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FIGURE 8.4 Pre- and postsynaptic differentiation without a
partner. A. An electron micrograph showing clustering of a2 adren-
ergic receptor (arrows) in a postnatal day 4 rat visual cortex neuron.
Both of the red-tinted structures are dendrites. B. An electron micro-
graph showing an apparent presynaptic terminal adjacent to
hemolymph. This terminal is made by a Drosophila motoneuron in a
mutant strain, twisted ( twi ), that does not generate postsynaptic
muscle cells. (Panel A from C. Aoki, unpublished observations;
Panel B adapted from Prokop et al., 1996)
Postnatal Days
FIGURE 8.3 Growth of neuronal elements during cat visual
cortex development. A. From birth until postnatal day 40 (P40), the
density of neuronal cell bodies decreases as gliogenesis and angio-
genesis occurs. During this same period, dendritic arbors are
expanding, and synaptic terminals (black dots) are accumulating on
the postsynaptic membrane. B. The total volume of visual cortex
occupied by each neuron increases by almost 10-fold during the first
postnatal month. When neuron packing density is taken into con-
sideration, the accumulation of synapses can be expressed as
synapses per neuron. As shown in the graph, there is a dramatic
increase in synapses from P10 to P30, and significant decline after
P108 (adapted from Cragg, 1975).
Therefore, glial cells may serve as gatekeepers, deter-
mining when and where synapses can be formed.
During the time when synapses are forming
between nerve cells, it is quite common to see pre- and
postsynaptic structures all by themselves: essentially,
synapses to nowhere. From the amphibian spinal cord
to the rodent olfactory cortex, presynaptic-like struc-
tures with an accumulation of vesicles apparently
develop in the absence of a postsynaptic cell. Similarly,
postsynaptic densities that are not in contact with a
presynaptic terminal have been found in the olfactory
bulb and cortex (Figure 8.4A). These lonesome struc-
tures indicate that growth cones and dendrites are
poised on the brink of differentiating into synaptic spe-
cializations (Hayes and Roberts, 1973; Newman-Gage
et al., 1987; Westrum, 1975; Hinds and Hinds, 1976).
Presynaptic terminal differentiation has even
been found to occur on nonneuronal cells. Transient
presynaptic-like terminals have been identified on glial
cells during axon ingrowth, particularly in areas
without dendritic processes (Hendrikson and Vaughn,
1974). In a Drosophila mutant that has no mesoderm,
and therefore no muscle, motor axons continue to form
presynaptic-like profiles on glia and other cells (Figure
8.4B). However, some cells appear to be crucial for
synapse differentiation. Ablation of selected embryonic
muscle precursors in Caenorhabditis results in gaps in a
set of dorsal muscles and prevents presynaptic vari-
cosity formation (Plunkett et al., 1996). Thus, growing
neuronal processes can display a synaptic morphology
with little encouragement from its appropriate partner.
Ultrastructural studies also provide the best
description of the time period when synapses are
added in the peripheral and central nervous system.
synapses are present in newborn tissue, while few axo-
somatic synapses are found until two to three weeks
later (Pappas and Purpura, 1964; Schwartz et al, 1968).
In fact, when a postsynaptic marker protein (PSD-95)
is visualized in dendrites as they grow within the
zebrafish midbrain, the new postsynaptic sites appear
first on dendritic filopodia (Niell et al., 2004). There-
fore, it is the most recently generated postsynaptic
structures that are first contacted by axonal growth
cones (Fiala et al., 1998).
It is also possible that some membrane compart-
ments are not available for contact. For example, exci-
tatory connections to an auditory brainstem nucleus
called the medial superior olive (MSO) are at first
restricted to the dendritic regions of the cell. At this
stage, MSO cell bodies are completely surrounded by
glial membrane. As the glial membrane regresses,
synapses are formed on the MSO cell bodies (Brunso-
Bechtold et al., 1992). In this regard, it is interesting
that elimination of a putative cell adhesion molecule
at the neuromuscular junction (cf, s-laminin) permits
glial processes to invade the synaptic region and
impeed synapse maturation (Noakes et al., 1995).
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