Feline Lindhout

1 General introduction 13 Another particularly striking feature of human neurodevelopment, is the significant prolonged development of human neurons compared to non-human neurons (Fig 1). Multiple studies reported that human neurogenesis in vitro occurs at slower rates, as this process takes about 1 week when derived from mouse embryonic stem cells (ESCs) and more than 3 months when derived from human ESCs (Gaspard et al. 2008; Shi, Kirwan, and Livesey 2012; Espuny-Camacho et al. 2013). This is consistent with the observed neoteny of the human neocortex, as in vivo neurogenesis also takes about 1 week in mice and upon 120 days in humans (Sousa et al. 2017). Likewise, induced pluripotent stem cells (iPSC)-derived neuronal stem cell (NSC) cultures from mouse and human with the exact same culture conditions showed that neurogenesis was initiated after 2 days in mice and only after 3 weeks in humans (Espuny-Camacho et al. 2013). This protracted development in humans is also reported to occur during NSC expansion prior to neurogenesis, as human NSCs are subjected to significantly more cell cycles before they undergo neuronal differentiation (Espuny-Camacho et al. 2013). Remarkably, these temporal developmental differences are still observed when comparing more closely related species. Neurogenesis occurred at a significant slower phase in iPSC-derived cells derived from human and chimpanzee compared to those from macaque, consistent with in vivo development (Otani et al. 2016). Moreover, these and these temporal species-specific differences were even maintained in co-cultures of human and macaque neurons (Otani et al. 2016). Neurodevelopment of human and chimpanzee neurons is overall highly similar, however, human NSCs showed prolonged transition time between prometaphase and metaphase (Mora-Bermudez et al. 2016). Also at later neurodevelopmental stages, human neurons continue to mature at slower rates, as cortical neurons reach mature adult-like stages in years whereas this takes a few months in macaque neurons (Huttenlocher 1979; Huttenlocher et al. 1982; Somel et al. 2009; Petanjek et al. 2011; Liu et al. 2012). Moreover, transplanted human iPSC- derived neurons in chimera mouse brains take about 9-12 months to fully develop, whereas this takes about 4 weeks for mouse iPSC-derived neurons (Espuny-Camacho et al. 2013; Maroof et al. 2013; Nicholas et al. 2013; Linaro et al. 2019). In summary, first insights begin to elucidate unique hallmarks of human neurodevelopment, however, the precise differences in neurodevelopment between human and other species remain largely unknown. This is mostly due to technical challenges, as access to living or post-mortem human brain tissues is limited. Nevertheless, with the development of neurons derived from human iPSCs it is now possible to overcome these limitations, and more insights in human-specific processes during neurodevelopment begin to emerge (Lancaster et al. 2013) (Chapter 2 and 3) . Axons: specialized structures and functions Axon morphology Axons are comprised of a unique morphology, marked by their excessive length and small diameter. These extreme axon lengths are necessary, as axons often need to reach long