Evolutionary perspectives from development of mesodermal components in the lamprey.

Lampreys, a jawless vertebrate species, lack not only jaws but also several other organs, including ventral migratory muscles shared by gnathostomes. In the lamprey embryo, the mesoderm consists primarily of unsegmented head mesoderm, segmented somites, and yet uncharacterized lateral plate mesoderm, as in gnathostomes. Although the adult lamprey possesses segmented myotomes in the head, the head mesoderm of this animal is primarily unsegmented, similar to that in gnathostomes. In the trunk, the large part of lamprey somites is destined to form myotomes, and the Pax3/7 gene expression domain in the lateral part of somites is suggested to represent a dermomyotome homologue. Lamprey myotomes are not segregated by a horizontal myoseptum, which has been regarded as consistent with the apparent absence of a migratory population of hypaxial muscles shared by gnathostomes. However, recent analysis suggests that lampreys have established the gene regulatory cascade necessary for the ventrally migrating myoblasts, which functions in part during the development of the primordial hypobranchial muscle. There have also been new insights on the developmental cascade of lamprey cartilages, in which the Sox family of transcription factors plays major roles, as in gnathostomes. Thus, mesoderm development in lampreys may represent the ancestral state of gene regulatory mechanisms required for the evolution of the complex and diverse body plan of gnathostomes.