Cheung Amanda F P, Pollen Alexander A, Tavare Aniket, DeProto Jamin, Molnár Zoltán
Department of Physiology, Anatomy and Genetics, University of Oxford, UK.
J Anat. 2007 Aug;211(2):164-76. doi: 10.1111/j.1469-7580.2007.00769.x. Epub 2007 Jul 17.
The mammalian neocortex consists of six layers. By contrast, the reptilian and avian cortices have only three, which are believed to be equivalent to layers I, V and VI of mammals. In mammals, the majority of cortical cell proliferation occurs in the ventricular and subventricular zones, but there are a small number of scattered individual divisions throughout the cortex. Neurogenesis in the cortical subventricular zone is believed to contribute to the supragranular layers. To estimate the proportions of different forms of divisions in reptiles and birds, we examined the site of proliferation in embryonic turtle (stages 18-25) and chick (embryonic days 8-15) brains using phospho-histone H3 (a G2 and M phase marker) immunohistochemistry. In turtle, only few scattered abventricular H3-immunoreactive cells were found outside the ventricular zone; the majority of the H3-immunoreactive cells were located in the ventricular zone throughout the entire turtle brain. Ventricular zone cell proliferation peaks at stages 18 and 20, before an increase of abventricular proliferation at stages 23 and 25. In turtle cortex, however, abventricular proliferation at any given stage never exceeded 17.5+/-2.47% of the total division and the mitotic profiles did not align parallel to the ventricular zone. Phospho-histone H3 immunoreactivity in embryonic chick brains suggests the lack of subventricular zone in the dorsal cortex, but the presence of subventricular zone in the ventral telencephalon. We were able to demonstrate that the avian subventricular zone is present in both pallial and subpallial regions of the ventral telencephalon during embryonic development, and we characterize the spatial and temporal organization of the subventricular zone. Comparative studies suggest that the subventricular zone was involved in the laminar expansion of the cortex to six layers in mammals from the three-layered cortex found in reptiles and birds. Within mammals, the number of neurons in a cortical column appears to be largely constant; nevertheless, there are considerable differences between the germinal zones in mammalian species. It is yet to be determined whether these elaborations of the subventricular zone may have contributed to cell diversity, tangential expansion or gyrus formation of the neocortex and whether it might have been the major driving force behind the evolution of the six-layered neocortex in mammals.
哺乳动物的新皮层由六层组成。相比之下，爬行动物和鸟类的皮层只有三层，据信这三层与哺乳动物的第I、V和VI层相对应。在哺乳动物中，大多数皮层细胞增殖发生在脑室区和脑室下区，但在整个皮层中也有少量分散的单个细胞分裂。皮层脑室下区的神经发生被认为有助于颗粒上层的形成。为了估计爬行动物和鸟类不同形式分裂的比例，我们使用磷酸化组蛋白H3（一种G2和M期标记物）免疫组织化学方法，研究了胚胎龟（18 - 25期）和鸡（胚胎第8 - 15天）大脑中细胞增殖的部位。在龟中，在脑室区外仅发现少量分散的脑室下区H3免疫反应性细胞；在整个龟脑中，大多数H3免疫反应性细胞位于脑室区。脑室区细胞增殖在18期和20期达到峰值，之后在23期和25期脑室下区增殖增加。然而，在龟皮层中，任何给定阶段的脑室下区增殖从未超过总分裂数的17.5±2.47%，且有丝分裂形态并不与脑室区平行排列。胚胎鸡大脑中的磷酸化组蛋白H3免疫反应性表明，背侧皮层缺乏脑室下区，但腹侧端脑存在脑室下区。我们能够证明，在胚胎发育过程中，鸟类的脑室下区存在于腹侧端脑的 pallial 和 subpallial 区域，并且我们描述了脑室下区的空间和时间组织。比较研究表明，脑室下区参与了哺乳动物皮层从爬行动物和鸟类的三层皮层扩展到六层皮层的过程。在哺乳动物中，一个皮层柱中的神经元数量似乎基本恒定；然而，哺乳动物物种的生发区之间存在相当大的差异。脑室下区的这些精细结构是否可能导致了新皮层的细胞多样性、切向扩展或脑回形成，以及它是否可能是哺乳动物六层新皮层进化背后的主要驱动力，还有待确定。
