigations to the discussed
problems of the division spindle, which ultimately determine the
behaviour of cell, i.e. whether to start dividing, to remain undivided and
to die, or to prolong its existence in differentiated state.
On the basis of all stated in this Section, the biological aging and
the neoplasms can be considered to be natural phenomena approved in
the course of the evolution of multicellular organisms. These are two
mutually connected and at the same time opposite processes. The one
of them has emerged as a result of cell specialization and differentiation
that have limited the number of divisions, and the other — returning to
unlimited division, which is a norm in the independently existing