Attempt to produce a chick/mouse heteroclass musculature

Mouse-chick chimera: an experimental system for study of somite development

As the mammalian embryo is implanted in the uterus and not readily accessible to direct observation or manipulation, much of our understanding of mammalian somite development is based on findings in lower vertebrates. One means of overcoming the difficulties raised by intrauterine development is to engraft mouse tissue in ovo. The experiments described in this chapter relate to the unilateral replacement of somites in chick embryo with those from mouse fetus. Mouse somites differentiate in ovo in dermis, cartilage, and skeletal muscle and are able to migrate into chick host limb. A LacZ transgenic mouse strain was used to ascertain the role of the implanted somites in forming epaxial and hypaxial muscle in the chick embryo. Myogenesis occurred normally in in ovo developing mouse somites, and muscle cells from mouse myotome formed neuromuscular contacts with chick motor axons. After fragments of fetal mouse neural primordium were transplanted into chick embryo, mouse neural tube contributed to the mechanism maintaining myogenesis in the somites of the host embryo. A recently developed double-grafting procedure involving neural tube and somites from knockout mouse strains should elucidate the molecular events involved in early somitogenesis.

Role of the brachial somites in the development of the appendicular musculature in rat embryos

DiI, a fluorescent lipophilic dye, was micro-injected into the brachial somites of 10.5 day rat embryos to determine whether these somites can contribute cells to the development of the fore-limb bud. The injected embryos were cultured and harvested at the 20-25-somite stage. The dye did not interfere with somitogenesis because, at the injection site, the DiI-labelled somites were able to differentiate into dermomyotome and sclerotome. We have analyzed cryo-sections of 20-21-somite stage embryos and were unable detect the presence of DiI-labelled cells in the fore-limb buds. However, at the 22-somite stage, a few DiI-positive cells were found in the proximal region of the limb bud. These labelled cells had migrated into the limb from the lateral border of the dermomyotome. From the 23-somite stage onwards, there were even more DiI-positive cells inside the limb. We have performed an additional set of experiments to confirm that the somitic cells do have the ability to invade and colonize the limb bud. This was achieved by first labelling newly formed somites isolated from the caudal region of 10.5 day embryos with DiI and then grafting them into corresponding regions in 8-11-somite stage hosts. The donor somites were not orientated when they were implanted into the host. However, this did not disrupt their ability to undergo normal somitogenesis. We have detected the presence of DiI-positive cells in the limb buds of approximately 71% of the 19-30-somite stage embryos that have been examined. This is similar to what we obtained for the injected embryos. Nevertheless, there is one slight difference and that is the stage the somitic cells begin their invasion of the limb. For the injected embryos, migration began at the 22-somite stage but in the transplanted embryos, it commenced as early as the 18-somite stage. We have also investigated the myogenic potential of the fore-limb bud at various stages of development to ascertain whether there is a correlation between the stage the somitic cells first appear in the limb bud and the stage the bud acquires the capacity to form skeletal muscles. This was realized by culturing fore-limb buds excised from 18-30-somite stage embryos conventionally and in the kidney capsules of adult rats. In both methods, bone and cartilage were present in all of the cultures whereas skeletal muscles were only present in cultured explants older than the 21-22-somite stage.(ABSTRACT TRUNCATED AT 400 WORDS)