[Problems of evolution of extraembryonic organs]

Complex of extraembryonic organs is a special functional system that is one of the earliest to arise in ontogenesis. Fetal development significantly depends upon successful ontogenesis of its extraembryonic organs, which possess their special evolutionary history. What are concrete mechanisms of "adjustment" of a developing organism to the requirements of an environment, which in many respects is new in comparison with those conditions that existed during the ontogenesis of parental forms? Authors believe that it is very promising to look for their morphological manifestations at the level of placenta which is the collector of homeostatic mechanisms. Taking into considerations the rapid course of noosphere evolution, the determination of the strategy of the evolution of the "mother-extraembryonic organs-fetus" system at all the stages of its formation, remains an urgent issue. However, the problem of interrelations between the extraembryonic organs both during their morphogenesis and, especially, in already formed extraembryonic provisional complex often escapes the attention of researchers. This article presents the examination and analysis of phylogenetic history of formation and integration of extraembryonic organ complex. The problems, perspectives and possible mechanisms of their evolutionary transformations are discussed. It appears that at placental level, the mechanisms of co-adaptation of maternal and fetal organisms are realized by trial-and-error process. Placenta plays a role of molecular peacemaker between these organisms. Specific mechanisms, in our opinion, have a clonal-selection nature, that is based on the phenomenon of an alternative splicing. Morphological equivalent of clonal diversity is associated with an enormous number of variations of morphotypes of cell nuclei at the level of trophoblastic elements with the reduction of this variability during their normal development. However, it must be preserved under conditions of ill-being as a morphological reaction, indicating the continuing search for new clones.