Millet S, Bloch-Gallego E, Simeone A, Alvarado-Mallart R M
INSERM U106, Hôpital de la Salpétrière, Bâtiment de Pédiatrie, Paris, France.
Development. 1996 Dec;122(12):3785-97. doi: 10.1242/dev.122.12.3785.
Segmentation of the neural tube has been clearly shown in the forebrain and caudal hindbrain but has never been demonstrated within the midbrain/hindbrain domain. Since the homeobox-containing gene Otx2 has a caudal limit of expression in this region, we examined, mainly in chick embryos, the possibility that this limit could represent an interneuromeric boundary separating either two cerebellar domains or the mesencephalic and cerebellar primordia. In situ hybridisation with chick or mouse Otx2 probes showed the existence of a transient Otx2-negative area in the caudal mesencephalic vesicle, between stages HH10 and HH17/18 in chick, and at embryonic day 9.5 in mice. The first post-mitotic neurons of the mesencephalon sensu stricto, as labelled with an anti-beta-tubulin antibody, overlay the Otx2-positive neuroepithelium with a perfect match of the caudal limits of these two markers at all embryonic stages analysed (until stage HH20). Chick/quail homotopic grafts of various portions of the midbrain/hindbrain domain have shown that the progeny of the cells located in the caudal mesencephalic vesicle at stage HH10 are found within the rhombomere 1 as early as stage HH14. Furthermore, our results indicate that the cells forming the HH20 constriction (coinciding with the caudal Otx2 limit) are the progeny of those located at the caudal Otx2 limit at stage HH10 (within the mesencephalic vesicle). As a result, the Otx2-positive portion of the HH10 mesencephalic vesicle gives rise to the HH20 mesencephalon, while the Otx2-negative portion gives rise to the HH20 rostral rhombomere 1. Long-survival analysis allowing the recognition of the various grisea of the chimeric brains strongly supports the view that, as early as stage HH10, the caudal limit of Otx2 expression separates mesencephalic from isthmo/cerebellar territories. Finally, this study revealed unexpected rostrocaudal morphogenetic movements taking place between stages HH10 and HH16 in the mediodorsal part of the caudal Otx2-positive domain.
神经管的分段在端脑和后脑尾部已得到明确显示，但在中脑/后脑区域内尚未得到证实。由于含同源异型框的基因Otx2在该区域的表达有一个尾侧界限，我们主要在鸡胚中研究了这个界限是否可能代表一个神经节间边界，该边界将两个小脑区域或中脑和小脑原基分隔开来。用鸡或小鼠的Otx2探针进行原位杂交显示，在鸡胚的HH10和HH17/18阶段之间，以及小鼠胚胎第9.5天，尾侧中脑泡中存在一个短暂的Otx2阴性区域。用抗β-微管蛋白抗体标记的严格意义上的中脑的第一批有丝分裂后神经元，在所有分析的胚胎阶段（直到HH20阶段）都与Otx2阳性神经上皮完美重叠，这两个标记的尾侧界限完全吻合。中脑/后脑区域不同部分的鸡/鹌鹑同位移植表明，HH10阶段位于尾侧中脑泡的细胞后代早在HH14阶段就出现在菱脑节1内。此外，我们的结果表明，形成HH20收缩处（与Otx2尾侧界限一致）的细胞是HH10阶段位于Otx2尾侧界限处（在中脑泡内）的细胞后代。因此，HH10中脑泡的Otx2阳性部分产生了HH20中脑，而Otx2阴性部分产生了HH20头侧菱脑节1。长期存活分析能够识别嵌合脑的各种灰质，有力地支持了这样一种观点，即早在HH10阶段，Otx2表达的尾侧界限就将中脑与峡部/小脑区域分隔开来。最后，这项研究揭示了在HH10和HH16阶段之间，在尾侧Otx2阳性区域的中背部分发生了意外的头尾形态发生运动。
