THE OPTIC TRACT IN EMBRYONIC HAMSTERS - FASCICULATION, DEFASCICULATION, AND OTHER REARRANGEMENTS OF RETINAL AXONS

The early development of the optic tract in hamsters was studied by la beling retinal axons with DiI applied to the eye, and then examining t he labeled axons in flatmount preparations of the rostral brain stem. This technique permits a panoramic view of the entire retinal projecti on, from the chiasm to the caudal end of the superior colliculus. In t he E11 embryo, retinal axons have reached the chiasm. They defascicula te as they emerge from the nerve, prior to reaching the ventral midlin e of the diencephalon, then converge again as they pass over to the op posite side. At the midline, many axonal trajectories crisscross, impl ying some shuffling of relative positions. Retinal axons are tightly b undled within the optic tract. Upon reaching the ventral border of the lateral geniculate body (LGB), they splay out over the nucleus, revea ling a wavefront of pioneer axons individually distributed across the rostro-caudal extent of the LGB. Later-emerging retinal axons course o ver the surface of the thalamus in waves; subsequent waves of axons in terdigitate between the lead fibers without fasciculating along them. Past the LGB, the axons undergo a second change in relative positions as the ribbon of fibers swerves caudally, prior to entering the superi or colliculus. Retinal axons are tipped with growth cones of varying m orphologies. No strong correlation is evident between the structural c omplexity of the growth cone and its position within the tract. In the majority of cases, ipsilaterally and contralaterally directed axons f ollow a similar developmental course along the optic tract, without an y indication of a temporal lag in the ipsilateral projection as claime d in earlier reports. Understanding the changes in spatial distributio n of embryonic retinal axons as they navigate along the optic tract pr ovides a further step towards elucidating how point-to-point projectio ns form in developing sensory systems.