Worm Breeder's Gazette 8(1): 16
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
It has been shown that vertebrate skeletal muscle cells, which fuse, are post-mitotic before any contractile protein expression is detected. However, heart muscle cells, which do not fuse, are known to undergo one or two rounds of mitosis after the contractile proteins are expressed. In the nematode Caenorhabditis odywall muscle cells do not fuse. However, independent studies of embryonic cell lineages (Sulston et al., pers. comm.) indicate that the muscle lineage cells may have reached their terminal division at or shortly before the stage when muscle specific proteins in these cells are detected immunocytochemically (4.5 hours after the 2-cell stage, Gossett et al., Cell 30:193, 1982). We therefore wanted to know, are these immunoreactive cells, in fact, post-mitotic? The answer to this question was facilitated by the availability of a monoclonal antibody that recognizes only cells in mitosis (Davis et al., PNAS 80:2926, 1983), which are otherwise too small to detect directly. Wild-type embryos were incubated for 4.5 hours at 25 C, placed on ice for 3 hours to arrest cells at mitosis, then squashed, fixed and stained. Some cells which stain for myosin also stain for the mitotic proteins. This indicates that differentiated nematode body-wall muscle cells, like vertebrate heart muscle cells, are not necessarily post-mitotic. Similar results were obtained with the embryonic arrest mutant emb-29 (b262), which arrests in G2 as early as the 150 cell stage at restrictive temperature (Hecht et al., these abstracts). Although cell division is arrested and no apparent endoreplication is observed, the ancestral body-wall precursor cells retain the potential to express myosin in cells capable of resuming mitosis. This expression still begins 4.5 hours beyond the 2-cell stage, which indicates that muscle specific protein expression is timed by events uncoupled from nuclear and cellular division.