Chapter 2 The chemotactic hypothesis of Cajal: a century behind

The chapter discusses the chemotactic hypothesis of Santiago Ramón y Cajal, a neuroscientist who showed that the nervous system is made up of billions of independent, richly and precisely interconnected nerve cells, organized in detailed networks. His studies on the architectural organization of the brain, and his prophetic predictions of its functions became the basis of neuroanatomy, neurophysiology, neuropathology, and “rational psychology.” This monumental work justifies his well-deserved title as founder of modern neuroscience. Cajal's chemotaxis hypothesis has become one of the most promising fields to study late steps of neurodevelopment: those guiding afferent axons to their proper postsynaptic partners. The chemotactic molecules were named “netrins,” and were mainly produced by intermediate targets. Other families of chemotropic molecules have been identified, such as secreted semaphorins, hepatocytegrowth factor/scatter factor, and Slits, that together with neurotrophins constitute the bulk of chemotactic molecules used by growth cones in their growth and elongation.