Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.
Institute of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.
Dev Dyn. 2024 Mar;253(3):283-295. doi: 10.1002/dvdy.657. Epub 2023 Sep 21.
Although vertebrae are the defining character of vertebrates, they are found only in rudimentary form in extant agnathans. In addition, the vertebrae of agnathans possess several unique features, such as elastin-like molecules as the main matrix component and late (post-metamorphosis) differentiation of lamprey vertebrae. In this study, by tracing the developmental process of vertebrae in lamprey, we examined the homology of vertebrae between lampreys and gnathostomes.
We found that the lamprey somite is first subdivided mediolaterally, with myotome cells differentiating medially and non-myotome cells emerging laterally. Subsequently, collagen-positive non-myotome cells surround the myotome. This pattern of somitogenesis is rather similar to that in amphioxi and sheds doubt on the presence of a sclerotome, in terms of mesenchyme cells induced by a signal from the notochord, in lamprey. Further tracing of non-myotome cell development revealed that fin cartilage develops in ammocoete larvae approximately 35 mm in body length. The development of the fin cartilage occurs much earlier than that of the vertebra whose development proceeds during metamorphosis.
We propose that the homology of vertebrae between agnathans and gnathostomes should be discussed carefully, because the developmental process of the lamprey vertebra is different from that of gnathostomes.
尽管脊椎是脊椎动物的特征，但在现存的无颌类动物中只发现了它们的原始形式。此外，无颌类动物的脊椎具有一些独特的特征，例如弹性蛋白样分子作为主要的基质成分，以及七鳃鳗脊椎的后期（变态后）分化。在这项研究中，通过追踪七鳃鳗脊椎的发育过程，我们检查了七鳃鳗和有颌类动物之间脊椎的同源性。
我们发现，七鳃鳗体节首先从中线向两侧分裂，肌节细胞向中线分化，非肌节细胞随后出现在外侧。随后，胶原蛋白阳性的非肌节细胞围绕肌节。这种体节发生模式与八目鳗非常相似，这使得我们对七鳃鳗中由脊索发出的信号诱导的中胚层细胞是否存在体节提出了质疑。对非肌节细胞发育的进一步追踪表明，在体长约 35 毫米的幼鳗中，鳍软骨发育。鳍软骨的发育比在变态过程中发育的脊椎早得多。
我们提出，无颌类和有颌类动物之间的脊椎同源性应该仔细讨论，因为七鳃鳗脊椎的发育过程与有颌类动物不同。
