Stepien Barbara K, Vaid Samir, Huttner Wieland B
Max Planck Institute of Molecular Cell Biology and Genetics, Max Planck Society (MPG), Munich, Germany.
Institute of Anatomy, Faculty of Medicine Carl Gustav Carus, School of Medicine, Technische Universität Dresden, Dresden, Germany.
Front Cell Dev Biol. 2021 May 13;9:676911. doi: 10.3389/fcell.2021.676911. eCollection 2021.
The neocortex, a six-layer neuronal brain structure that arose during the evolution of, and is unique to, mammals, is the seat of higher order brain functions responsible for human cognitive abilities. Despite its recent evolutionary origin, it shows a striking variability in size and folding complexity even among closely related mammalian species. In most mammals, cortical neurogenesis occurs prenatally, and its length correlates with the length of gestation. The evolutionary expansion of the neocortex, notably in human, is associated with an increase in the number of neurons, particularly within its upper layers. Various mechanisms have been proposed and investigated to explain the evolutionary enlargement of the human neocortex, focussing in particular on changes pertaining to neural progenitor types and their division modes, driven in part by the emergence of human-specific genes with novel functions. These led to an amplification of the progenitor pool size, which affects the rate and timing of neuron production. In addition, in early theoretical studies, another mechanism of neocortex expansion was proposed-the lengthening of the neurogenic period. A critical role of neurogenic period length in determining neocortical neuron number was subsequently supported by mathematical modeling studies. Recently, we have provided experimental evidence in rodents directly supporting the mechanism of extending neurogenesis to specifically increase the number of upper-layer cortical neurons. Moreover, our study examined the relationship between cortical neurogenesis and gestation, linking the extension of the neurogenic period to the maternal environment. As the exact nature of factors promoting neurogenic period prolongation, as well as the generalization of this mechanism for evolutionary distinct lineages, remain elusive, the directions for future studies are outlined and discussed.
新皮层是一种六层神经元的脑结构，出现在哺乳动物进化过程中，为哺乳动物所特有，是负责人类认知能力的高级脑功能所在。尽管其起源相对较近，但即使在亲缘关系密切的哺乳动物物种中，它在大小和折叠复杂性方面也表现出显著的变异性。在大多数哺乳动物中，皮层神经发生在产前进行，其持续时间与妊娠期长度相关。新皮层的进化扩张，尤其是在人类中，与神经元数量的增加有关，特别是在其上层。人们提出并研究了各种机制来解释人类新皮层的进化扩大，尤其关注与神经祖细胞类型及其分裂模式有关的变化，这在一定程度上是由具有新功能的人类特异性基因的出现所驱动的。这些导致祖细胞池大小的扩增，从而影响神经元产生的速率和时间。此外，在早期的理论研究中，还提出了另一种新皮层扩张机制——神经发生期的延长。随后的数学建模研究支持了神经发生期长度在决定新皮层神经元数量方面的关键作用。最近，我们在啮齿动物中提供了实验证据，直接支持延长神经发生以特异性增加上层皮层神经元数量的机制。此外，我们的研究考察了皮层神经发生与妊娠之间的关系，将神经发生期的延长与母体环境联系起来。由于促进神经发生期延长的因素的确切性质，以及这种机制在进化上不同谱系中的普遍性仍然难以捉摸，因此概述并讨论了未来研究的方向。
