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Journal of Electron Microscopy 48(1): 55-62 (1999)
© 1999 Oxford University Press

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

Masayuki Iwasaki1,*, Makoto Mizunami2, Michiko Nishikawa1, Tsunao Itoh1 and Yoshiya Tominaga1

1Division of Biology, Faculty of Science, Fukuoka University Fukuoka 814-0180
2Laboratory of Neuro-Cybernetics, Research Institute for Electronic Science, Hokkaido University Sapporo 060-0812, Japan

*To whom correspondence should be addressed. E-mail: iwasaki{at}ssat.fukuoka-u.ac.jp

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 {alpha} and ß 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.

Keywords     pedunculus, slab, Kenyon cell, axon diameter, electron microscopy, Periplaneta americana

Received     18 June 1998, accepted 13 October 1998


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