Onai Takayuki, Adachi Noritaka, Kuratani Shigeru
Morphology Laboratory, RIKEN center for Developmental Biology, Chuo-ku, Kobe, Japan.
Int J Dev Biol. 2017;61(10-11-12):621-632. doi: 10.1387/ijdb.170121to.
The vertebrate head characteristically exhibits a complex pattern with sense organs, brain, paired eyes and jaw muscles, and the brain case is not found in other chordates. How the extant vertebrate head has evolved remains enigmatic. Historically, there have been two conflicting views on the origin of the vertebrate head, segmental and non-segmental views. According to the segmentalists, the vertebrate head is organized as a metameric structure composed of segments equivalent to those in the trunk; a metamere in the vertebrate head was assumed to consist of a somite, a branchial arch and a set of cranial nerves, considering that the head evolved from rostral segments of amphioxus-like ancestral vertebrates. Non-segmentalists, however, considered that the vertebrate head was not segmental. In that case, the ancestral state of the vertebrate head may be non-segmented, and rostral segments in amphioxus might have been secondarily gained, or extant vertebrates might have evolved through radical modifications of amphioxus-like ancestral vertebrate head. Comparative studies of mesodermal development in amphioxus and vertebrate gastrula embryos have revealed that mesodermal gene expressions become segregated into two domains anteroposteriorly to specify the head mesoderm and trunk mesoderm only in vertebrates; in this segregation, key genes such as delta and hairy, involved in segment formation, are expressed in the trunk mesoderm, but not in the head mesoderm, strongly suggesting that the head mesoderm of extant vertebrates is not segmented. Taken together, the above finding possibly adds a new insight into the origin of the vertebrate head; the vertebrate head mesoderm would have evolved through an anteroposterior polarization of the paraxial mesoderm if the ancestral vertebrate had been amphioxus-like.
脊椎动物的头部通常呈现出一种复杂的模式，包括感觉器官、大脑、成对的眼睛和颌部肌肉，而脑壳在其他脊索动物中并不存在。现存脊椎动物的头部是如何进化的仍然是个谜。历史上，关于脊椎动物头部的起源有两种相互冲突的观点，即分段观点和非分段观点。根据分段论者的观点，脊椎动物的头部被组织成一种分节结构，由与躯干中的节段相当的节段组成；脊椎动物头部的一个体节被认为由一个体节、一个鳃弓和一组脑神经组成，因为人们认为头部是从类似文昌鱼的祖先脊椎动物的头部前段进化而来的。然而，非分段论者认为脊椎动物的头部不是分段的。在这种情况下，脊椎动物头部的原始状态可能是非分段的，文昌鱼的头部前段可能是后来获得的，或者现存的脊椎动物可能是通过对类似文昌鱼的祖先脊椎动物头部的彻底改造而进化的。对文昌鱼和脊椎动物原肠胚胚胎中胚层发育的比较研究表明，中胚层基因表达仅在脊椎动物中前后分离，以确定头部中胚层和躯干中胚层；在这种分离中，参与节段形成的关键基因，如delta和hairy，在躯干中胚层中表达，但在头部中胚层中不表达，这强烈表明现存脊椎动物的头部中胚层不是分段的。综上所述，上述发现可能为脊椎动物头部的起源增添了新的见解；如果祖先脊椎动物类似文昌鱼，那么脊椎动物的头部中胚层可能是通过轴旁中胚层的前后极化进化而来的。
