Fig. 4. Cardiac cell types in CycA-;CycB-double mutants and in stg mutants. (A-F,I-J) A2-A7 hemisegments of stage 13 embryos are labeled for (A,B,I) Mef2 (green) and Eve (red), (C,D,J) Tinman (red) and Eve (green), or (E,F) Lbe (red) alone. (G,H) Stage 15 embryos labeled for Tinman (red). (A,B) In CycA;CycB double mutants, two instead of six myocardial cells are present in each hemisegment, DA1 muscles but no EPCs are formed. (C,D) Double labeling for Tinman and Eve shows that only one of the two myocardial cells per hemisegment in the CycA;CycB double mutant expresses tinman (D). (E,F) Lbe staining shows that the single tinman-expressing myocardial cells in each hemisegment are not Lbe positive. (G,H) About three tinman-expressing myocardial cells remain in each T3-A1 hemisegment in the mutant (H), but only one in A2-A7. Note that some hemisegments show one TMC cell and one tinman-expressing pericardial cell. (I,J) In stg^- mutants, the overall mesodermal segmentation is significantly affected, in addition to the arrest at cell cycle 14. The Mef2-, tinman- and eve-expressing cells are reduced dramatically. However, the cells that maintain eve expression often appear in pairs and co-express Mef2, but not tinman, indicating that they acquire a myogenic cell fate.

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