Fig. 4. Somites are not formed in the absence of medial PSM cells. (A,E) The type of PSM ablation performed on 15 to 20 somite stage embryos. (B) Dorsal view of an embryo cultured for 9 hours after lateral PSM ablation. The black arrowheads indicate somites formed during the in vitro culture period. The same number of somites is formed both in operated and control sides, although in the former the size of the somites appears smaller. (C) Lateral PSM ablated embryo hybridised with Delta-1. Somites formed in culture express Delta-1 in its normal pattern of expression. (D) Lateral PSM ablated embryo hybridised with Tbx6L present a normal pattern of expression. (F) Dorsal view of an embryo cultured for 9 hours, whose medial PSM has been removed. The brackets indicate the operated side where no somites have been formed in the absence of medial PSM tissue. (G) Medial PSM ablated embryo hybridised with Delta-1 reveals that the remaining lateral PSM does not express the Delta-1 gene and that it is restricted to a more caudal domain, when compared with the control PSM. (H) Medial PSM ablated embryo hybridised with Tbx6L clearly shows that lateral PSM tissue remains in the operated side (arrowhead). Scale bars: 200 µm in B-F; 300 µm in C,D,G,H.