The Questions of Developmental Biology

Upon entering the brain, mammalian retinal ganglion axons reach the optic chiasm,
where they have to "decide" if they are to continue straight or if they are to turn 90° and enter the
other side of the brain (Figure 13.28D). It appears that those axons that are not destined to cross
to the other side of the brain are repulsed from doing so when they enter the optic chiasm
(Godement et al. 1990), but the molecular basis of this repulsion is not known. However, two
guidance molecules, the L1 adhesion molecule (which promotes the crossing of the chiasm) and
the CD44 protein (which inhibits crossing), are expressed on the brain neurons in the region of
the chiasm. On their way to the optic tectum, the axons travel on a pathway (the optic tract) over
glial cells whose surfaces are coated with laminin (Figure 13.28E). Very few areas of the brain
contain laminin, and the laminin in this pathway exists only when the optic nerve fibers are
growing on it (Cohen et al. 1987). At this point, the retinal ganglion axons have reached the optic
region of the brain (Figure 13.26F), and target selection begins.
Target selection.
When the axons come to the end of the laminin-lined optic tract, they spread out and find
their specific targets in the optic tectum. Studies on frogs and fishes (in which retinal neurons
from each eye project to the opposite side of the brain) have indicated that each retinal ganglion
axon sends its impulse to one specific site (a cell or small group of cells) within the tectum
(Sperry 1951). As shown in Figure 13.29, there are two optic tecta in the frog brain. The axons
from the right eye enter the left optic tectum, while those from the left eye form synapses with the
cells of the right optic tectum. The growth of axons in the Xenopus optic tract appears to be
mediated by fibroblast growth factors secreted by the cells lining the tract.
The retinal ganglion axons express FGF
receptors in their growth cones. However, as
the axons reach the tectum, the amount of FGF
rapidly diminishes, perhaps slowing down the
axons and allowing them to find their
targets (McFarlane et al. 1995).
The map of retinal connections to
the frog optic tectum (the retinotectal
projection) was detailed by Marcus
Jacobson (1967). Jacobson defined this
map by shining a narrow beam of light on
a small, limited region of the retina and noting,
by means of a recording electrode in the tectum,
which tectal cells were being stimulated.
The retinotectal projection of Xenopus laevis is
shown in Figure 13.29. Light illuminating the
ventral part of the retina stimulates cells on the
lateral surface of the tectum. Similarly, light
focused on the posterior part of the retina
stimulates cells in the caudal portion of the
tectum. These studies demonstrated a point-for-point correspondence between the cells of the
retina and the cells of the tectum. When a group of retinal cells is activated, a very small and
specific group of tectal cells is stimulated. We also can observe that the points form a continuum;
in other words, adjacent points on the retina project onto adjacent points on the tectum.
This arrangement enables the frog to see an unbroken image. This intricate specificity caused
Sperry (1965) to put forward the chemoaffinity hypothesis: