Neurobiology Unit, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0201, USA; Department of Neurosciences, School of Medicine, University of California, San Diego, La Jolla, CA 92093-0201, USA.
Integr Comp Biol. 2008 Nov;48(5):611-9. doi: 10.1093/icb/icn065. Epub 2008 Jun 28.
The morphology of the vertebrate head is extremely complex and comprises numerous iterative structures that arise from each of the embryonic germ layers. The search for a fundamental plan uniting all of these serial structures spans ∼200 years. The earliest attempt to identify a common plan was J. W. Goethe's vertebral theory of skull organization, in which the skull was interpreted as being formed by a series of trunk vertebrae. This theory was rejected by T. H. Huxley in the 1858 Croonian Lecture and was replaced by the segmented mesodermal model of Francis Balfour, which was elaborated subsequently by A. Marshall, Gavin de Beer, and Edwin Goodrich. This model assumes that the head of the earliest vertebrates consisted of eight segments. It further assumes that each segment contained dorsal muscles arising from the somitic mesoderm, and ventral muscles arising from lateral plate mesoderm, except for the first segment, which lacked ventral muscles derived from the lateral plate mesoderm. The muscles of each head segment were believed to be innervated by two pairs of cranial nerves, homologous to the dorsal and ventral spinal nerves of lampreys. The validity of this theory, known as the Goodrich model, came into question, however, after the discovery that the branchiomeric muscles associated with each pharyngeal arch do not arise from lateral plate mesoderm, as initially proposed by Marshall and subsequently accepted by Goodrich and de Beer, but, rather, arise from paraxial mesoderm. Furthermore, segmentation of the brain into some 14 neuromeres cannot be accommodated by any model involving eight segments. Finally, there is also clear evidence that at least one, if not two, additional series of placodally derived sensory nerves occurs in the head and has no counterpart in the trunk. At present, there is no theory of segmentation that can account for all cephalic iterative structures.
脊椎动物头部的形态极其复杂，包含许多由胚胎的各个胚层衍生而来的迭代结构。寻找一个将所有这些连续结构统一起来的基本模式已经有大约 200 年的历史了。最早试图识别一个共同计划的是 J.W.歌德的颅骨组织的脊椎理论，在这个理论中，颅骨被解释为由一系列躯干椎骨组成。这个理论在 1858 年的克罗恩演讲中被 T.H.赫胥黎所拒绝，并被弗朗西斯·贝尔福的节状中胚层模型所取代，该模型随后由 A.马歇尔、加文·德·比尔和埃德温·古德里奇进一步阐述。这个模型假设最早的脊椎动物的头部由八个节段组成。它进一步假设每个节段都包含来自体节中胚层的背肌，以及来自侧板中胚层的腹肌，除了第一个节段，它缺乏来自侧板中胚层的腹肌。每个头部节段的肌肉被认为是由两对颅神经支配的，这与七鳃鳗的背侧和腹侧脊神经同源。然而，当发现与每个咽弓相关的鳃支肌肉不是像马歇尔最初提出的那样，也不是像古德里奇和德比尔后来接受的那样，来自侧板中胚层，而是来自轴旁中胚层时，这个被称为古德里奇模型的理论的有效性就受到了质疑。此外，将大脑分为大约 14 个神经节也不能被任何涉及 8 个节段的模型所容纳。最后，也有明确的证据表明，至少有一个，如果不是两个，额外的一系列由基板衍生的感觉神经发生在头部，在躯干中没有对应物。目前，没有一个关于节段的理论可以解释所有的头部分批迭代结构。
