How multicellular organisms came into the world, happily inhabited by unicellular organisms? Perhaps the explanation is self-organization of individual cells into multicellular agglomerations, Russian biologist believes.

Vsevolod Brodsky, doctor of biological sciences and professor of the Institute of Developmental Biology, suggests that signal molecules – neurotransmitters – that are responsible for behaviour of mammals, could have launched abovementioned mechanism of self-organization. These molecules are serotonin, dopamine and insulin, and their function is coordinating cell behaviour within a population, in other words, making it “social” – cells react on signals from other members of a unicellular “community”.

Recent research showed that noradrenaline and serotonin in mammals still organize intercellular cooperation. Ancient mechanisms of cell self-organization can affect many natural and pathological processes in human tissues during inflammation, healing, tumor growth, ageing and so on.

Individual cells used to inhabit ancient Earth. Bacteria were pioneers, and then came protista – unicellular organisms with nucleus. These organisms synthesized signal factors, which helped cells coordinate their behaviour, for instance, with help of said factors cells synchronized biochemical reactions, respiration or movements. Well-coordinated behaviour of bacteria and protista resulted in formation of temporary multicellular aggregates and multinuclear plasmodia.

Community of interacting unicellular organisms is a very useful adaptation to the environment, which provides easier utilization of food and enhances community resistance to environmental changes. A plasmodium, for instance, can digest a large piece of food, too huge for a single cell to eat. After joint digestion, plasmodium splits into individual cells. However, a myxogastrid Dictyostelium discoideum has a life cycle, in which amoebas obligatory form a multicellular organism. Cell colonies move like an individual organism or form separate groups of cells.

“Long-living” cell communities constantly form bonds between cells and have well-developed intercellular signal system. Sometimes researches spotted cell differentiation, in other words, aggregated interacting cells turned into a multicellular organism with its own regulatory, motor, trophic and protective systems. The main difference of multicellular organisms from temporary multicellular aggregates is that first ones have differentiated cells.

Cells “communicate” by means of a variety of signals: yeasts use acetaldehyde; mucous amoebas – cyclic adenosine monophosphate; mammalian cells – gangliosides. Researchers found signal factors that are common for microbes, protista and mammals – biogenic amines: serotonin, dopamine, insulin and noradrenaline. These amines appeared in cells at the dawn of evolution and regulated biochemical reactions, as well as provided cell-cell interactions. Later these chemical compounds became neuromediators; however, they still regulate cell metabolism, for instance, noradrenaline and serotonin intensify cell aggregation, and dopamine slows down cell growth. Since signal systems, common for all organisms, exist, scientists suggest that self-organization of cell populations can take part in evolution of multicellular organisms.

Despite the fact that multicellular organisms developed a special regulatory system (nervous system), direct intercellular interactions are still very important even in mammalian tissues. For instance, such interactions synchronize protein synthesis in hepatocytes (liver cells) and are responsible for many circahoralian rhythms. The ways inductors operate are sophisticated and sometimes not clear, however, their existence cannot be doubted. A research continues.

Source: Science News

Kizilova Anna