One of the major features of vertebrates which do not have any counterpart in say Drosophila is the formation of a notochord. A picture of this formation is illustrated with the frog, zenopus laevis. Here, the cells that were near the involute first, turning inward and then moving up toward the animal pole to form the most anterior part of the gut. As they near the , these leading cells will signal to the overlying to define the anterior extremity of the head. A slender rod of cells with endoderm below it, ectoderm above it and mesoderm on either side is the notochord.
In a sheet of ectoderm overlying the notochord other cell movements occur in a process known as neurulation. First, a broad central region of ectoderm, called the neural plate, thickens, rolls up into a tube, and pinches off from the rest of the cell sheet. The tube created is called the neural tube and it will give rise to the central nervous system (brain and spinal cord). Meanwhile a sheet on the dorsal side of the neural tube, called the neural crest migrates away from the surface of the neural tube and gives rise to a diverse set of cell types of the peripheral nervous system.
The neural tube consists initially of a single layered epithelium. The epithelia cells are progenitors of and cells. The locations where neural cells are born are called germinal zones. Prenatally, this zone is the ventricular zone/neural tube. Postnatally, the zones are the subventricular zone/Olfactory bulb (OB) and hippocampus.
Newborn neurons go through their final division close to the inner surface of the neural tube and then often migrate outward by crawling along radial before sending out axons and dendrites. The first born neurons settle closest to their birthplace, while neurons born later crawl past them to settle farther out. Thus successive generations of neurons occupy different layers in the cortex.
The long held belief that neuron generation is restricted to the embryonic period has been altered with evidence that new neurons continue to be added to certain regions like the hippocampus and olfactory bulb of the adult vertebrate brain. This generation of new neurons or neurogenesis has been shown in human postmortem brain tissue treated with the thymidine analog, bromodeoxyuridine (BrdU) that labels DNA during S phase. Using immunofluorescent labeling for BrdU and neuronal markers, new neurons as defined by these markers are generated from dividing progenitor cells in the adult human hippocampus. Enhancers of neurogenesis include physical activity and antidepressants. Inhibitors of such neurogenesis include aging and stress.