Royal Cancer Hospital SUMMARY

An analysis was made of the differentiative capabilities of somites isolated from all regions of stage-8 (4-somite) to stage-18 (36-somite) chick embryos, with particular reference to the incidence of spontaneous chondrogenesis. A chorioallantoic grafting technique with Millipore filter as a graft vehicle was employed. The youngest somites which spontaneously differentiated cartilage were the anterior somites of stage 11. Cartilage obtained from grafts of stage 8-10 anterior somites was probably derived from lateral rather than somite mesoderm. Younger, posterior somites consistently failed to differentiate cartilage in spite of the demonstrable viability of the grafts, as evidenced by the appearance of other differentiated derivatives. Increasing the number of non-chondrifying somites grafted, from 4 to 16, did not result in their spontaneous chondrification when all such somites were derived from the same 4-somite region of embryos of the same stage. All embryos from stage 12 to stage 18 showed a gradient of chondrogenic potential along the somite axis, with anterior somites always showing a higher incidence of cartilage than posterior somites of the same stage. Bone and striated muscle derived from somite mesoderm, ganglion cells derived from trunk neural crest, and nephric tubules derived from nephrotome cells were also found in the grafts. Each differentiated type showed a distinct pattern of incidence along the somite axis. The incidences of nephric tubules and ganglion cells were unrelated to those of the differentiated derivatives of somite mesoderm, suggesting that the results reflect true differences of intrinsic differentiative potential