The cell constitutes the fundamental biological unit of all living organisms. In multicellular organisms, cells are heterogeneous in nature and are specialized in achieving a specific set of functions. This diversity in cell types prompted scientists to determine the cellular composition of organisms, throughout their development or in their adult form. This endeavour did not only help understanding the normal function a cell subserves, but did also shed light on how its malfunction engenders diseases. But what is a cell type? Though a cell can be characterized by its morphology and physiology, these two properties are mostly determined by its gene expression profile. The transcriptomic characterization of cell types was long hindered by the limitations of the tools available, this until the development of a revolutionary technique over a decade ago, namely single-cell RNA sequencing (scRNA-seq).

During my thesis, I applied scRNA-seq to three subsystems of the mammalian nervous system. The work I performed did not only offer a description of the cell types pertaining to these subsystems, but also provided the necessary tools to explore the organisation and function of these systems.