Ultrastructural analyses of modular subunits in the mushroom bodies of the cockroach

The mushroom body is a higher centre of the insect brain involved in sensory association, memory, and some forms of motor control. Recently we have reported that 15 repetitive modular subunits are present in the output regions of the mushroom body in the cockroach. Each modular subunit consists of light and dark slabs. Seeking to clarify morphological organization and function of this unit structure, we compared the ultrastructure of the light and the dark slabs. The pedunculus comprises a huge number of tightly packed Kenyon cell axons (KCAs), glial processes, and extrinsic fibres. The extrinsic fibres and synaptic vesicles in KCAs occur much less frequently in the pedunculus than in α and β lobes. Nevertheless, the pedunculus also reveals light and dark slabs as well as lobes. Ultrastructural quantitative analyses of transversely sectioned KCAs in the pedunculus show that average diameters of KCAs in the light slabs are 5–18% larger than those in the adjacent dark slabs, which are significantly different. These results suggest that the diameter of KCAs is the major factor contributing to the varied density of slabs, at least in the pedunculus. Difference in conduction velocities due to varied diameters of KCAs may be utilized for signal processing.