Horigome N, Myojin M, Ueki T, Hirano S, Aizawa S, Kuratani S
Department of Biology, Okayama University Faculty of Science, 3-1-1 Tsushimanaka, Okayama, 700-8530, Japan.
Dev Biol. 1999 Mar 15;207(2):287-308. doi: 10.1006/dbio.1998.9175.
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.
神经嵴细胞对脊椎动物头部形态发生有广泛贡献，其起源是理解有头类动物头部进化过程中需要解决的重要问题。在现存的无颌类脊椎动物之一——日本七鳃鳗的胚胎中，研究了头部嵴细胞的分布模式。在20.5期时，在神经棒的背侧观察到假定的嵴细胞的最初出现，这些细胞的腹侧扩展首先出现在吻侧体节水平。与有颌类动物一样，头部嵴细胞在表面外胚层下方迁移，并形成三个主要细胞群，每个细胞群在菱脑节（r）3和r5水平处分开。神经嵴最初似乎在除最前端区域外的所有神经轴水平产生，头部嵴细胞随后从r3和r5水平被排除。这种嵴细胞分布模式预示着脑神经原基的形态。第二或中间的嵴细胞群穿过耳囊内侧，这意味着耳囊不作为分隔嵴细胞群的屏障。三个头部嵴细胞群在腹侧填充咽弓，最前端的细胞群覆盖下颌前区域和下颌弓，从侧面覆盖咽中胚层。第三细胞群相当于鸡的环咽嵴细胞，其流入咽区域先于耳后咽弓的形成。DiI的局部注射揭示了在神经胚期神经嵴中存在前后组织，该组织对应于每个咽区域。来自表达LjOtxA基因（Otx2同源基因）的中脑后部的嵴细胞被证明迁移到下颌弓，这一模式与有颌类动物胚胎相同。得出的结论是，七鳃鳗胚胎的头部区域与有颌类动物胚胎具有一组共同的形态特征，并且有颌类动物和七鳃鳗之间下颌弓的形态差异并非基于早期胚胎模式形成。
