The organization of the adult mammalian nervous system is the result of developmental, functional, evolutionary, and biomechanical constraints. Our current understanding of its architecture originated with the pioneering studies of Santiago Ramón y Cajal, who mapped microscopic neuroanatomy in exquisite detail. The adult brain is organized into dorsoventral and rostrocaudal compartments, which result from patterning of the early neural tube (Rubenstein and Rakic, 2013). However, many neurons (e.g., telencephalic interneurons), glia (e.g., oligodendrocyte precursor cells [OPCs]), and vascular and immune cells migrate long distances during embryogenesis and thus end up in a location different from their place of birth. Furthermore, convergent functional specialization occurs in many parts of the nervous system: for example, dopaminergic neurons are found both in the midbrain and in the olfactory bulb, and noradrenergic neurons are found in the sympathetic ganglia, as well as the hindbrain.
