Development of human brain neurons The earliest stages of human brain development are very difficult to monitor, but using induced pluripotent stem cells (iPSCs) can help to elucidate the process. Real et al. transplanted neural progenitors derived from human iPSCs into the brains of adult mice. They used intravital imaging to visualize how resulting neurons grew and connected. The human cells produced neurons that integrated and developed synaptic networks with oscillatory activity. Dendritic pruning was observed and involved a process of branch retraction, not degeneration. Cells derived from individuals with Down syndrome, upon transplantation into the mouse brain, produced neurons that grew normally but showed reduced dendritic spine turnover and less network activity. Science, this issue p. eaau1810 Transplantation of human neuronal progenitors into the mouse brain opens a window into normal and abnormal brain development. INTRODUCTION Scientists are building detailed maps of the cellular composition in the human brain to learn about its development. In the human cortex, the largest area of the mammalian brain, neural circuits are formed through anatomical refinement, including axon and synaptic pruning, and the emergence of complex patterns of network activity during early fetal development. Cellular analyses in the human brain are restricted to postmortem material, which cannot reveal the process of development. Model organisms are, therefore, commonly used for studies of brain physiology, development, and pathogenesis, but the results from model organisms do not always translate to humans. RATIONALE Systems to model human neuron dynamics and their dysfunction in vivo are needed. While biopsy specimens and the generation of neurons from induced pluripotent stem cells (iPSCs) could provide the necessary human genetic background, two- and three-dimensional cultures lack factors that normally support neuronal development, including blood vessels, immune cells, and interaction with innervating neurons from other brain areas. On the basis of previous stem cell transplantation studies in mice, we reasoned that the physiological microenvironment of the adult mouse brain could support the growth of human cortical tissue grafts that had been generated from iPSC-derived neuronal progenitors. With human neurons implanted into the mouse brain, high-resolution, real-time in vivo monitoring of human neuron dynamics for periods of time spanning the range from subseconds to several months becomes feasible. RESULTS We found that transplanted human iPSC–derived neuronal progenitors consistently assembled into vascularized territories with complex cytoarchitecture, mimicking key features of the human fetal cortex, such as its large size and cell diversification. Single-cell-resolution intravital microscopy showed that human neuronal arbors were refined via branch-specific retraction, rather than degeneration. Human synaptic networks restructured over the course of 4 months, while maintaining balanced rates of synapse formation and elimination. Human functional neurons rapidly and consistently acquired oscillatory population activity, which persisted over the 5-month observation period. Lastly, we used cortical tissue grafts derived from the fibroblasts of two individuals with Down syndrome, caused by supernumerary chromosome 21. We found that neuronal synapses in cells derived from these individuals were overly stable and that oscillatory neural activity was reduced in these grafts, revealing in vivo cellular phenotypes not otherwise apparent. CONCLUSION By combining live imaging in a multistructured tissue environment in mice with a human-specific genetic background, we provide insights into the earliest stages of human axon, synaptic, and network activity development and uncover cellular phenotypes in Down syndrome. Our work provides an alternative experimental system that can be used to study other disorders affecting the developing human cortex. Human neuron dynamics imaged in vivo. We combined a human-specific genetic background with live imaging in cortical tissue grafts to investigate the earliest stages of human axon, synaptic, and network activity development and model Down syndrome. Harnessing the potential of human stem cells for modeling the physiology and diseases of cortical circuitry requires monitoring cellular dynamics in vivo. We show that human induced pluripotent stem cell (iPSC)–derived cortical neurons transplanted into the adult mouse cortex consistently organized into large (up to ~100 mm3) vascularized neuron-glia territories with complex cytoarchitecture. Longitudinal imaging of >4000 grafted developing human neurons revealed that neuronal arbors refined via branch-specific retraction; human synaptic networks substantially restructured over 4 months, with balanced rates of synapse formation and elimination; and oscillatory population activity mirrored the patterns of fetal neural networks. Lastly, we found increased synaptic stability and reduced oscillations in transplants from two individuals with Down syndrome, demonstrating the potential of in vivo imaging in human tissue grafts for patient-specific modeling of cortical development, physiology, and pathogenesis.

中文翻译：

---

人体神经元动力学和唐氏综合症的体内建模

人脑神经元的发育 人脑发育的最早阶段很难监测，但使用诱导多能干细胞 (iPSC) 可以帮助阐明这一过程。真实等人。将源自人类 iPSC 的神经祖细胞移植到成年小鼠的大脑中。他们使用活体成像来可视化生成的神经元是如何生长和连接的。人类细胞产生的神经元整合并发展出具有振荡活动的突触网络。观察到树枝状修剪并涉及分支回缩的过程，而不是退化。来自唐氏综合症个体的细胞，在移植到小鼠大脑后，产生的神经元生长正常，但树突棘更新减少，网络活动减少。科学，本期第 3 页。eaau1810 将人类神经元祖细胞移植到小鼠大脑中打开了一扇了解正常和异常大脑发育的窗口。引言 科学家们正在构建人脑细胞组成的详细地图，以了解其发育过程。在人类大脑皮层（哺乳动物大脑的最大区域）中，神经回路是通过解剖学上的细化形成的，包括轴突和突触修剪，以及在胎儿早期发育过程中出现的复杂网络活动模式。人脑中的细胞分析仅限于死后材料，无法揭示发育过程。因此，模式生物通常用于大脑生理学、发育和发病机制的研究，但模式生物的结果并不总是转化为人类。基本原理 需要用于模拟人类神经元动力学及其体内功能障碍的系统。虽然活检标本和诱导多能干细胞 (iPSC) 产生的神经元可以提供必要的人类遗传背景，但二维和三维培养物缺乏通常支持神经元发育的因素，包括血管、免疫细胞和与神经支配的相互作用来自其他大脑区域的神经元。基于先前在小鼠中进行的干细胞移植研究，我们推断成年小鼠大脑的生理微环境可以支持由 iPSC 衍生的神经元祖细胞产生的人类皮质组织移植物的生长。将人类神经元植入小鼠大脑，高分辨率，在从亚秒到几个月的时间段内对人类神经元动态进行实时体内监测变得可行。结果 我们发现移植的人类 iPSC 衍生的神经元祖细胞始终组装成具有复杂细胞结构的血管化区域，模仿人类胎儿皮层的关键特征，例如其大尺寸和细胞多样化。单细胞分辨率活体显微镜显示人类神经元乔木是通过分支特异性收缩而不是退化来精制的。人类突触网络在 4 个月内重组，同时保持突触形成和消除的平衡速率。人类功能性神经元快速且持续地获得振荡种群活动，这种活动在 5 个月的观察期内持续存在。最后，我们使用了来自两个唐氏综合症患者的成纤维细胞的皮质组织移植物，这是由 21 号染色体引起的。我们发现来自这些个体的细胞中的神经元突触过于稳定，并且这些移植物中的振荡神经活动降低，这在体内揭示了其他不明显的细胞表型。结论 通过将小鼠多结构组织环境中的实时成像与人类特定的遗传背景相结合，我们提供了对人类轴突、突触和网络活动发展的最早阶段的见解，并揭示了唐氏综合症的细胞表型。我们的工作提供了一种替代实验系统，可用于研究影响发育中的人类皮层的其他疾病。体内成像的人类神经元动力学。我们将人类特定的遗传背景与皮质组织移植物中的实时成像相结合，以研究人类轴突、突触和网络活动发展的最早阶段以及唐氏综合症模型。利用人类干细胞的潜力来模拟皮质电路的生理学和疾病需要监测体内的细胞动力学。我们表明，移植到成年小鼠皮层的人类诱导多能干细胞 (iPSC) 衍生的皮层神经元始终组织成具有复杂细胞结构的大型（高达约 100 mm3）血管化神经元-胶质区域。超过 4000 个移植的发育中的人类神经元的纵向成像显示，神经元乔木通过分支特异性收缩进行了细化。人类突触网络在 4 个月内大幅重组，突触形成和消除的平衡速率；和振荡的种群活动反映了胎儿神经网络的模式。最后，我们发现两名唐氏综合症患者的移植物突触稳定性增加，振荡减少，这证明了人体组织移植物中的体内成像在皮质发育、生理学和发病机制的患者特异性建模中的潜力。