Stepwise enforcement of the notochord and its intersection with the myoseptum: an evolutionary path leading to development of the vertebra?
Stepwise enforcement of the notochord and its intersection with the myoseptum: an evolutionary path leading to development of the vertebra?
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DOI：
10.1111/j.1469-7580.2006.00618.x
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发表时间：
2006-09-01
影响因子：
2.4
通讯作者：
Totland, Geir K.
中科院分区：
文献类型：
--
作者：
Grotmol, Sindre;Kryvi, Harald;Totland, Geir K.
脊索构成的主要轴向支持在胚胎和幼虫阶段，和胶原纤维的安排内的脊索鞘被认为发挥决定性作用，在确定其功能特性作为纤维缠绕的静水骨架。我们已经发现，在早期个体发育在大西洋鲑鱼逐步发生变化的配置中的胶原纤维缠绕的脊索鞘。鞘由基底层、II型胶原层和界定脊索的外弹性膜组成;这些成分以特定的顺序分泌。最初，胶原原纤维垂直于纵轴周向排列，并且这种特定的空间原纤维构型保持直到当胶原重组成不同的层或薄片时的孵化。在每个薄片内，原纤维彼此平行，围绕脊索的纵轴形成螺旋，与颅尾轴的切线角为75-80度。螺旋几何形状在相邻的薄片之间移动，分别形成对映的左手和右手线圈，从而加强鞘。观察到的纤维缠绕配置的变化可能反映了脊索的适应功能的需求，在个体发育阶段。当椎体最初形成为腱索中心时，椎体区域中鞘的胶原层通过矿物质沉积而固定;然而，在椎间区域中，它们代表了为椎间关节提供扭转稳定性的预适应。因此，鞘的这些修改将脊索本身转变为功能性脊柱。包裹脊索的外弹性纤维具有锯齿状表面，向内连接到鞘的II型胶原，向外连接到脊索周围的间充质结缔组织的I型胶原。以类似的方式，神经和血管弓软骨的胶原基质被紧密地锚定到弹性膜的外表面。因此，弹性膜可以用作脊索和相邻结构之间的界面，具有与从肌肉组织传递张力相关的基本功能。脊索和myosepta之间的相互联系进行了讨论的功能和演变的弓和脊椎。与目前的理解相反，这项研究还表明，脊索空泡化不会导致胚胎延长，这与II型胶原蛋白的圆形排列一致，II型胶原蛋白可能只会在空泡化后限制周长的增加，而不会帮助延长。由于空泡化发生在卵期，这种类型的胶原蛋白沉积，与弹性外膜结合，也可能促进胚胎的柔韧性和卷曲。
The notochord constitutes the main axial support during the embryonic and larval stages, and the arrangement of collagen fibrils within the notochord sheath is assumed to play a decisive role in determining its functional properties as a fibre-wound hydrostatic skeleton. We have found that during early ontogeny in Atlantic salmon stepwise changes occur in the configuration of the collagen fibre-winding of the notochord sheath. The sheath consists of a basal lamina, a layer of type II collagen, and an elastica externa that delimits the notochord; and these constituents are secreted in a specific order. Initially, the collagen fibrils are circumferentially arranged perpendicular to the longitudinal axis, and this specific spatial fibril configuration is maintained until hatching when the collagen becomes reorganized into distinct layers or lamellae. Within each lamella, fibrils are parallel to each other, forming helices around the longitudinal axis of the notochord, with a tangent angle of 75-80 degrees to the cranio-caudal axis. The helical geometry shifts between adjacent lamellae, forming enantiomorphous left- and right-handed coils, respectively, thus enforcing the sheath. The observed changes in the fibre-winding configuration may reflect adaptation of the notochord to functional demands related to stage in ontogeny. When the vertebral bodies initially form as chordacentra, the collagen lamellae of the sheath in the vertebral region are fixed by the deposition of minerals; in the intervertebral region, however, they represent a pre-adaptation providing torsional stability to the intervertebral joint. Hence, these modifications of the sheath transform the notochord per se into a functional vertebral column. The elastica externa, encasing the notochord, has serrated surfaces, connected inward to the type II collagen of the sheath, and outward to type I collagen of the mesenchymal connective tissue surrounding the notochord. In a similar manner, the collagen matrix of the neural and haemal arch cartilages is tightly anchored to the outward surface of the elastic membrane. Hence, the elastic membrane may serve as an interface between the notochord and the adjacent structures, with an essential function related to transmission of tensile forces from the musculature. The interconnection between the notochord and the myosepta is discussed in relation to function and to evolution of the arches and the vertebra. Contrary to current understanding, this study also shows that notochord vacuolization does not result in an increased elongation of the embryo, which agrees with the circular arrangement of type II collagen that probably only enables a restricted increase in girth upon vacuolization, not aiding elongation. As the vacuolization occurs during the egg stage, this type of collagen disposition, in combination with an elastica externa, also probably facilitates flexibility and curling of the embryo.