Blanton M G, Kriegstein A R
Stanford University Medical Center, California 94305.
J Comp Neurol. 1991 Aug 22;310(4):558-70.
As a starting point for understanding the development of the cerebral cortex in reptiles and for determining how reptilian cortical development compares to that in other vertebrate classes, we studied the appearance and morphological differentiation of cerebral cortical neurons in embryonic turtles. 3H-thymidine birthdate labeling and focal injections of horseradish peroxidase (HRP) in in vitro cortical slices revealed that replicating cells occupy the outer ventricular zone, and subsequently migrate to the ventricular surface where they divide. Postmitotic neurons begin differentiating and elaborating neurites while migrating back through the ventricular zone. On their arrival at the top of the ventricular zone, pyramidal and nonpyramidal neurons can be distinguished morphologically. Cells with multipolar apical dendritic tufts ascending in the marginal zone resemble immature pyramidal neurons. Neurons morphologically similar to these early pyramidal cells were retrogradely labeled by injections of the lipophilic tracer 1,1-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (diI) in a known pyramidal cell target, the thalamus. Nonpyramidal neurons, resembling Cajal-Retzius cells, had horizontally oriented long axons and dendrites coursing in the plexiform primordium, the future marginal zone. With further development morphological differences between cell types became accentuated, and pyramidal cell somata were segregated into a single cellular layer flanked by zones containing predominantly nonpyramidal cells. Axon elaboration occurred early in embryonic development, as pyramidal cells sent axonal branches to the septum, thalamus, and cortical targets soon after their generation, and the intracortical axonal plexus became increasingly dense during embryonic life. Over a similar time course the distribution of projecting neurons labeled by thalamic diI injections changed from an initial homogeneous distribution to a preferential location in the superficial half of the cellular layer. Results from this study demonstrate several features of cortical differentiation that are conserved in reptiles and mammals, including similar early morphological differentiation events, the early distinction of principal cell types, and the parallel development of pyramidal and nonpyramidal neurons. The context in which these similar developmental events occur, however, differs profoundly in reptiles and mammals, with differences in the timing and location of neurite elaboration and differences in the appearance and architectonic organization of the cortex. Comparison of cortical developmental patterns between reptiles and mammals shows that similar functional cortical circuits with balanced excitation and inhibition can emerge in diverse cortical structures.
作为理解爬行动物大脑皮层发育以及确定爬行动物皮层发育与其他脊椎动物类群相比情况的起点，我们研究了胚胎龟大脑皮层神经元的出现和形态分化。在体外皮层切片中进行³H-胸腺嘧啶核苷出生日期标记和辣根过氧化物酶（HRP）的局部注射显示，正在复制的细胞占据外侧脑室区，随后迁移到脑室表面并在那里分裂。有丝分裂后的神经元在通过脑室区向后迁移时开始分化并长出神经突。当它们到达脑室区顶部时，可在形态上区分出锥体细胞和非锥体细胞。在边缘区有多极顶端树突丛向上延伸的细胞类似于未成熟的锥体细胞。通过向已知的锥体细胞靶区丘脑注射亲脂性示踪剂1,1-二辛基-3,3,3',3'-四甲基吲哚羰花青高氯酸盐（diI），对形态上与这些早期锥体细胞相似的神经元进行了逆行标记。非锥体细胞类似于Cajal-Retzius细胞，具有水平取向的长轴突和在丛状原基（未来的边缘区）中走行的树突。随着进一步发育，细胞类型之间的形态差异变得更加明显，锥体细胞的胞体被分隔成一个单层细胞层，两侧是主要包含非锥体细胞的区域。轴突的发育在胚胎发育早期就开始了，因为锥体细胞在产生后不久就将轴突分支发送到隔区、丘脑和皮层靶区，并且在胚胎期内皮层内的轴突丛变得越来越密集。在相似的时间进程中，通过丘脑diI注射标记的投射神经元的分布从最初的均匀分布变为在细胞层浅半部的优先定位。这项研究的结果表明了爬行动物和哺乳动物中皮层分化的几个保守特征，包括相似的早期形态分化事件、主要细胞类型的早期区分以及锥体细胞和非锥体细胞的平行发育。然而，这些相似发育事件发生的背景在爬行动物和哺乳动物中差异很大，在神经突发育的时间和位置以及皮层的外观和结构组织方面存在差异。爬行动物和哺乳动物之间皮层发育模式的比较表明，具有平衡兴奋和抑制的相似功能性皮层回路可以在不同的皮层结构中出现。
