The fate of the first avian somite.

We have studied the derivatives of the first somite using the quail-chick marking technique. After transplantation of the somite, the chick embryos were reincubated for periods ranging from 4 h to 11 days. Coronal and sagittal sections of the embryos were prepared for parallel staining with Feulgen-reaction, anti-quail antibody, anti-desmin antibody and QH-1 antibody. The first somite consists of an epithelial envelope surrounding somitocoele cells. Like other somites, it forms sclerotome, dermatome and myotome. Cells contribute to the occipital and parasphenoid bone, the meninges, the dermis in the occipital region and the pharyngeal connective tissue. The contribution of the first somite to bones, meninges, dermis and pharyngeal connective tissue is characterised by sharp anterior and posterior boundaries. In contrast, other derivatives such as connective tissue surrounding the vagus nerve, the carotid artery, and jugular vein exceed 10 to 18 segments. This is also true for myogenic cells participating in the formation of the cucullaris capitis muscle that extends from the temporal bone to the shoulder. In one third of the embryos, myocytes of the intrinsic laryngeal muscles are derived from the grafted first somite. Moreover, endothelial cells originate from this somite and migrate into the head (hind-brain, meninges, dermis), neck (pharynx, connective tissue surrounding the vagus nerve, carotid artery and jugular vein) and thorax. With respect to differentiation and derivatives the first somite is similar to other somites.