Migration of LHRH neurons born in the olfactory placode into the forebrain in the chick embryo

(1)用荧光碳菁染料DiI标记鸡胚的嗅觉胎盘上皮细胞，时间为3.5-4.0天。DiI染毒后1天，在嗅觉上皮细胞和嗅神经中首次检测到DiI标记细胞，2~4天后，标记细胞依次出现在前脑吻侧内侧和视前隔区域。DiI标记细胞的分布模式与LHRH神经元非常相似。DiI和LHRH的双重染色表明，DiI标记的细胞共表达LHRH。这些结果为LHRH神经元在发育过程中从嗅觉胎盘向视前隔区的实际迁移提供了证据。(2)在不完全切除胎盘的胚胎中，在嗅觉上皮碎片和NCAM-H阳性的嗅神经残留物中检测到少量表达NCAM-H的LHRH免疫反应(Ir)细胞，这些细胞停止了轴突向前脑的延伸。嗅神经中枢投射缺失导致LHRH-ir细胞停滞，前脑未见LHRH-ir细胞。此外，迁移的LHRH-ir细胞偏离发育不良的嗅神经，迁移到三叉神经眼神经NCAM-H阳性的鼻内侧支。这些结果提示LHRH神经元迁移的结构支持是重要的，LHRH神经元的迁移途径和靶细胞在嗅觉定位的LHRH神经元的前体细胞中并不是完全被编程的。由于迁移的LHRL-ir细胞在体内和体外从来都不依赖于NCAM-H阳性的神经细胞，因此LHRH神经元的迁移可能需要NCAM-H之间的相互作用才能成功迁移。

(1)The epithelial cells of the olfactory placode of the chick embryoswere labeled with a fluorescent carbocyanine dye DiI at embryonic days 3.5-4.0. The DiI-labeled cells were first detected in the olfactory epithelium and olfactory nerve one day after the application of DiI.Two or four days after the dye application, the labeled cells sequentially appeared in the rostral medial forebrain, and in the septo-preoptic area. The distribution pattern of DiI-labeled cells closely resembled that of LHRH neurons. Double staining for DiI and LHRH demonstrated that DiI-labeled cells co-expressed LHRH.These results provide evidence for the actual migration of LHRH neurons from the olfactory placode to the septo-preoptic area during the development.(2)In the embryos with a incomplete placodectomy, a small number of LHRH-immunoreactive (ir) cells expressing NCAM-H were detected in the olfactory epithelial fragments and in the NCAM-H-positive olfactory nerve remnants which ceased to extend their axons to the forebrain. The lack of the central projection of the olfactory nerve caused stagnation of LHRH-ir cells, no LHRH-ir cells being found in the forebrain. Furthermore, the migrating LHRH-ir cells deviated from the poorly developed olfactory nerve and migrated into the NCAM-H-positive medial nasal branch of the ophthalmic nerve of the trigeminal nerve. These results suggest the importance of the structual support for the migration of LHRH neurons, and that the migration route and target of LHRH neurons are not exactly programd in the precursor cells of LHRH neurons in the olfactory placode. Since the migrating LHRL-ir cells were never independent of NCAM-H-positive neural elements in vivo and in vitro, the possible interaction between the migration of LHRH neurons and NCAM-H may be needed for successful LHRH neuronal migration.