Wnt3a diminution and RA activity in the chick tail bud. RA detection using F9-1.8 cells cultured in (A) RA-free media, (B) with 1 nM RA or co-cultured with (C) somitic tissue, (D) tail bud tissue from a HH stage 20, (E) HH stage 22 and (F) HH stage 26. (G) Schematic representation. From HH stage 21 there is an onset of c-Raldh2 in the proximity of the precursor cells located in the chick tail bud. This local RA activity is likely to negatively affect the expression of c-Wnt3a, which subsequently affects the growth of the precursor population and the speed of somite formation, and might also impinge on the observed loss of Notch activity associated with the somitogenesis oscillator. (H-J) Dorsal view of chick embryo analysed by in situ hybridisation using a c-Fgf8 probe. (H) control, (I) exposure to DMSO and (J) 1.5 mM RA soaked beads. (K) Scheme of experiment with RA soaked beads. Red circle shows the position of the DMSO/RA soaked bead in the tail bud; yellow circle represents the DMSO reference bead used as a means of measuring PSM size before and after treatment. (L-M) 1.5-10 mM RA soaked beads in the tail bud (L) slow down the growth of the PSM and (M) reduces the number of somites formed in a defined culture period. Blue bars denote DMSO treated samples and green bars denote RA treated samples. Error bars represent standard deviation. Abbreviations: som (somites) and tb (tail bud).