Summary: The cranial muscle is a critical component in the vertebrate head for a predatory lifestyle.However, its evolutionary origin and possible Roasting Bags segmental nature during embryogenesis have been controversial.In jawed vertebrates, the presence of pre-otic segments similar to trunk somites has been claimed based on developmental observations.However, evaluating such arguments has been hampered by the paucity of research on jawless vertebrates.
Here, we discovered different cellular arrangements in the head mesoderm in lamprey embryos (Lethenteron camtschaticum) using serial block-face scanning electron and laser scanning microscopies.These cell populations were morphologically and molecularly different from somites.Furthermore, genetic comparison among deuterostomes revealed that mesodermal gene expression domains were segregated antero-posteriorly in vertebrates, whereas such segregation was not recognized Blood Pressure - Blood Pressure Units in invertebrate deuterostome embryos.These findings indicate that the vertebrate head mesoderm evolved from the anteroposterior repatterning of an ancient mesoderm and developmentally diversified before the split of jawless and jawed vertebrates.