Engrailed Expression during Development of a Lamprey, Lampetra japonica: A Possible Clue to Homologies between Agnathan and Gnathostome Muscles of the Mandibular Arch

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Abstract
Developing lampreys were fixed at frequent intervals between the gastrula stage (6 days) and the earliest ammocoete larva (31 days). Expression of lamprey engrailed (en) gene was studied by labeling with a polyclonal antiserum (αEnhb-1) raised against mouse en protein. Western blotting of proteins from developing lampreys reveals a major band (40±103 Mr ), which is probably lamprey en protein. Expression domains of en were demonstrated in developing lampreys by immunohistochemistry of whole mounts and histological sections. Expression of en first becomes detectable at the head protrusion stage (11/12 days) in neural tube cells at the mid/hindbrain boundary and soon thereafter in some mesodermal cells of the mandibular arch. These en-expressing cells of the mandibular arch are located in the walls of vesicles of paraxial mesoderm that originate by enterocoely on either side of the pharynx. At the tailbud stage (15 days), en expression is also detectable in mesodermal cells of the anterior lip and in some mesodermal and epidermal cells in the region of the tailbud. By the eye spot stage (18 days), detectable en expression in the mandibular arch becomes limited to cells of the velothyroideus muscles, which drive the power stroke of the recently formed velum. At later stages, while the preceding expression domains fade, en expression begins in some epidermal cells associated with the lip papillae, gill slits, and nostril. We suggest that the velothyroideus muscles of lampreys may be homologous to certain jaw muscles of teleosts-namely, the levator arcus palatini and the dilator operculi, which express en continuously while differentiating from the myogenic mesoderm into identifiable muscle types.