Abstract：
Cephalochordates occupy a key phylogenetic position for deciphering the origin and evolution of chordates, since they diverged earlier than urochordates and vertebrates. The notochord is the most prominent feature of chordates. The amphioxus notochord features coin-shaped cells bearing myofibrils. Notochord-derived hedgehog signaling contributes to patterning of the dorsal nerve cord, as in vertebrates. However, properties of constituent notochord cells remain unknown at the single-cell level. We examined these properties using Iso-seq analysis, single-cell RNA-seq analysis, and in situ hybridization (ISH). Gene expression profiles broadly categorize notochordal cells into myofibrillar cells and non-myofibrillar cells. Myofibrillar cells occupy most of the central portion of the notochord, and some cells extend the notochordal horn to both sides of the ventral nerve cord. Some notochord myofibrillar genes are not expressed in myotomes, suggesting an occurrence of myofibrillar genes that are preferentially expressed in notochord. On the other hand, non-myofibrillar cells contain dorsal, lateral, and ventral Müller cells, and all three express both hedgehog and Brachyury. This was confirmed by ISH, although expression of hedgehog in ventral Müller cells was minimal. In addition, dorsal Müller cells express neural transmission-related genes, suggesting an interaction with nerve cord. Lateral Müller cells express hedgehog and other signaling-related genes, suggesting an interaction with myotomes positioned lateral to the notochord. Ventral Müller cells also expressed genes for FGF- and EGF-related signaling, which may be associated with development of endoderm, ventral to the notochord. Lateral Müller cells were intermediate between dorsal/ventral Müller cells. Since vertebrate notochord contributes to patterning and differentiation of ectoderm (nerve cord), mesoderm (somite), and endoderm, this investigation provides evidence that an ancestral or original form of vertebrate notochord is present in extant cephalochordates.
摘要：
头孢属在破译脊索的起源和进化方面占据着关键的系统发育位置，因为它们比尿囊和脊椎动物更早分化。脊索是脊索的最突出特征。文昌鱼脊索具有带有肌原纤维的硬币形细胞。Notochord衍生的刺猬信号有助于背神经索的图案化，如脊椎动物。然而,组成脊索细胞的特性在单细胞水平上仍然未知。我们使用Iso-seq分析检查了这些性质，单细胞RNA-seq分析，和原位杂交(ISH)。基因表达谱将脊索细胞广泛分类为肌原纤维细胞和非肌原纤维细胞。肌原纤维细胞占据了脊索的大部分中央部分，一些细胞将脊索角延伸到腹侧神经索的两侧。一些脊索肌原纤维基因在肌小体中不表达，提示在脊索中优先表达的肌原纤维基因的发生。另一方面,非肌原纤维细胞含有背侧，横向,和腹侧穆勒细胞，这三个人都表达了刺猬和Brachyury。ISH证实了这一点，尽管刺猬在腹侧Müller细胞中的表达很少。此外,背侧Müller细胞表达神经传递相关基因，表明与神经索的相互作用。外侧Müller细胞表达hedgehog和其他信号相关基因，表明与位于脊索外侧的肌小体相互作用。腹侧Müller细胞还表达了FGF和EGF相关信号的基因，这可能与内胚层的发育有关，腹向脊索。外侧Müller细胞位于背侧/腹侧Müller细胞之间。由于脊椎动物脊索有助于外胚层（神经索）的形成和分化，中胚层（精岩），和内胚层，这项调查提供了证据，证明现存的头颅中存在脊椎动物脊索的祖先或原始形式。
