Strikingly, single cortical progenitor cells isolated in culture generate distinct types of cortical neurons and glial cells, with the timing and characteristics appropriate to specific cortical layers. These observations suggest that the neural progenitors in the developing mammalian cortex, much like the Drosophila neuroblasts, step through an intracellular developmental program that generates the ordered succession of different nerve cell types.
Figure 21–69 A schematic cross section of the spinal cord of a chick embryo, showing how cells at different levels along the dorsoventral axis acquire different characters. (a) Signals that direct the dorsoventral pattern. Sonic hedgehog protein from the notochord and the floor plate (the ventral midline of the neural tube) and BMp and Wnt proteins from the roof plate (the dorsal midline) act as morphogens to control gene expression.
(B) the resulting patterns of cell fates in the developing spinal cord. Different groups of proliferating neural progenitor cells (in the ventricular zone, close to the lumen of the neural tube) and of differentiating neurons (in the mantle zone, further out) express different combinations of transcription regulators. Neurons expressing different transcription regulators will form connections with different partners and may make different combinations of neurotransmitters and receptors. colors represent different cell types and combinations of regulatory proteins.
outer surface of cell body of radial glial cell inner surface of developing neural tube 10 µm the Growth cone pilots axons along Specific routes toward their targets
According to the character assigned to it during its early development, a neuron will proceed to make connections with specific partners. This phase of neural development involves a type of morphogenesis unique to the nervous system, in which axons and dendrites extend along specific routes toward their target cells. A typical neuron sends out one long axon and many dendrites, which are usually shorter. The axon projects to distant target cells to which the neuron will eventually send signals. The dendrites will receive incoming signals from axon terminals  of other neurons. Axons and dendrites extend by growth at their tip, where one  sees an irregular, spiky enlargement called a growth cone (Figure 21–72 and Movie 21.6). The growth cone is both the engine that produces the crawling movement and the steering apparatus that directs the tip along the proper path. Cytoskeletal machinery in the growth cone creates active protrusions, in the form of filopodia and lamellipodia (see Chapter 16 for details): when such a protrusion layers of cortical neurons
Figure 21–70 Migration of immature neurons. Before sending out axons and dendrites, newborn neurons often migrate from their birthplace and settle in another location. the diagrams are based on reconstructions from sections of the cerebral cortex (part of the neural tube) of a monkey and rely on a staining technique that picks out at random a small subset of the whole dense mass of neuroepithelial cells. the neurons go through their final cell division close to the inner, lumenal face of the neural tube (in the ventricular proliferative zone) and then migrate outward by crawling along radial glial cells that form a scaffold. each of these latter cells extends from the inner to the outer surface of the tube, a distance that may be as long as 2 cm in the cerebral cortex of the developing brain of a primate.  the radial glial cells can be considered as persisting cells of the original columnar epithelium of the neural tube that become extraordinarily stretched as the wall of the tube thickens. they also serve as neural stem cells: depending on stage and region, the newborn neurons can be generated from radial glial cells that undergo mitosis while their nuclei are close to the inner surface of the tube, or they can be generated from a nearby class of specialized progenitors in the ventricular proliferative zone. (after p. rakic, J. Comp. Neurol. 145:61–84, 1972. With permission from John Wiley & Sons, Inc.)