ABSTRACT
An experimental study of the morphogenesis of the thoracic cage of the chick embryo: mechanisms and origins
In the chick embryo, the ribs develop at the level extending from somite 19 to somite 26. According to several investigations, only the dorsal third of the body wall originates from the somitic mesenchyme, whereas more recent works have shown that the whole costal and intercostal material is of somitic origin. In the present paper, the origin of the material that forms the body wall as well as the mechanisms of thoracic morphogenesis have been studied.
The evolution of somites 21-22 to 25, grafted into the coelome of a host embryo under different experimental conditions, and the morphogenesis of the thorax after radiodestruction of the same somitic level at different stages have been examined.
The results have shown that:
The whole costal and intercostal material and the abdominal musculature originate from the somitic mesenchyme, whereas the sternum and the associated pectoral musculature originate from the lateral plate.
The morphogenesis of the ribs and the intercostal muscles do not depend upon the lateral plate, nor upon the neural tube and notochord. Ribs and intercostal muscles are formed by a lateral outgrowth of somitic mesenchyme.
The morphogenesis of the sternum along with the associated pectoral muscles and the organogenesis of the thoracic dorsal region are completely independent.
The lateral outgrowth of the somitic mesenchyme that forms the thoracic ribs and the corresponding intercostal muscles starts antero-posteriorly in embryos which have reached the 22-27 somite stages.