Johanson Zerina, Sutija Margareta, Joss Jean
Palaeontology, Australian Museum, 6 College Street, Sydney, NSW 2010, Australia.
J Exp Zool B Mol Dev Evol. 2005 May 15;304(3):229-37. doi: 10.1002/jez.b.21048.
Differentiation of the axial skeleton into distinct regions, once thought to be characteristic of the Tetrapoda, also occurs in the actinopterygian Danio rerio. In these taxa, the boundary between the cervical-thoracic regions correlates with Hoxc6 expression and morphological features such as position of the pectoral fin and associated nerves, and the absence of ribs. In the lungfish Neoceratodus, a member of the extant sister taxon to the Tetrapoda, the first vertebral element to chondrify is situated well posterior to the skull, developing from somites 6 and 7 (6/7) and associated with an enlarged cranial rib and nerves innervating the pectoral fin. Two vertebral elements develop later and more anteriorly, associated with somites 4/5 and 5/6. These three elements become incorporated into the occipital region of the skull during Neoceratodus ontogeny, until the cranial rib itself articulates to the rear of the skull. These features of early development indicate a regionalization of the Neoceratodus vertebral column: the cranial rib marks the boundary between the cervical and thoracic regions, the two more anterior vertebrae lacking ribs represent the cervical region, while somites 1-4 (cranial half), lacking any vertebral development, represent the occipital region. However, the cervical region of the vertebral column is effectively lost during ontogeny of Neoceratodus. A recognizable cervical region in the tetrapod vertebral column, as in zebrafish, suggests that cervical vertebrae are not incorporated into the skull but maintained as distinct elements of the column, representing an important shift in relative developmental timing and the influence of heterochrony in this region during the fish-tetrapod transition.
轴骨骼分化为不同区域，这一曾被认为是四足动物特征的现象，也出现在辐鳍鱼纲的斑马鱼中。在这些分类单元中，颈胸区域之间的边界与Hoxc6的表达以及形态特征相关，如胸鳍和相关神经的位置，以及肋骨的缺失。在肺鱼新角齿鱼（Neoceratodus）中，它是现存四足动物姐妹分类单元的成员，第一个软骨化的椎骨元件位于头骨后方很远的位置，由体节6和7（6/7）发育而来，并与一根扩大的颅肋以及支配胸鳍的神经相关。后来又有两个椎骨元件在更靠前的位置发育，与体节4/5和5/6相关。在新角齿鱼个体发育过程中，这三个元件被并入头骨的枕部区域，直到颅肋本身与头骨后部相连。早期发育的这些特征表明新角齿鱼脊柱存在区域化：颅肋标志着颈胸区域之间的边界，前面两个没有肋骨的椎骨代表颈部区域，而体节1 - 4（颅侧半部）没有任何椎体发育，代表枕部区域。然而，在新角齿鱼个体发育过程中，脊柱的颈部区域实际上消失了。在四足动物脊柱中可识别的颈部区域，就像在斑马鱼中一样，表明颈椎没有并入头骨，而是作为脊柱的不同元件保留下来，这代表了在鱼类 - 四足动物过渡过程中，该区域相对发育时间的重要转变以及异时性的影响。
