The Journal of Neuroscience, October 1, 2002, 22(19):8514-8522
Stereological Analysis Reveals Striking Differences in the
Structural Plasticity of Two Readily Identifiable Glomeruli in the
Antennal Lobes of the Adult Worker Honeybee
Sheena M.
Brown1, 2,
Ruth
M.
Napper2,
Caryn M.
Thompson3, and
Alison R.
Mercer1
Departments of 1 Zoology, 2 Anatomy and
Structural Biology, and 3 Mathematics and Statistics,
University of Otago, Dunedin, 9001 New Zealand
The primary antennal sensory centers (antennal lobes) in the brain
of the honeybee are highly compartmentalized into discrete spheres of
synaptic neuropil called glomeruli, many of which can be identified
according to their predictable size and location. Glomeruli undergo
significant changes in volume during the lifetime of the adult worker
bee, at least some of which are activity dependent. This study tests
the commonly expressed assumption that increases in neuropil volume are
accompanied by an underlying increase in the number of synapses present
in the tissue. A combination of light and electron microscopy was used
to determine total synapse number within two glomeruli, T1-44 and
T4-2(1). The Cavalieri direct estimator of volume was applied to 1.5 µm sections of resin-embedded brains. Selected sections were then
re-embedded and prepared for transmission electron microscopy. Synapse
densities were determined using the physical disector method on
electron micrographs. Synapse density and glomerulus volume
were combined to give an unbiased estimate of the total number of
synapses. In glomerulus T1-44, a significant increase in volume was
accompanied by a significant increase in the total number of synapses.
In contrast, synapse counts in T4-2(1) remained unchanged, despite a
significant increase in the volume of this glomerulus. These results
demonstrate that synapse proliferation in antennal lobes of the adult
worker bee is highly site specific. Although volumetric changes and
changes in synapse number both contribute to the structural plasticity of the antennal lobes, these two components of plasticity appear to be
independent processes.
Key words:
Apis mellifera; olfactory bulbs; stereology; transmission electron microscopy; structural plasticity; synapse
Copyright © 2002 Society for Neuroscience 0270-6474/02/22198514-09$05.00/0