Retinal ganglion cell (RGC) axons travel in radial routes unerringly toward
the optic disk, their first intermediate target in the center of the eye.
The path of the RGC growth cone is restricted to a narrow zone subjacent to
the endfeet of Muller glial cells and the vitreal basal lamina. The presen
t survey indicates that RGC growth cones are guided by many molecular cues
along their pathway which are recognized by receptors on their surface. Gro
wth-promoting molecules on Muller glial endfeet and in the basal lamina ass
ist growth cones in maintaining contact with these elements. The repellant
character of deeper retinal laminae discourages them from escaping the RGC
axon layer. Cell adhesion/recognition proteins enable growth cones to fasci
culate with preformed axons in their vicinity. It is still unclear whether
the optic disk emits long range guidance components which enable the growth
cones to steer toward it. Recent evidence in fish indicates the existence
of an axonal receptor (neurolin) for a guidance component of unknown identi
ty. Receptor blockade causes RCC axons to course in aberrant routes before
they reach the disk. At the disk, axons receive signals to exit the retina.
Contact with netrin-1 at the optic disk/nerve head encourages growth cones
to turn into the nerve. This response requires the axonal netrin receptor
DCC, laminin-1, beta-integrin and most likely the UNC5H netrin receptors wh
ich convert the growth encouraging signal into a repulsive one which drives
growth cones into the nerve. (C) 2000 Elsevier Science Ltd. All rights res
erved.