Luskin M B, Shatz C J
J Comp Neurol. 1985 Dec 22;242(4):611-31. doi: 10.1002/cne.902420409.
The 3H-thymidine method of birth-dating was used to determine when the cells belonging to each of the principal cellular layers of the cat's primary visual cortex are generated. In order to detect systematic differences in the position of radioactively labeled cells following 3H-thymidine administration at different prenatal ages, a geometric method was devised to represent the distribution of labeled cells in the form of depth histograms. Results show that visual cortical neurogenesis occurs largely during the second half of gestation between embryonic day 31 (E31) and E57. Cells of layer 6 are generated early, between E31 and E38, whereas cells destined for successively more superficial layers are generated at progressively later times. Layer 4 cells, the principal targets of geniculocortical afferents, are generated between E37 and E44. In addition, a special population of cells embedded in the white matter below layer 6 was found to be produced throughout the week-long period immediately prior to the onset of layer 6 neurogenesis. Overall, this radial pattern of cortical neurogenesis closely resembles the inside-first, outside-last, spatiotemporal sequence of development described for the monkey's primary visual cortex (Rakic, '74). In addition to finding this pronounced gradient in the radial dimension, we were also able to detect a less pronounced gradient along the tangential dimension: neurons destined for any given layer in the anterior part of the cortex (inferior visual field representation) are generated slightly in advance of neurons destined for more posterior regions (superior visual field). However even our more quantitative histogram analysis failed to reveal a mediolateral (central to peripheral visual field) gradient within area 17. In the cat, layers 6, 5, and 4 each take about a week to be generated, although their total cell numbers and packing densities differ in the adult. About 2 weeks are required to produce the cells of layers 2 and 3 combined. Furthermore, we found that neurons belonging to different layers and different morphological classes can be generated simultaneously. This suggests that the identity of a cortical neuron is not solely a function of the time of neurogenesis.
采用³H-胸腺嘧啶核苷出生时间标记法来确定猫初级视皮层各主要细胞层的细胞是何时产生的。为了检测在不同产前年龄给予³H-胸腺嘧啶核苷后放射性标记细胞位置的系统差异，设计了一种几何方法，以深度直方图的形式表示标记细胞的分布。结果表明，视皮层神经发生主要发生在妊娠后半期，即胚胎第31天（E31）至E57天之间。第6层细胞在E31至E38之间较早产生，而注定要进入逐渐更浅表层的细胞则在逐渐更晚的时间产生。第4层细胞是膝状体皮质传入纤维的主要靶细胞，在E37至E44之间产生。此外，发现在第6层神经发生开始前的整个一周时间内，有一群特殊的细胞嵌入第6层下方的白质中。总体而言，这种皮层神经发生的径向模式与为猴初级视皮层所描述的由内向外、由后向前的时空发育序列非常相似（拉基奇，1974年）。除了在径向维度上发现这种明显的梯度外，我们还能够检测到沿切线维度的不太明显的梯度：皮层前部（下视野代表区）中注定进入任何给定层的神经元比注定进入更后部区域（上视野）的神经元略早产生。然而，即使我们更定量的直方图分析也未能揭示17区内的中外侧（中央视野到周边视野）梯度。在猫中，第6层、第5层和第4层各自大约需要一周时间产生，尽管它们在成体中的总细胞数和堆积密度不同。产生第2层和第3层的细胞总共需要约两周时间。此外，我们发现属于不同层和不同形态类别的神经元可以同时产生。这表明皮层神经元的身份不仅仅是神经发生时间的函数。
