Effects of cytochalasin B on the morphogenesis of explanted early chick embryos.

Effects of cytochalasin B (CB) on chick embryos explanted at stages 4-5 and cultured for 24 hours were studied. CB (2-4 mug/ml) inhibited blastodermal expansion and neural tube closure in over 90% of the embryos. Somite formation was inhibited only where neural tissue was very degenerate. The development of other structures was usually unaffected. Cellular degeneration occurred in severely affected neuroepithelium, but cells at various phases of mitosis were found throughout its thickness, suggesting that interkinetic nuclear migration had been inhibited. Electron microscopic studies of the flattened neural tube showed that CB disrupted microfilaments and reduced the number of cytoplasmic extensions, but had no apparent effect on microtubules and other organelles. These effects were reversible. Futhermore, dimethyl sulfoxide, at 0.2-0.4% (concentrations present in 2-4 mug/ml CB medium), had no adverse effect, indicating that the observed abnormalities were direct consequences of CB treatment.