Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1206 Chapter 21: Development of Multicellular Organisms
explant
tectal membrane
anterior
posterior
posterior
explant
retina
tectum
anterior
posterior
anterior
explant
(A)
growing axon
(B)
Figure 21–78 Selectivity of retinal axons growing over tectal membranes. (A) Diagram of an experiment performed with
cells from a chick embryo. The culture substratum is coated with alternating stripes of membrane prepared either from posterior
tectum or from anterior tectum. Axons from posterior retina grow on anterior tectal membrane but are repelled by posterior
tectal membrane. Axons from anterior retina show different (less selective) behavior. (B) Photograph of results. The retinal axons,
growing out from the left, are made visible by staining them with a fluorescent marker. The selective pattern of outgrowth shows
that anterior tectum differs from posterior tectum, and anterior retina correspondingly differs from posterior retina. In the intact
organism, this serves to orient a retinotopic map; the map is refined by subsequent competitive interactions among the anterior
and posterior retinal axons, which push the anterior retinal cells off anterior tectal territory. (From J. Walter et al., Development
101:685–696, 1987.With permission from the Company of Biologists.)
retinal axons because the tectal cells also carry positional labels. Thus, the neural
map depends on a correspondence between two MBoC6 systems m22.106/22.79 of positional markers,
one in the retina and the other in the tectum.
How are these markers used to make the map? When posterior axons are
allowed to grow out over a carpet of anterior or posterior tectal membranes in
a culture dish, they show selectivity. Posterior axons strongly prefer the anterior
tectal membranes, as in vivo, whereas anterior axons show no preference or prefer
posterior tectal membranes (Figure 21–78). The key difference between anterior
and posterior tectum is not an attractive factor on the anterior tectum but a
repulsive factor on the posterior tectum, to which posterior retinal axons are sensitive
but anterior retinal axons are not. If a posterior retinal growth cone touches
posterior tectal membrane, it collapses its filopodia and withdraws.
In this system, as in others that we have mentioned, the repulsive interactions
are mediated by ephrin–Eph signaling—specifically, EphrinA–EphA signaling
for the anteroposterior axis (Figure 21–79). An analogous mechanism based on
EphB–EphrinB signaling orients the dorsoventral axis of the retinotopic map.
These mechanisms serve to orient the map along both axes, but they are not
enough by themselves to ensure accurate point-to-point detail. This is brought
about through a long process of adjustment that fills in and refines the map
through interactions among the RGC axon terminals as they compete for territory
on the tectum. This refinement of the pattern of connections involves electrical
signaling in the system of developing synapses—a topic that we return to shortly.
Both Dendrites and Axonal Branches From the Same Neuron
Avoid One Another
Axons and dendrites from different neurons can repel one another, or they can
cohere; they can collaborate to form synapses, or they can compete. Remarkably,