Background

Development of the visual projection

The neural connections between the retinal ganglion cells and the higher order visual neurons in the midbrain (e.g. the optic tectum) or in the forebrain (e.g. lateral geniculate nucleus) develop in coherent topographic patterns. To establish the visual projections, individual ganglion cells send out their axons (the optic fibers) which grow towards the optic disc near the centre of the retina. The axons of ganglion cells exit the eyeball at its posterior pole and form a thick ensheathed bundle, the optic nerve. The growing tips of these axons select particular routes to reach their appropriate target zones along the visual pathways and eventually form synapses with higher order visual neurons.

Since the complexities of the visual pathways vary from one species to another we will discuss here a relatively simple example of visual projections in the goldfish. The optic fibers of goldfish make a complete cross at the optic chiasm and invade only the contralateral lobe of the optic tectum in the midbrain. Before they enter the rostral pole of the tectum the ingrowing axons from the ventral retina segregate from those originating from the dorsal area of the retina. The former select the medial branch of the optic tract and the latter the lateral branch. The optic fibers within the tectal tissue course through a superficial layer (the stratum opticum) towards the caudal pole along the spheroidal circumferences of the optic tectum. Most growing tips of retinal ganglion cell axons terminate in the subjacent layer (the stratum fibrosum et griseum superficiale) and form synapses with visual neurons of the tectum.