The triple origin of skull in higher vertebrates: a study in quail-chick chimeras.

We have used the quail-chick chimera technique to study the origin of the bones of the skull in the avian embryo. Although the contribution of the neural crest to the facial and visceral skeleton had been established previously, the origin of the vault of the skull (i.e. frontal and parietal bones) remained uncertain. Moreover formation of the occipito-otic region from either the somitic or the cephalic paraxial mesoderm had not been experimentally investigated. The data obtained in the present and previous works now allow us to assign a precise embryonic origin from either the mesectoderm, the paraxial cephalic mesoderm or the five first somites, to all the bones forming the avian skull. We distinguish a skull located in front of the extreme tip of the notochord which reaches the sella turcica and a skull located caudally to this boundary. The former ('prechordal skull') is derived entirely from the neural crest, the latter from the mesoderm (cephalic or somitic) in its ventromedial part ('chordal skull') and from the crest for the parietal bone and for part of the otic region. An important point enlighten in this work concerns the double origin of the corpus of the sphenoid in which basipresphenoid is of neural crest origin and the basipostsphenoid is formed by the cephalic mesoderm. Formation of the occipito-otic region of the skeleton is particularly complex and involves the cooperation of the five first somites and the paraxial mesoderm at the hind-brain level. The morphogenetic movements leading to the initial puzzle assembly could be visualized in a reproducible way by means of small grafts of quail mesodermal areas into chick embryos. The data reported here are discussed in the evolutionary context of the 'New Head' hypothesis of Gans and Northcutt (1983, Science, 220, 268-274).