*Marine Biology Research Division, Scripps Institution of Oceanography, University of California-San Diego, La Jolla CA 92093-0202, USA; Department of Anatomy, Howard University College of Medicine, 520 W Street NW, Washington, DC 20059, USA.
Integr Comp Biol. 2008 Nov;48(5):630-46. doi: 10.1093/icb/icn060. Epub 2008 Jun 21.
Whether or not the vertebrate head is fundamentally segmented has been controversial for over 150 years. Beginning in the late 19th century, segmentalist theories proposed that the vertebrate head evolved from an amphioxus-like ancestor in which mesodermal somites extended the full length of the body with remnants of segmentation persisting as the mesodermal head cavities of sharks and lampreys. Antisegmentalists generally argued either that the vertebrate ancestors never had any mesodermal segmentation anteriorly or that they lost it before the origin of the vertebrates; in either case, the earliest vertebrates had an unsegmented head and the embryonic cranial mesoderm of vertebrates is at best pseudo-segmented, evolving independently of any pre-vertebrate segmental pattern. Recent morphologic studies have generally confirmed the accuracy of the major classical studies of head development in lampreys and sharks, yet disagree with their theoretical conclusions regarding the evolution of head segmentation. Studies of developmental genes in amphioxus and vertebrates, which have demonstrated conservation of the mechanisms of anterior-posterior patterning in the two groups, have shed new light on this controversy. Most pertinently, some homologs of genes expressed in the anterior amphioxus somites, which form as outpocketings of the gut, are also expressed in the walls of the head cavities of lampreys, which form similarly, and in their major derivatives (the velar muscles) as well as in the eye and jaw muscles of bony gnathostomes, which derive from unsegmented head mesoderm. These muscles share gene expression with the corresponding muscles of the shark, which derive from the walls of head cavities that form, not as outpocketings of the gut, but as secondary cavities within solid blocks of tissue. While molecular data that can be compared across all the relevant taxa remain limited, they are consistent with an evolutionary scenario in which the cranial paraxial mesoderm of the lamprey and shark evolved from the anterior somites of an amphioxus-like ancestor. Although, bony vertebrates have lost the mesodermal head segments present in the shark and lamprey, their remnants persist in the muscles of the eye and jaw.
脊椎动物的头部是否从根本上分段已经争论了 150 多年。从 19 世纪末开始，分段论者提出脊椎动物的头部是从文昌鱼样的祖先进化而来的，在文昌鱼样的祖先中，中胚层体节延伸到身体的全长，分段的残余物作为鲨鱼和七鳃鳗的中胚层头部腔 persists。反分段论者通常认为脊椎动物的祖先在前面没有任何中胚层分段，或者它们在脊椎动物起源之前就失去了它；在任何一种情况下，最早的脊椎动物都没有分段的头部，脊椎动物的胚胎颅中胚层充其量是假分段的，独立于任何前脊椎动物的分段模式进化。最近的形态学研究普遍证实了文昌鱼和鲨鱼头部发育的主要经典研究的准确性，但不同意它们关于头部分段进化的理论结论。文昌鱼和脊椎动物发育基因的研究表明，两组动物的前后模式形成机制是保守的，这为这场争论带来了新的曙光。最相关的是，一些在文昌鱼前体节中表达的基因的同源物，这些基因形成了肠道的外分泌体，也在七鳃鳗的头部腔壁中表达，这些腔壁也以类似的方式形成，以及在它们的主要衍生物（帆状肌肉）以及硬骨颌骨动物的眼睛和颌骨肌肉中表达，这些肌肉是由不分节的头部中胚层衍生而来的。这些肌肉与鲨鱼的相应肌肉具有相同的基因表达，而鲨鱼的肌肉则来自于不是肠道外分泌体形成的头部腔壁，而是组织块中的次生腔。虽然可以在所有相关分类群中比较的分子数据仍然有限，但它们与一个进化情景一致，即七鳃鳗和鲨鱼的颅轴中胚层是从文昌鱼样祖先的前体节进化而来的。尽管硬骨脊椎动物失去了鲨鱼和七鳃鳗中存在的中胚层头部节段，但它们的残余物存在于眼睛和颌骨的肌肉中。
