Fig. 2. Neurites and the axon are generated in a stereotypical sequence. (A,B) Time-lapse video microscopy of individual neurons showing that they extend the first two sprouts opposite to each other. (A) An example of a neuron in which the first sprout becomes the axon (arrowheads). (B) Example of a neuron that extended the axon (2; arrowheads) opposite to the first sprout (1). (C) Neurons migrating radially from an aggregate culture in Matrigel. The neurons exit the explant by extending one leading neurite (open arrowhead, 0'). The oppositely located trailing neurite is visible (solid arrowhead). (D) Migrating neurons in Matrigel cultures were fixed and stained with the axonal marker tau-1. Open arrow, leading neurite; arrowhead, trailing neurite. (E,F) Individual neurons were followed from the formation of the first bud (inset) until stage2. Stage-2 cells were fixed and the mean intensity of APC immunoreactivity measured in all neurite growth cones. In 70% of the observed neurons (33 cells) APC mean fluorescence was maximal either in the neurite appearing first (one example shown in E) or in the neurite appearing second (one example shown in F) neurite. Bars, 10 µm.