The Questions of Developmental Biology

Each neurotrophin can promote the growth of some
axons to its source while inhibiting other axons. For instance,
BDNF can stop the growth of certain sensory neurons from the
rat dorsal root ganglia, and these same neurons can be induced
to turn toward a source of NT-3 (Figure 13.24).
Although genetic and biochemical techniques enable us
to look at the effect of one type of molecule at a time, we must
remember that any growth cone is sensing a wide range of
chemotactic and chemorepulsive molecules, both in solution
and on the substratum upon which it migrates. Growth cones do not rely on a single type of
molecule to recognize their target. Rather, they integrate the simultaneously presented attractive
and repulsive cues and select their targets based on the combined input of these multiple cues
(Winberg et al. 1998).
Address selection:
activitydependent
development
When an axon
contacts its target (usually
either a muscle or another
neuron), it forms a
specialized junction called a
synapse. Neurotrans-mitters
from the axon terminal are
released at these synapses
to depolarize or
hyperpolarize the
membrane of the target cell
across the synaptic cleft.
The construction of
the synapse involves
several steps (Figure 13.25;
Burden 1998). When motor
neurons in the spinal cord
extend axons to muscles,
growth cones that contact newly formed muscle cells migrate over
their surfaces. When the growth cone first adheres to the muscle
cell membrane, no specializations can be seen in either membrane.
However, the axon terminals soon begin to accumulate
neurotransmitter-containing synaptic vesicles, the membranes of
both cells thicken at the region of contact, and the synaptic cleft
between the cells fills with extracellular matrix that includes a
specific form of laminin. This muscle-derived laminin specifically
binds the growth cones of motor neurons and may act as a "stop
signal" for axonal growth (Martin et al. 1995; Noakes et al. 1995).
In at least some neuron-to-neuron synapses, the synapse is
stabilized by N-cadherin.