CHANGES IN MORPHOLOGY AND BEHAVIOR OF RETINAL GROWTH CONES BEFORE ANDAFTER CROSSING THE MIDLINE OF THE MOUSE CHIASM - A CONFOCAL MICROSCOPY STUDY

The growth of retinal axons was investigated in different regions of t he optic chiasm in C57 pigmented mouse embryos aged embryonic day 13 ( E13) to E15. Individual retinal axons and their growth cones were labe lled anterogradely by Dir and imaged using a confocal imaging system. In aldehyde-fixed embryos, retinal growth cones display a simple form in the optic nerve and become more complex in morphology in the chiasm , The complex form is particularly prominent in those axons that turn to the ipsilateral tract in the premidline region of chiasm. Moreover, complex growth cones are also commonly found in axons in the postmidl ine chiasm, which are markedly different in morphology from those axon s in the premidline region, suggesting that the postmidline chiasm con tains a novel environment for the pathfinding of retinal axons. In ano ther experiment, the dynamic growth of retinal axons is studied in a b rain slice preparation of the living retinofugal pathway. Retinal axon s show an intermittent growth across the premidline and postmidline ch iasm. Extensive remodelling of growth cone form followed by a shift in growth direction is commonly seen during the pause periods, indicatin g that signals that guide axon growth across the chiasm are not restri cted to the midline, but are laid down throughout the chiasm, Moreover , dramatic changes in axon trajectory are noted first at the premidlin e chiasm where the uncrossed axons segregate from the crossed axons, a nd second at the postmidline chiasm where specific sorting of retinal axons according to their position in the dorsal ventral retinal axis a nd their ages are known to take place. These results show that there a re two distinct environments, separated by the midline in the chiasm, where axons show different responses to local guidance cues and develo p the distinct fibre orders.