Neural Development Group, Division of Cell & Developmental Biology, College of Life Science, University of Dundee, Dundee DD1 5EH, Scotland, UK.
BMC Neurosci. 2011 May 9;12:37. doi: 10.1186/1471-2202-12-37.
The vertebrate peripheral nervous system contains sensory neurons that arise from ectodermal placodes. Placodal cells ingress to move inside the head to form sensory neurons of the cranial ganglia. To date, however, the process of placodal cell ingression and underlying cellular behavior are poorly understood as studies have relied upon static analyses on fixed tissues. Visualizing placodal cell behavior requires an ability to distinguish the surface ectoderm from the underlying mesenchyme. This necessitates high resolution imaging along the z-plane which is difficult to accomplish in whole embryos. To address this issue, we have developed an imaging system using cranial slices that allows direct visualization of placode formation.
We demonstrate an effective imaging assay for capturing placode development at single cell resolution using chick embryonic tissue ex vivo. This provides the first time-lapse imaging of mitoses in the trigeminal placodal ectoderm, ingression, and intercellular contacts of placodal cells. Cell divisions with varied orientations were found in the placodal ectoderm all along the apical-basal axis. Placodal cells initially have short cytoplasmic processes during ingression as young neurons and mature over time to elaborate long axonal processes in the mesenchyme. Interestingly, the time-lapse imaging data reveal that these delaminating placodal neurons begin ingression early on from within the ectoderm, where they start to move and continue on to exit as individual or strings of neurons through common openings on the basal side of the epithelium. Furthermore, dynamic intercellular contacts are abundant among the delaminating placodal neurons, between these and the already delaminated cells, as well as among cells in the forming ganglion.
This new imaging assay provides a powerful method to analyze directly development of placode-derived sensory neurons and subsequent ganglia formation for the first time in amniotes. Viewing placode development in a head cross-section provides a vantage point from which it is possible to study comprehensive events in placode formation, from differentiation, cell ingression to ganglion assembly. Understanding how placodal neurons form may reveal a new mechanism of neurogenesis distinct from that in the central nervous system and provide new insight into how cells acquire motility from a stationary epithelial cell type.
脊椎动物的周围神经系统包含源自外胚层基板的感觉神经元。基板细胞进入头部内部，形成颅神经节的感觉神经元。然而，到目前为止，基板细胞进入的过程和潜在的细胞行为还知之甚少，因为这些研究依赖于对固定组织的静态分析。要观察基板细胞的行为，需要能够区分表面外胚层和下面的间质。这就需要在 z 平面上进行高分辨率成像，而这在整个胚胎中很难实现。为了解决这个问题，我们开发了一种使用颅切片的成像系统，可以直接观察基板的形成。
我们展示了一种有效的成像测定法，用于在单细胞分辨率下捕获鸡胚组织外显体的基板发育。这提供了三叉神经基板外胚层有丝分裂、基板细胞进入和细胞间接触的首次延时成像。在基板外胚层的整个顶端-基底轴上都发现了具有不同方向的细胞分裂。基板细胞在进入时最初具有短的细胞质突起，作为年轻神经元，随着时间的推移成熟，在间质中形成长的轴突突起。有趣的是，延时成像数据显示，这些正在分层的基板神经元很早就从外胚层内部开始进入，在那里它们开始移动，并继续作为单个或成串的神经元通过上皮细胞基底侧的共同开口离开。此外，在正在分层的基板神经元之间、这些神经元与已经分层的细胞之间以及在正在形成的神经节中，细胞间的动态接触非常丰富。
这种新的成像测定法为首次在羊膜动物中直接分析基板衍生的感觉神经元的发育和随后的神经节形成提供了一种强大的方法。从头部横切面观察基板的发育，可以提供一个有利的视角，从这个视角可以研究基板形成的全面事件，从分化、细胞进入到神经节组装。了解基板神经元的形成方式可能会揭示一种不同于中枢神经系统的新的神经发生机制，并为细胞如何从静止的上皮细胞类型获得运动性提供新的见解。
