Fig. 5. Differential regulation between segment border formation and its maintenance. The border formation in nascent somites of embryos of various mespb genotypes was compared in serial horizontal sections. In all embryos (A-E, A'-E') except for the Mesp2-null embryo (F,F'), the initial segmental borders are generated. However, the borders are not maintained in the neo-mespb/– embryo (E,E'). Various levels of border fusion were observed in mespb/– and neo-mespb/neo-mespb embryos. The initial segmental borders are indicated by black arrowheads. Partial fusion between segmented epithelial somites is indicated by green arrowheads. All specimens were prepared at 11.5 d.p.c., but the AP level of these samples are not always same. (A'-F') Higher magnification of the boxed areas in A-F.