![]() ![]() However, this model does not explain how the first axon developed. This is known as the "touch and go" model. At this point, the neurite will begin to differentiate into an axon. It has been proposed that a minor neurite could extend outward until it touches an already developed axon of another neuron. 10% of the time, the neurite that will become the axon protrudes from the cell body simultaneously with one or more other neurites. ![]() 30% of the time, a neurite not destined to become the axon protrudes from the cell body first. It is known that 60% of the time the first neurite that protrudes from the cell body will become the axon. In vivo Ī neurite growing in vivo is surrounded by thousands of extracellular signals which in turn can be modulated by hundreds of intracellular pathways, and the mechanisms for how these competing chemical signals effect the ultimate differentiation of neurites in vivo is not precisely understood. By day 7, the neuron should be completely polarized, with a functional dendrites and an axon. On days 4 to 7, the remaining minor neurites will begin differentiating into dendrites. This neurite will eventually become the axon. Sometime between day 1.5 and day 3, one of the minor neurites begins to outgrow the other neurites significantly. 0.5 to 1.5 days after being plated in culture, several minor neurites will begin to protrude out from the cell body. Įstablishing polarity In vitro Īn undifferentiated mammalian neuron placed in culture will retract any neurites that it has already grown. Co-culture of neurons with electrically aligned glial tissue also directs neurite outgrowth, as it is rich in neurotrophins that promote nerve growth. Weak endogenous electric fields may be used to both facilitate and direct the growth of projections from cell soma neurites, EFs of moderate strength have been used to direct and enhance neurite outgrowth in both murine, or mouse, and xenopus models. There are several software kits available to facilitate neurite tracing in images. The neural cell adhesion molecule N-CAM simultaneously combines with another N-CAM and a fibroblast growth factor receptor to stimulate the tyrosine kinase activity of that receptor to induce the growth of neurites. This prevents the development of multiple axons. In all other neurites however, the actin filaments are stabilized by myosin. Actin filaments retain their dynamic properties in the neurite that will become the axon in order to push the microtubules bundles outward to extend the axon. Even after the microtubules have stabilized, the cytoskeleton of the neuron remains dynamic. Tau proteins can aid in the stabilization of microtubules by binding to the microtubules, protecting them from microtubule severing proteins. Young neurites are often packed with microtubule bundles, the growth of which is stimulated by neurotrophic factors, such as nerve growth factor (NGF). Among the known extracellular growth signals are netrin, a midline chemoattractant, and semaphorin, ephrin and collapsin, all inhibitors of neurite growth. While not all of the growth signals are known, several have been identified and characterized. The developing neurite sums together all of these growth signals in order to determine which direction the neurite will ultimately grow towards. At every given point along a developing neurite, there are receptors detecting both positive and negative growth cues from every direction in the surrounding space. The development of a neurite requires a complex interplay of both extracellular and intracellular signals. The term is frequently used when speaking of immature or developing neurons, especially of cells in culture, because it can be difficult to tell axons from dendrites before differentiation is complete. This projection can be either an axon or a dendrite. Any projection from the cell body of a neuronĪ neurite or neuronal process refers to any projection from the cell body of a neuron. ![]()
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