Cell movement in somite formation and development in the chick: inhibition of segmentation.

The contribution of active cell movement to somite formation (segmentation) and the later dispersal of the somite sclerotome was examined using cytochalasin D (CD). Stage 14-16 chick embryos were grown over liquid medium. After 8 hr in culture, control embryos had an average of six additional pairs of somites while CD (l-2 *g/ml dissolved in DMSO)-treated embryos had no new somites. DMSO alone had no effect on somitogenesis. CD-treated embryos transferred to drug-free medium recovered and segmentation resumed. Normal and CD-treated segmental plates were examined by SEM. Drug-treated segmental plate cells rounded up, consistent with the interaction of CD on contractile microfilaments. Embryos cultured 8 hr with or without CD were fractured through somite pair 20 and examined by SEM. In untreated embryos the sclerotome had dispersed and was migrating toward the notochord. CD stopped sclerotome dispersal. To test whether CD interfered with elaboration of extracellular matrix material associated with somite development, incorporation of [3Hjglucosamine and Na2%301 by somites and segmental plate was determined. There was no difference in total label incorporation. Molecular-weight profiles of proteoglycan obtained using controlled-pore glass-bead columns showed only small proteoglycans for both treated and control tissues. Therefore, the alteration of segmentation and somite morphogenesis by CD was not due to detectable changes in proteoglycan synthesis.