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(
Fig. 1
D). Gap junctions, or electrical synapses, are formed between selective
neuron-neuron, neuron-muscle, or neuron-glial partners (
Fig. 1
E). Reanalysis of
the original EMmicrographs have recently identified close to 7000 chemical synapses
and 500 gap junctions; a web-based interactive wiring diagram is found on
http://
www.wormatlas.org/neuronalwiring.html
. The cellular complexity and anatomical
details make the C. elegans nervous system an ideal model system to study neuronal
development
in vivo at the resolution of single-cell and single-synapse.
B. Overview of Nervous System Development
All neurons are descendants of the founder cell AB. Individual neurons are
produced in an invariant lineage fashion. The identity of a neuron is determined
based on the position, axon morphology, and unique features such as neurotrans-
mitter type, dye-filling ability, and distinct molecular labels. Questions concerning
nervous system development of the nervous system generally fall into several
categories, reflecting the developmental sequences of a neuron (
Fig. 2
). The early
steps of neuronal development concern neurogenesis and specification of neurons.
The late steps address neuronal differentiation, which include a series of events from
neuritogenesis, polarity determination, axon path finding or guidance, to synapse
formation. Recently, the maintenance and plasticity of the nervous system have also
emerged as a fascinating topic in the analysis of neuronal development.
Early approaches to the analysis of neuronal development relied heavily on
lineage studies using Nomarski microscopy, combined with labeling methods such
as dye filling and antibody immunostaining. Morphological changes in genetic
Fig. 2
Illustration of the major steps of development from a neuronal precursor cell to a mature neuron.
(For color version of this figure, the reader is referred to the web version of this topic.)
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