Department of Zoology, School of Natural Sciences & Regenerative Medicine Institute (REMEDI), National University of Ireland, University Road, Galway, Ireland.
Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.
Dev Biol. 2014 May 1;389(1):98-119. doi: 10.1016/j.ydbio.2014.01.019. Epub 2014 Feb 1.
Cranial placodes are evolutionary innovations of vertebrates. However, they most likely evolved by redeployment, rewiring and diversification of preexisting cell types and patterning mechanisms. In the second part of this review we compare vertebrates with other animal groups to elucidate the evolutionary history of ectodermal patterning. We show that several transcription factors have ancient bilaterian roles in dorsoventral and anteroposterior regionalisation of the ectoderm. Evidence from amphioxus suggests that ancestral chordates then concentrated neurosecretory cells in the anteriormost non-neural ectoderm. This anterior proto-placodal domain subsequently gave rise to the oral siphon primordia in tunicates (with neurosecretory cells being lost) and anterior (adenohypophyseal, olfactory, and lens) placodes of vertebrates. Likewise, tunicate atrial siphon primordia and posterior (otic, lateral line, and epibranchial) placodes of vertebrates probably evolved from a posterior proto-placodal region in the tunicate-vertebrate ancestor. Since both siphon primordia in tunicates give rise to sparse populations of sensory cells, both proto-placodal domains probably also gave rise to some sensory receptors in the tunicate-vertebrate ancestor. However, proper cranial placodes, which give rise to high density arrays of specialised sensory receptors and neurons, evolved from these domains only in the vertebrate lineage. We propose that this may have involved rewiring of the regulatory network upstream and downstream of Six1/2 and Six4/5 transcription factors and their Eya family cofactors. These proteins, which play ancient roles in neuronal differentiation were first recruited to the dorsal non-neural ectoderm in the tunicate-vertebrate ancestor but subsequently probably acquired new target genes in the vertebrate lineage, allowing them to adopt new functions in regulating proliferation and patterning of neuronal progenitors.
颅颊类器官是脊椎动物的进化创新。然而，它们很可能是通过重新部署、重新布线和分化预先存在的细胞类型和模式形成机制而进化的。在本综述的第二部分，我们将脊椎动物与其他动物群体进行比较，以阐明外胚层模式形成的进化历史。我们表明，几个转录因子在脊椎动物和无脊椎动物的背腹和前后区域化的外胚层中具有古老的双边作用。来自文昌鱼的证据表明，祖先脊索动物随后将神经分泌细胞集中在前最前端的非神经外胚层中。这个前原颅颊类器官区域随后在前口虹吸原基中产生了被囊动物（失去了神经分泌细胞）和脊椎动物的前（腺垂体、嗅觉和晶状体）颅颊类器官。同样，被囊动物的心房虹吸原基和脊椎动物的后（耳、侧线和咽上）颅颊类器官可能是从被囊动物-脊椎动物祖先的后原颅颊类器官区域进化而来的。由于被囊动物的两个虹吸原基都产生了稀疏的感觉细胞群，因此这两个原颅颊类器官区域可能也在被囊动物-脊椎动物祖先中产生了一些感觉受体。然而，只有在脊椎动物谱系中，适当的颅颊类器官才从这些区域进化而来，这些器官产生高密度排列的专门感觉受体和神经元。我们提出，这可能涉及到 Six1/2 和 Six4/5 转录因子及其 Eya 家族共因子上游和下游的调控网络的重新布线。这些在神经元分化中起古老作用的蛋白质首先被招募到被囊动物-脊椎动物祖先的背侧非神经外胚层，但随后可能在脊椎动物谱系中获得了新的靶基因，使它们能够在调节神经元祖细胞的增殖和模式形成中发挥新的功能。
