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Lamellipodium
Filopodium
Immature
synapse
Tight
junction
Mature
synapse
extracellular
matrix
Addition of membrane
and surface glycoprotein
Recognition
signal
Golgi
apparatus
FIGURE 8.5 Stages of synapse formation. When a presynaptic growth cone comes into contact with the
postsynaptic membrane, its filipodia retract (left), and the membranes become tightly apposed to one another.
Vesicles are found in both pre- and postsynaptic elements, possibly to add membrane and extracellular gly-
coproteins. The immature synapse may display a few vesicles presynaptically and a small postsynaptic
density (center). The mature synapse (right) exhibits an accumulation of presynaptic vesicles, a dense extra-
cellular matrix in the cleft, and a postsynaptic density. (Adapted from Rees, 1978)
Bursts of synapse formation are found throughout the
nervous system, but the timing and duration varies
greatly (Vaughn, 1989). In the mouse olfactory bulb,
synaptic profiles can first be recognized in electron
micrographs at embryonic (E) day 14. The total
number of synaptic profiles increases exponentially
through the first postnatal week and then continues to
increase at a lower rate over the next several weeks
(Figure 8.5). Therefore, new synaptic contacts continue
to be manufactured over a long time period after axons
invade their target. One reason for this extended
period of synaptogenesis is that dendrites are still
growing, and the addition of postsynaptic membrane
may attract new contacts. It is also likely that certain
afferent projections may arborize at different times. In
the rat visual cortex, where the synaptic profiles of
excitatory and inhibitory synapses can be recognized,
their increase in number occurs at different times.
Other areas display a steady increase in synapse
number, such as the the rat superior cervical ganglion,
where the process occurs gradually from innervation
at E14 to over one month after birth (Smolen, 1981).
An important caution is that neither anatomy nor
physiology alone is sufficient to identify a developing
synapse. Purely anatomical measures of synapse for-
mation can be misleading because synaptic physiology
can develop rapidly (see below), with little evidence of
specialized morphology. On the other hand, an exclu-
sively functional assay of synapse formation may create
problems because there is evidence that “silent” synapses
exist in the CNS, which nonetheless display normal
structure. Therefore, a precise chronology of synapse
addition is still missing for most regions of the CNS.
THE FIRST SIGNS OF
SYNAPSE FUNCTION
At the moment a growth cone comes in contact with
its postsynaptic target, it begins a metamorphosis,
transforming from a spindly pathfinding organelle to
a bulbous presynaptic terminal. One surprise is that
the growth cone comes equipped with a rudimentary
transmitter-releasing mechanism (Young and Poo,
1983; Hume et al., 1983). This was first demonstrated
in a primary culture of Xenopus spinal neurons and
myocytes. During the first two days of culture, spinal
neurons produce growth cones, extend neurites, and
form functional cholinergic synapses with neighboring
muscle cells. To detect the release of ACh, a special
type of recording electrode is manufactured (Figure
8.6). The electrode has an excised piece of muscle cell
membrane at its tip, and this membrane contains ACh
 
 
 
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