FIGURE 2. Stereotyped axon and vessel navigation.

a, Photographs of axons terminating in a growth cone, containing numerous filopodia¹⁰²). b, Scanning electron micrograph of an axonal growth cone with characteristic filopodial extensions (from ref. 103). c, Primitive vascular labyrinth of similarly sized vessels in the retina of a neonatal mouse. Note how the tip endothelial cells at the leading front of the growing blood vessels extend multiple filopodia (arrows) into the avascular retina. d, Stereotyped pattern of intersomitic vessels (ISVs), branching off from the dorsal aorta in a 22-h-old zebrafish embryo. Note the numerous filopodial extensions at the leading endothelial tip cells. e, Schematic representation of endothelial tip and stalk cell (reproduced with permission from ref. 6). (Labels C, B, A in a and T and M in b indicate details not discussed here.)