Mediolateral semitic origin of ribs and dermis determined by quail-chick chimeras

Somites are transient mesodermal structures giving rise to all skeletal mus cles of the body, the axial skeleton and the dermis of the back. Somites ar ise from successive segmentation of the presomitic mesoderm (PSM), They app ear first as epithelial spheres that rapidly differentiate into a ventral m esenchyme, the sclerotome, and a dorsal epithelial dermomyotome. The sclero tome gives rise to vertebrae and ribs while the dermomyotome is the source of all skeletal muscles and the dorsal dermis, Quail-chick fate mapping and diI-labeling experiments have demonstrated that the epithelial somite can be further subdivided into a medial and a lateral moiety, These two subdoma ins are derived from different regions of the primitive streak and give ris e to different sets of muscles. The lateral semitic cells migrate to form t he musculature of the limbs and body wall, known as the hypaxial muscles, w hile the medial somite gives rise to the vertebrae and the associated epaxi al muscles, The respective contribution of the medial and lateral semitic c ompartments to the other semitic derivatives, namely the dermis and the rib s has not been addressed and therefore remains unknown. We have created qua il-chick chimeras of either the medial or lateral part of the PSM to examin e the origin of the dorsal dermis and the ribs. We demonstrate that the who le dorsal dermis and the proximal ribs exclusively originates from the medi al semitic compartment, whereas the distal ribs derive from the lateral com partment.