Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 106

growth cones, and in vitro lamellipodia can mediate guidance decision in
the absence of filopodial contact with a localized guidance cue ( Gallo et al.,
1997 ). Thus, lamellipodia may be able to compensate for filopodia during
growth cone guidance. Nonetheless, the intriguing study by Dwivedy et al.
(2007) provides exciting data and indicates that further analysis of the role
of filopodia in axon guidance is warranted.
Some neuronal circuits exhibit the property of dendritic tiling
( Gao, 2007 ). Tiling refers to the formation of spatially nonoverlapping fields
of dendritic arborization by neurons with shared functions within the cir-
cuit, or more generally, the formation of nonoverlapping dendritic fields
between adjacent neurons. Tiling arises through contact-mediated repulsion
between nascent dendrites and is mediated by multiple cell surface signals
on the membranes of dendrites (e.g. dsCAMs; Hattori et al., 2008 ). Simi-
larly, dendrites from the same neuron can also exhibit tiling and form
nonoverlapping fields. The repulsion between dendrites is mediated by con-
tacts initiated by dendritic filopodia in vivo ( Baker and Macagno, 2007 ).
Similarly, the repulsion of axonal growth cones induced by repellent signals
(e.g. semaphorins) can be mediated by contact of a single filopodium with a
point source of the repellent ( Fan and Raper, 1995 ) of the surface of a cell
with repellent properties ( Oakley and Tosney, 1993 ). Thus, neuronal filopo-
dia serve as receptor structures for both attractant and repellent signals.
2.3. Formation of Axon Collateral Branches and Dendrite
Branches
Although consolidation (Section 2.2 ) greatly decreases protrusive activity
along the axon shaft proximal to the growth cone neck, filopodia can also
form along the axons shaft. These axonal filopodia have a fundamental role
in neurodevelopment, the initiation of axon collateral/interstitial branches
( Dwivedy et al., 2007 ; Gallo, 2011 ). The formation of collateral branches is
required for the proper development of neuronal circuitry and is consid-
ered to be a major mechanism underlying the ability of the injured nervous
system to rewire damaged circuitry ( Fawcett, 2009 ; Gibson and Ma, 2011 ;
Onifer et al., 2011 ). During development, neurons form one axon but need
to establish synaptic connections with up to thousands of other neurons. In
order to achieve this high order of connectivity, the single axon generates
numerous side branches each contributing to the establishment of subsets
of the final set of synapses formed by the neuron. Collateral branching
allows a single neuron to connect to disparate regions of the nervous system
and/or cover a large extent of territory within a single target region. Thus,