Stereological Analysis Reveals Striking Differences in the Structural Plasticity of Two Readily Identifiable Glomeruli in the Antennal Lobes of the Adult Worker Honeybee

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 microm 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.