FitzGibbon T, Reese B E
Department of Human Anatomy, University of Oxford, United Kingdom.
J Comp Neurol. 1992 Sep 8;323(2):153-66. doi: 10.1002/cne.903230203.
Optic axons are added to the retinal nerve fibre layer of fish along its vitreal border in a chronotopic manner. Likewise, the optic tract of all vertebrate species acquires axons preferentially along the superficial surface of the pathway. We have examined the developing retina of fetal ferrets (Mustela putorius furo) aged between embryonic day 27 (E27) and E34 to see whether a similar segregation of growth cones is apparent within the mammalian retinal nerve fibre layer. The distributions of growth cone, "wrist" (thick trailing portion of the growth cone), axonal, and glial profiles were determined from electron micrographs, and expressed as a percentage of neural profiles for several retinal locations. The retinal nerve fibre layer of fetal ferrets contains radially elongated bundles of fibres composed of axonal, wrist, and growth cone profiles. Glial processes of varying density divide the adjacent bundles, occasionally subdividing them in the plane of the retina, and give rise to endfeet lining the basal lamina and separating the optic axons from the latter. Growth cones within the developing fibre layer represented about 2.4% of profiles at E28, while at later developmental stages (E34), this value fell to about 0.6%. During this period of axonal outgrowth, growth cones were not preferentially segregated toward the vitreal basal lamina or the glial endfeet within the nerve fibre layer. Rather, they were found scattered throughout the axon bundles of the fibre layer. While there were differences in the proportion of immature profiles found within the vitreal half compared to the scleral half of the fibre layer, such that more growth cones and wrists were found vitreally, there was no clear accumulation of them in association with features of the vitreal margin. The present results show that young and old optic axons course together throughout the depth of the nerve fibre layer. A chronotopic mode of pathway genesis such as seen in the optic fibre layer of fish or in the optic tract of mammals is not present in the nerve fibre layer of ferrets. Differences in growth cone behaviour in the optic fibre layer and tract indicate that the mechanisms governing pathway formation differ along its course.
视轴以时间顺序排列的方式沿着玻璃体边界添加到鱼类视网膜神经纤维层。同样，所有脊椎动物物种的视束优先沿着通路的表面获得轴突。我们研究了胚胎第27天（E27）至E34天的胎儿雪貂（Mustela putorius furo）发育中的视网膜，以观察在哺乳动物视网膜神经纤维层内是否有类似的生长锥分离现象。从电子显微镜照片中确定生长锥、“腕部”（生长锥的粗大拖尾部分）、轴突和神经胶质形态的分布，并将其表示为几个视网膜位置神经形态的百分比。胎儿雪貂的视网膜神经纤维层包含由轴突、腕部和生长锥形态组成的径向拉长的纤维束。不同密度的神经胶质突起将相邻的纤维束分开，偶尔在视网膜平面内将它们细分，并产生衬于基膜的终足，将视轴突与后者分开。发育中的纤维层内的生长锥在E28时约占形态的2.4%，而在发育后期（E34），该值降至约0.6%。在轴突生长的这段时间里，生长锥在神经纤维层内并没有优先向玻璃体基膜或神经胶质终足分离。相反，它们散布在纤维层的轴突束中。虽然在纤维层的玻璃体半侧与巩膜半侧发现的未成熟形态比例存在差异，使得在玻璃体侧发现更多的生长锥和腕部，但它们并没有与玻璃体边缘的特征明显聚集。目前的结果表明，年轻和年老的视轴突在神经纤维层的整个深度一起走行。在雪貂的神经纤维层中不存在像在鱼类视纤维层或哺乳动物视束中看到的那种时间顺序排列的通路发生模式。视纤维层和视束中生长锥行为的差异表明，控制通路形成的机制在其过程中是不同的。
