Fig. 3. Alterations of Her6 and Her4 expression disrupt paraxial mesoderm segmentation and AP somitic polarity without affecting the specification of A or P somitic compartments. Dorsal view of whole-mounted (A-C,H-J) or flat-mounted embryos at 7-15 ss. Anterior is towards the top. Two-cell stage embryos were injected in one cell only. In all cases, the injected side is on the right, the control non-injected side on the left. Red (D,E,G,K,L,N) or brown (O) staining marks the tracer co-injected with the various mRNAs. (A-C,H-J) Embryos injected with wt-her6 or her6VP16 mRNAs, have abnormal somites with irregular boundaries. Living embryos injected with wt-her6 (A) or her6VP16 (H) and lacZ mRNA as a lineage tracer. (B,I) The same embryos as in A and H, respectively, after fixation and X-gal staining. (C,J) Higher magnification views of the anterior somites of living embryos injected with wt-her6 and her6VP16, respectively. (D,K,O) In embryos injected with wt-her6, her6VP16 and wt-her4, respectively, the segmental expression of myod in the paraxial mesoderm is irregular. (E,F,L,M,P) Expression of A (lfng, papc) and P (ephrinB2) somite compartment markers is disrupted in embryos with altered expression and/or function of Her6 and Her4. Identical abnormalities are observed for lfng in wt-her6-injected embryos (E) and for ephrinB2 in her6VP16-injected embryos (L). Markers are irregularly expressed throughout the segmented paraxial mesoderm. The segmental expression pattern of papc, which is normally restricted to the prospective A compartment of the forming and newly formed somites, is disorganised in embryos injected with wt-her6 (F), her6-VP16 (M) or wt-her4 (P). (G,N) Alterations of Her6 expression or function do not affect the determination of the A somitic compartment. Injection of wt-her6 (G) or her6-VP16 (N) both disrupt the expression pattern of the A compartment specification gene, mespb, without leading to its up- or downregulation compared with the control noninjected side.