Serotonin distribution in early Ophryotrocha embryos was investigated with fluorescence microscopy based on formaldehyde gas treatment of the embryos, and with light- and electron-microscopic autoradiography after the embryos had been treated withH-5-hydroxytryptophan.Sections of early cleavage embryos showed serotonin-specific fluorescence all over the blastomeres, but it was mainly concentrated on yolk granules, and to a lesser degree on lipid drops and vacuoles. In 2-8 cell embryos, marked regional concentration of serotonin fluorescence was noticeable along the completed cleavage furrows.The autoradiographs confirmed the picture of the yolk granules as the principal site of serotonin formation and serotonin accumulation; considerable amounts were also associated with their decomposition products, i.e. lipid drops, vacuoles, and vesicles, whereas major cell organelles, e.g. mitochondria, were almost totally lacking. Of cytoplasmic structures in the blastomeres without apparent yolk granule origin, only microfilaments, particularly those amassed along the cleavage furrow, showed consistent and significant association with formed serotonin. This suggests a connexion between serotonin and microfilaments and might imply that in early embryo cells the fundamental contractile machinery is controlled by serotonin gradually released from the yolk granules.Within the blastomere nuclei, moderate amounts of serotonin were demonstrated with both fluorescence microscopy and autoradiography.The monoamine oxidase (MAO) inhibitor catron (phenylisopropylhydrazine), used to intensify the autoradiographic picture of serotonin in the Ophryotrocha embryos, markedly increased intragranular serotonin accumulation, but also retarded yolk granule disintegration and delayed the cell cleavage process. In embryos barely able to cleave after treatment with catron, ultrastructural analysis demonstrated that membrane formation at cell cleavage depends on influx of material from the nearby disintegrating yolk granules.
