Dunlop S A, Tee L B, Lund R D, Beazley L D
Department of Zoology, The University of Western Australia, Nedlands.
J Comp Neurol. 1997 Jul 21;384(1):26-40.
We have examined the development of retinal projections in a diminutive polyprotodont marsupial, the fat-tailed dunnart, Sminthopsis crassicaudata. Here, we document the most immature mammalian visual system at birth described to date. At postnatal day (P) 0, the retinal ganglion cell layer has yet to form, and axons have not entered the optic stalk. By P4, the retinal ganglion cell layer could be distinguished at the posterior pole, and the front of growing axons extended one-third the length of the optic stalk, a distance of approximately 150 microm; a few pioneer growth cones had grown beyond the main axon group but had still to reach the midline. Axons had decussated at the optic chiasm by P10 to penetrate the base of the contralateral optic tract and, by P15, had reached the dorsal lateral geniculate nucleus (dLGN), superior colliculus (SC), and accessory optic system (AOS); ipsilaterally projecting axons matured slightly later. From P20, axons had reached the caudal SC both contralaterally and ipsilaterally and terminated throughout the depth of the retinorecipient layers. After P30, the projections gradually refined. Within the rostral dLGN, segregation into four contralateral and four ipsilateral bands occurred by P50, approximately 5 days after eye opening. The projection to the ipsilateral SC underwent refinement by P50, becoming restricted to its rostral pole, and presented as discrete patches within the stratum opticum. At birth, the dunnart visual system is comparable to early to midembryonic stages [embryonic day (E) 12, E14, E19, E24, and E30, respectively] in the mouse, rat, ferret, cat, and monkey. The extreme immaturity of the neonatal dunnart together with the observation that the entire development of the primary optic pathway occurs postnatally over a protracted period make this marsupial especially valuable for investigating factors that control pathway formation in the early developing mammalian primary visual system.
我们研究了一种小型多门齿有袋动物——肥尾袋鼬（Sminthopsis crassicaudata）视网膜投射的发育过程。在此，我们记录了迄今为止所描述的出生时最不成熟的哺乳动物视觉系统。在出生后第（P）0天，视网膜神经节细胞层尚未形成，轴突也未进入视柄。到P4时，视网膜神经节细胞层可在后极区分出来，生长轴突的前端延伸至视柄长度的三分之一，约150微米的距离；少数先驱生长锥已生长至主要轴突群之外，但仍未到达中线。轴突在P10时于视交叉处交叉，穿透对侧视束基部，到P15时，已到达背外侧膝状体核（dLGN）、上丘（SC）和副视系统（AOS）；同侧投射的轴突成熟稍晚。从P20开始，轴突在对侧和同侧均已到达尾侧SC，并在视网膜接受层的整个深度终止。P30之后，投射逐渐精细化。在吻侧dLGN内，到P50时，即睁眼后约5天，分离为四条对侧带和四条同侧带。到P50时，对同侧SC的投射精细化，局限于其吻极，并在视层内呈现为离散的斑块。出生时，袋鼬的视觉系统与小鼠、大鼠、雪貂、猫和猴胚胎早期至中期阶段[分别为胚胎第（E）12、E14、E19、E24和E30天]相当。新生袋鼬的极度不成熟，以及初级视路的整个发育过程在出生后长期发生这一观察结果，使得这种有袋动物对于研究控制早期发育的哺乳动物初级视觉系统中通路形成的因素特别有价值。
