Search

Evolution of the vertebrate jaw has been reviewed and discussed based on the developmental pattern of the Japanese marine lamprey, Lampetra japonica. Though it never forms a jointed jaw apparatus, the L. japonica embryo exhibits the typical embryonic structure as well as the conserved regulatory gene expression patterns of vertebrates. The lamprey therefore shares the phylotype of vertebrates, the conserved embryonic pattern that appears at pharyngula stage, rather than representing an intermediate evolutionary state. Both gnathostomes and lampreys exhibit a tripartite configuration of the rostral-most crest-derived ectomesenchyme, each part occupying an anatomically equivalent site. Differentiated oral structure becomes apparent in post-pharyngula development. Due to the solid nasohypophyseal plate, the post-optic ectomesenchyme of the lamprey fails to grow rostromedially to form the medial nasal septum as in gnathostomes, but forms the upper lip instead. The gnathostome jaw may thus have arisen through a process of ontogenetic repatterning, in which a heterotopic shift of mesenchyme-epithelial relationships would have been involved. Further identification of shifts in tissue interaction and expression of regulatory genes are necessary to describe the evolution of the jaw fully from the standpoint of evolutionary developmental biology.

Neural crest cells contribute extensively to vertebrate head morphogenesis and their origin is an important question to address in understanding the evolution of the craniate head. The distribution pattern of cephalic crest cells was examined in embryos of one of the living agnathan vertebrates, Lampetra japonica. The initial appearance of putative crest cells was observed on the dorsal aspect of the neural rod at stage 20.5 and ventral expansion of these cells was first seen at the level of rostral somites. As in gnathostomes, cephalic crest cells migrate beneath the surface ectoderm and form three major cell populations, each being separated at the levels of rhombomeres (r) 3 and r5. The neural crest seems initially to be produced at all neuraxial levels except for the rostral-most area, and cephalic crest cells are secondarily excluded from levels r3 and r5. Such a pattern of crest cell distribution prefigures the morphology of the cranial nerve anlage. The second or middle crest cell population passes medial to the otocyst, implying that the otocyst does not serve as a barrier to separate the crest cell populations. The three cephalic crest cell populations fill the pharyngeal arch ventrally, covering the pharyngeal mesoderm laterally with the rostral-most population covering the premandibular region and mandibular arch. The third cell population is equivalent to the circumpharyngeal crest cells in the chick, and its influx into the pharyngeal region precedes the formation of postotic pharyngeal arches. Focal injection of DiI revealed the existence of an anteroposterior organization in the neural crest at the neurular stage, destined for each pharyngeal region. The crest cells derived from the posterior midbrain that express the LjOtxA gene, the Otx2 cognate, were shown to migrate into the mandibular arch, a pattern which is identical to gnathostome embryos. It was concluded that the head region of the lamprey embryo shares a common set of morphological characters with gnathostome embryos and that the morphological deviation of the mandibular arch between the gnathostomes and the lamprey is not based on the early embryonic patterning., (Copyright 1999 Academic Press.)