Miller B, Chou L, Finlay B L
Department of Psychology, Cornell University, Ithaca, New York 14853.
J Comp Neurol. 1993 Sep 1;335(1):16-41. doi: 10.1002/cne.903350103.
The early development of thalamocortical and corticothalamic projections in hamsters was studied to compare the specificity and maturation of these pathways, and to identify potential sources of information for specification of cortical areas. The cells that constitute these projections are both generated prenatally in hamsters and they make reciprocal connections. Fluorescent dyes (DiI and DiA) were injected into the visual cortex or lateral geniculate nucleus in fixed brains of fetal and postnatal pups. Several issues in axonal development were examined, including timing of axon outgrowth and target invasion, projection specificity, the spatial relationship between the two pathways, and the connections of subplate cells. Thalamic projections arrive in the visual cortex 2 days before birth and begin to invade the developing cortical plate by the next day. Few processes invade inappropriate cortical regions. By postnatal day 7 their laminar position is similar to mature animals. By contrast, visual cortical axons from subplate and layer 6 cells reach posterior thalamus at 1 day after birth in small numbers. By 3 days after birth many layer 5 cell projections reach the posterior thalamus. On postnatal day 7, there is a sudden increase in the number of layer 6 projections to the thalamus. Surprisingly, these layer 6 cells are precisely topographically mapped with colabeled thalamic afferents on their first appearance. Subplate cells constitute a very small component of the corticothalamic projection at all ages. Double injections of DiI and DiA show that the corticofugal and thalamocortical pathways are physically separate during development. Corticofugal axons travel deep in the intermediate zone to the thalamic axons and are separate through much of the internal capsule. Their tangential distribution is also distinct. The early appearance of the thalamocortical pathway is consistent with an organizational role in the specification of some features of cortical cytoarchitecture. The specific initial projection of thalamocortical axons strongly suggests the recognition of particular cortical regions. The physical separation of these two pathways limits the possibility for exchange of information between these systems except at their respective targets.
研究了仓鼠丘脑皮质和皮质丘脑投射的早期发育，以比较这些通路的特异性和成熟度，并确定用于指定皮质区域的潜在信息来源。构成这些投射的细胞在仓鼠出生前就已产生，并且它们形成相互连接。将荧光染料（DiI和DiA）注入胎儿和出生后幼崽固定大脑的视觉皮层或外侧膝状体核。研究了轴突发育中的几个问题，包括轴突生长和靶标侵入的时间、投射特异性、两条通路之间的空间关系以及板下层细胞的连接。丘脑投射在出生前两天到达视觉皮层，并在第二天开始侵入发育中的皮质板。很少有突起侵入不适当的皮质区域。到出生后第7天，它们在层内的位置与成熟动物相似。相比之下，来自板下层和第6层细胞的视觉皮质轴突在出生后1天少量到达丘脑后部。到出生后3天，许多第5层细胞的投射到达丘脑后部。在出生后第7天，投射到丘脑的第6层细胞数量突然增加。令人惊讶的是，这些第6层细胞在首次出现时就与标记的丘脑传入纤维精确地进行了拓扑映射。在所有年龄段，板下层细胞在皮质丘脑投射中只占非常小的一部分。DiI和DiA的双重注射表明，在发育过程中皮质离心和丘脑皮质通路在物理上是分开的。皮质离心轴突在中间带深处通向丘脑轴突，并在大部分内囊中分开。它们的切线分布也不同。丘脑皮质通路的早期出现与在皮质细胞构筑某些特征的指定中起组织作用一致。丘脑皮质轴突的特定初始投射强烈表明对特定皮质区域的识别。这两条通路在物理上的分离限制了这些系统之间除了在各自靶标处之外进行信息交换的可能性。
