Generation of cortical neurons from endogenous progenitor cells is limited to prenatal development. In the human brain the process starts in the first trimester and the new neurons integrate in the developing neuronal networks early on. Disturbance of this early developmental process is indicated to result in neuronal network dysfunction manifesting as psychiatric conditions later in life. Studying how these processes are regulated and what cells are involved would help us understand how new cortical neurons can be generated and how the establishment of new synaptic connections can be facilitated. In my talk I am going to present our findings on the regulation of self-renewing cortical neural stem cells and the capture of a novel immune cell state involved in the establishment of active synapses between cortical neurons.