Abnormalities of somite development in the absence of retinoic acid

We describe the effects of an absence of retinoic acid (RA) on the developm ent of somites in the quail embryo. RA was removed by generating vitamin A deficient quail embryos whereupon the resulting defects in the embryos can be analysed. The effect on the somites is threefold. Firstly, they are half the size of normal, but the total number of somites is the same as normal. There has therefore been some global regulation event. Secondly, by TUNEL staining and TEM we show that the lateral halves of all of the somites unde rgo apoptosis between stages 11 and 14. This effect is confined to the scle rotome of the somites. Thirdly, some of the genes involved in somite differ entiation are down-regulated such as fgf-4, fgf-8, engrailed and myogenin w hereas others we examined such as cek-8, Delta, follistatin and myf5 are no t affected. These studies reveal remarkably specific effects of RA on devel opmental gene pathways in the embryo.