Growing optic nerve fibers follow neighbors during embryogenesis.

The embryonic development of the optic nerve of the zebrafish, Brachydanio rerio, was studied by three-dimensional computer reconstruction from serial section electron micrographs. Growing fibers from retinal ganglion cells had growth cones in contact with more mature fibers from adjacent cell bodies. In the observed growth pattern, the optic fibers immediately behind the eye were ordered in such a way that the rectangular coordinates of the fiber positions were approximately proportional to the polar coordinates of their cell body positions. We suggest that this transformation is achieved by a simple following mechanism that translates the time and position of ganglion cell differentiation into a well-defined spatial organization within the optic nerve.