Inhibition of cleavage and hatching of sea urchin embryos by serotonin

The addition of low levels of serotonin (20–50 μg/ml) to the rearing medium of sea urchin embryos specifically inhibits the hatching process by blocking the synthesis of an active hatching enzyme. Other developmental processes seem to be unaffected. Embryos are sensitive to serotonin during the first three cleavage divisions only. The uptake of RNA and protein precursors, and the rate of RNA synthesis are unaffected by serotonin, whereas the rate of protein synthesis is elevated by approximately 45% during the 2‐cell to 16‐cell interval of development. The addition of higher levels of serotonin, 75 and 100 μg/ml, causes retardation and blockage of cleavage division respectively.