FIG. 4.

Expression of slow and fast TnI genes during development. Cross sections of crural muscles from mouse embryos and neonates were probed for the expression of both TnI isoforms by in situ hybridization as described in Materials and Methods. (A and B) TnIs and TnIf transcripts are detected in all muscle fibers at E15. (C and D) By E18, TnIs expression has increased selectively in prospective slow fibers, most notably in the soleus muscle, while beginning to decrease in superficial areas of peripheral fast muscles such as the tibialis. Up to this stage, TnIf transcripts continue to be abundant in all muscles (E and F). By P7, expression of TnIs and TnIf is largely confined to slow and fast muscle, respectively. S, soleus; E, EDL; G, gastrocnemius; T, tibialis.