Feline Lindhout

1 12 this extensive growth relies on drastic changes in the local cytoskeleton of the future axon, which specifically employs microtubule stabilization and actin depolymerization (Yu and Baas 1994; Bradke and Dotti 1999; Geraldo et al. 2008; Witte, Neukirchen, and Bradke 2008; Neukirchen and Bradke 2011; Zhao et al. 2017). Accordingly, the formation of multiple axons is observed when inducingmicrotubule stabilization using pharmacological treatments in stage 2/3 neurons (Witte, Neukirchen, and Bradke 2008). The observed microtubule stabilization in growth cones is accompanied with increased local actin dynamics, further implying that a controlled interplay between these different cytoskeletal components is at play in developing axons (Zhao et al. 2017). In stage 4 neurons, the remaining neurites will transform in dendrites, and further mature as dendrites undergo extensive arborization. Finally, in stage 5, neurons will undergo synaptogenesis, which involves the formation of specialized synaptic connections betweenneurons. These synapses generally arisebetween axonal boutons, swellings along the axon shaft or terminals, and dendritic shafts or spines, typically mushroom-shaped protrusions at dendrites (Fig 2). These neurodevelopmental stages identified in in vitro rodent neurons largely coincide with neurodevelopment in vivo , thereby indicating that these developmental processes are mostly driven by cell-intrinsic mechanisms. In developing brains, the temporal coordination of the neuronal migration and transition through neurodevelopmental stages is steered by the structural organization and molecular composition of neuronal tissues (Stoeckli 2018). In conclusion, studies in rodent model systems reveal that the development of complex neuronal networks is orchestrated by cell-intrinsic neurodevelopmental stages, which are further guided by extracellular processes in highly organized neuronal structures. Developing neurons: insights from humans Most insights in neurodevelopmental processes come from studies using non-human model systems (e.g. rodents, zebrafish, flies, worms). Many basic principles of brain development are found to be conserved throughout evolution. However, the human brain also shows striking anatomical differences compared to other species. This is mostly highlighted by a larger cortex containing gyri and sulci, which is accompanied with increased cognitive functions in humans (Emery and Clayton 2005; Pollard et al. 2006). Humans and other larger-brained species (e.g. ferrets, sheep, cats, and apes) contain an additional cortical layer, the outer subventricular zone (oSVZ), which is populated by the primate-specific outer radial glia cells (oRGCs), a subtype of neuronal precursor cells (Fish et al. 2008; Fietz et al. 2010; Hansen et al. 2010; Reillo et al. 2011; Dehay, Kennedy, and Kosik 2015; De Juan Romero and Borrell 2015). Also at the single-cell level, there are similarities as well as profound differences between the development of human and non-human neurons. Similar to rodents, human neurons also proceed through the five neurodevelopmental stages that were first identified in dissociated rodent neuron cultures (Dotti, Sullivan, and Banker 1988; Powell et al. 1997; Lancaster et al. 2013) ( Chapter 2 ). However, there are also human- specific morphologies observed in neurons, as marked by larger spine sizes and more complex dendritic arborization compared to non-human primates (Benavides-Piccione et al. 2002; Defelipe 2011).