The organization of our nervous system defines the ways we behave, perceive, think and feel. The development of the nervous system, once believed to be too complicated to possibly understand, has finally begun to reveal its secrets.  Efforts to understand how the nervous system develops are proceeding at a breathtaking pace. Here some of  the cellular and molecular processes influencing neural development will be summarized. All tissues come from one of three germ layers: ectoderm (skin, nervous system), mesoderm (e.g. skeleton, muscle, kidney, heart, blood) and endoderm (e.g. gut, liver, lungs) (Figure 1). The origin of the nervous system thus lies in the ectoderm of the embryo. The ectoderm differentiates into two major body components: the epidermis (the outer layer of the skin) and the nervous system. Ectodermal cells are epithelial. Although the highly branched, specialized nerve cells do not resemble a layer of simple epithelial cells, these neuronal cells do retain some of the characteristic properties of the epithelial cells from which they originate (for example, a clear polarity). The development of the nervous system is incredibly complex, but can be broken down into roughly four phases: 1) competence and segregation of neuronal cells from ectoderm, 2) regional specification and differentiation of different kinds of neuronal cells, 3) axonal pathfinding to establish connections, and 4) maintenance of connections among neuronal cells.

Figure 1. Differentiation of human tissues.

Neurulation is the formation of the hollow, dorsal neural tube, which gives rise to both the spinal cord and the brain. Once the neural tube has formed, the cells within the neural tube still must divide and then ultimately differentiate into cells with specific fates along the anterior/posteior and dorsal/ventral axis of the tube. The brain and the spinal cord both develop from the neural tube. The brain can initially be categorized into 3 regions (forebrain, midbrain and hindbrain), which are subsequently divided into the 5 functional units of the brain (telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon), and then further divided as development proceeds (see figures below).