DERMOMYOTOMAL ORIGIN OF THE RIBS AS REVEALED BY EXTIRPATION AND TRANSPLANTATION EXPERIMENTS IN CHICK AND QUAIL EMBRYOS

To elucidate role of the dermomyotome in the formation of the axial sk eleton, we performed extirpation and transplantation experiments on th e dermomyotomes in chick and quail embryos. When the thoracic dermomyo tomes of chick embryos were removed, the intercostal muscles and the d istal ribs were deficient, while the proximal ribs were more or less n ormal. Quail tissues including the dermomyotome, the ectoderm and the medial edge of lateral plate, were transplanted to replace chick dermo myotomes. In these chimeras, the ribs, which would be deficient withou t the back-transplantation, were recovered, The cells of the recovered part of the ribs as well as the intercostal muscles were derived from the quail transplants. These findings suggest that the distal rib ori ginated from the dermomyotomes and not the sclerotome as previously be lieved. To localize the origin of the distal rib further, we removed r estricted regions of the dermomyotomes along the mediolateral rostroca udal axis. The more lateral the part of the dermomyotomes that we remo ved, the more distal the part of the ribs affected. On the contrary, w hen the rostral and caudal edges of the dermomyotomes were removed, on ly the vertebral ribs showed extensive deficiencies while removal of t he middle part between the edges caused less deficiency, The sternal r ibs were not deficient in either case, but were extensively affected w hen the entire lateral edge of dermomyotomes was included in the regio n removed. We conclude that the lateral edges of the dermomyotomes are the primordia of the sternal ribs, and the rostral and/or caudal edge s of the medial part of dermomyotomes are the primordia of the distal part and not of the proximal part of the vertebral ribs.