The development of humanized neural chimeric mouse models consists in grafting human neuronal progenitor cells (NPC) or differentiated neurons derived from induced pluripotent stem cells (iPSC) into the mouse brain at different timepoints. As the main features of cortical networks and their alterations during brain development cannot be reproduced in vitro, these in vivo models have been used to investigate the mechanisms by which reprogrammed human neurons and non-neuronal cells integrate and migrate into the mouse brain at early stages of brain development and display functional activities several months after their grafting. Here, we describe the neonatal grafting technique of human NPC which we use in our laboratory.We also present a new method based on the grafting of human NPC into the brain of mouse embryos, in utero. A third method consists in the stereotaxic grafting of human NPC into selected brain regions of adult mice. The iPSC technology combined with the use of chimeric mouse models offers numerous possibilities to study human neural development within wellcontrolled and defined temporal windows and to model neuropathological disorders.

The transplantation of neural progenitors into a host brain represents a useful tool to evaluate the involvement of cell-autonomous processes and host local cues in the regulation of neuronal differentiation during the development of the mammalian brain. Human brain development starts at the embryonic stages, in utero, with unique properties at its neotenic stages. We analyzed the engraftment and differentiation of human neuronal progenitor cells (hNPCs) transplanted in utero into the mouse brain. The influence of the environment was studied by transplanting human NPCs within the lateral ventricles (LV), compared with the prefrontal cortex (PFC) of immunocompetent mice. We developed a semi-automated method to accurately quantify the number of cell bodies and the distribution of neuronal projections among the different mouse brain structures, at 1 and 3 months post-transplantation (MPT). Our data show that human NPCs can differentiate between immature “juvenile” neurons and more mature pyramidal cells in a reproducible manner. Depending on the injection site, LV vs. PFC, specific fetal local environments could modify the synaptogenesis processes while maintaining human neoteny. The use of immunocompetent mice as host species allows us to investigate further neuropathological conditions making use of all of the engineered mouse models already available.