Myogenic specification of somites is mediated by diffusible factors

Specification of myogenesis in early chicken somites and segmental plate was studied using transfilter explant cultures to determine if myogenic specification by axial structures is mediated by cell-cell contact. Formation of muscle fibers that express myosin heavy chain was assessed in somites transfilter from neural tube and notochord. Either the neural tube or the notochord from early chick embryos (ED 2) induces myogenesis in unspecified somites (Hamburger and Hamilton (HH) stages 11-14) when these tissues are separated by a 0.2- or a 0.05-micron pore filter. The ventral neural tube is found to be a strong inducer of myogenesis, while the dorsal neural tube is found to have low inducing activity. The reduced myogenic inducing activity of the dorsal neural tube is associated with an activity that inhibits myogenic differentiation in specified somites, somites containing cells already committed to myogenesis. Only 6-8 hr of transfilter exposure to neural tube is required to initiate somite myogenesis that is sustained in the absence of neural tube. Other tissues such as specified somites, heart, or ectoderm from the same aged embryo do not induce myogenesis in unspecified somites. Removal of the prospective floor plate of the caudal neural plate at HH stage 8 or 9 does not eliminate the inducing activity of the neurectoderm on unspecified somites (HH stage 11-14). Recombination of somites with neural tube or spinal cord from progressively older embryos (ED 4-20) showed myogenesis inducing activity at all ages, though the activity waned as development proceeded. Paraxial mesoderm need not segment into somites to respond to the inducing activity of the neural tube. We conclude that induction of myogenesis in somites does not require cell-cell contact between either the neural tube or the notochord and therefore induction is mediated by diffusible factor(s); that the inducing activity is localized principally to the notochord and ventral half of the neural tube, but not necessarily to the floor plate; that the myogenic response is specific to neural tube and notochord; and that the dorsal neural tube weakly induces myogenesis and contains an activity that inhibits myogenic cells from differentiating.