[A "micromere model" of cellular interactions in sea urchin embryos]

It has been shown that the isolation of sea urchin blastomeres before "pos-division adhesion" leads mainly to the formation of similar blastomeres at the stage of the 4th cleavage division, whereas after adhesion it results in the formation of micromeres simultaneously with intact embryos. Similar results were obtained in five sea urchin species. It has been concluded that there exists a critical point in the cleavage process, when blastomeres exchange information that determines further pattern of cleavage. It has been shown on this "micromere model" that serotonin and its analogues influence the cleavage pattern of half embryos. These data served as a basis for the hypothesis of "protosynapse", a double-sided symmetric structure in which both blastomeres are not only source and a target of the signal but also a passive obstacle for the leakage of the signal substance from the interblastomere cleft to the milieu. Such a structure is also able to provide the primary asymmetry of blastomeres. The micromere model may be useful in specific pharmacological screening.