A description of caudal migration during growth leading to the formation of the pericardial and pleural coeloms, to caudal movement of the aortic arches, and to development of the shoulder.
Formation of the shoulder in the chick embryo results from movement of cells into the base of the wing from more cranial regions. Between stage 23 and stage 27, the lateral body wall overlying the pleural coelom is caused to move into the base of the wing because the cranial margin of the pleural coelom moves from the level of the tenth somite to the level of the 16th somite. Thus, formation of the shoulder is related to the formation of the pleural coelom. Textbook descriptions of the formation of the pleural coelom did not permit this relationship to be delineated. Therefore, investigation of the relationship between the formation of the shoulder and the formation of the pleural coelom seemed warranted. At stage 12 the coelom is lateral to the gut from the level of the second somite to the level of the 16th somite and is continuous with the extraembryonic coelom. During stage 13 the coelom obtains a ventral margin at the level of the third and fourth somites as the heart fuses with the lateral body wall to form the lateral mesocardium. Between stage 16 and stage 23, the second (cervical) flexure develops, causing the lateral mesocardium to form a ventral margin of the coelom from the level of the first to the 17th somites. Between stage 16 and stage 23, the coelom becomes ventral to the pharynx and lung to the level of the ninth somite. From stage 23 to stage 27, the lateral body wall from the cranial margin of the lung to the caudal margin of the hyoid arch moves in a caudal direction. During stage 27, the lateral body wall fuses with the lateral mesocardium to form the pleuro-pericardial membrane. The wing remains ventral to the 16th to 19th somites, causing a pileup of cells at the cranial base of the wing, the shoulder.
D000818:Animals; D001013:Aorta, Thoracic / Q000196:embryology*; D002465:Cell Movement; D002642:Chick Embryo / Q000033:anatomy & histology / Q000254:growth & development*; D010496:Pericardium / Q000196:embryology*; D010994:Pleura / Q000196:embryology*; D012782:Shoulder / Q000196:embryology*