After a brief review of the main concepts developed in the first lecture, in particular the strategy of neuron differentiation in the primitive nervous system of the Drosophila embryo and the transcription factors involved, this second lecture begins with an overview of the phenomenon of quiescence and re-entry into the cell cycle that characterizes the transition from the first to the second wave of neurogenesis in the fly.
Next, studies of neurogenesis during the development of the medulla (part of the Drosophila optic system) are described, showing a similar strategy but involving different transcription factors, necessary and sufficient to fix a " temporal identity " to the cells produced and thus setting the tempo for neuronal differentiation. The second of these studies involves a new and analytically powerful technology ; single-cell RNA sequencing, and accompanying conceptual tools such as the notions of " trajectories " and " pseudo-time ". These concepts are explained and the lesson concludes with some remarks on the existence of such neuronal differentiation processes over time in the mammalian brain and retina.
