The terminal nerve ganglion (TNG) is a well-known structure of the peripheral nervous system in cartilaginous and teleost fishes. It derives from the olfactory placode during embryonic development. While the differentiation and migration of gonadotropin releasing hormone (GnRH)-expressing neurons from the olfactory placode has been well documented, the TNG has been neglected in birds and mammals, and its development is less well described. Here we describe the formation of a ganglion-like structure from migratory olfactory placodal cells in chicken. The TNG is surrounded by neural crest cells, but in contrast to other cranial sensory ganglia, we observed no neural crest corridor, and olfactory unsheathing cells appear only after the onset of neuronal migration. We identified Isl1 and Lhx2 as two transcription factors that label neuronal subpopulations in the forming TNG, distinct from GnRH1⁺ cells, thereby revealing a diversity of cell types during the formation of the TNG. We also provide evidence for extensive apoptosis in the terminal nerve ganglion shortly after its formation, but not in other cranial sensory ganglia. Moreover, at later stages placode-derived neurons expressing GnRH1, Isl1 and/or Lhx2 become incorporated in the telencephalon. The integration of TNG neurons into the telencephalon together with the earlier widespread apoptosis in the TNG might be an explanation why the TNG in mammals and birds is much smaller compared to other vertebrates.

在软骨和硬骨鱼中，末梢神经节（TNG）是周围神经系统的众所周知的结构。它源自胚胎发育过程中的嗅觉平台。尽管已经充分证明了从嗅斑中表达促性腺激素释放激素（GnRH）的神经元的分化和迁移，但在鸟类和哺乳动物中TNG却被忽略了，其发展还缺乏很好的描述。在这里，我们描述了鸡迁徙嗅觉性睑板腺细胞神经节样结构的形成。TNG被神经rest细胞包围，但是与其他颅感觉神经节相反，我们没有观察到神经rest通道，嗅鞘细胞仅在神经元迁移开始后出现。⁺细胞，从而揭示了TNG形成过程中细胞类型的多样性。我们还提供了形成终末神经节后广泛凋亡的证据，而其他颅内感觉神经节则没有。此外，在后期阶段，表达GnRH1，Isl1和/或Lhx2的源于placode的神经元被整合到端脑中。TNG神经元整合到端脑中，以及TNG中较早的广泛凋亡，可能解释了为什么哺乳动物和鸟类中的TNG比其他脊椎动物要小得多。