Somites and the pattern formation of trunk muscles: a study in quail-chick chimera.

The somites are metameric structures. Although these structures appear uniform, other types of muscles differentiate from them along the rostro-caudal body axis. This study investigated whether the somites are committed to producing level specific muscles by heterotopic transplantations between quail and chick embryos. Our results were as follows: 1) Cervical somites, when transplanted into the thoracic or lumbar region, produced the ectopic ventral muscle mass characteristic of the cervical region. Cervical somites did not develop ribs or body wall muscles associated with the rib in the thoracic region. 2) Thoracic somites did not produce the ventral muscle mass at the cervical region, but produced ectopic ribs and muscles associated with them. These muscles were invested by graft-derived fasciae, but did not show the lamination of body wall muscles. Thoracic somites produced ectopic ribs in the lumbar region, but muscles associated with ectopic ribs showed normal lumbar lamination. These muscles were invested by host derived fasciae. From these results, we propose that somites are committed to producing the level specific trunk muscles, but that the arrangement of the muscles is determined by the fasciae.