Axon fasciculation

Cell adhesion molecules are responsible for axonal fasciculation 117... 

FIGURE 7-6 Neural cell adhesion molecule (NCAM) carries different levels of polysialic acid. (A) NCAM is depicted on a growing axon, without polysialic acid. Under these conditions, NCAM and LI molecules interact homophilically and axons fasciculate. (B) NCAM carries polysialic acid (PSA), and NCAM and LI molecules are far apart and cannot, therefore, interact. Growing axons do not fasciculate under these circumstances. 

When antibodies against fasciclin II are applied to the grasshopper embryo, and the embryo allowed to develop further in a tissue culture medium, the initial growth of the MPl axon along non-neuronal substrates is not affected. However, when the antibodies are applied just before, or after the MPl axon fasciculates with the MPl/dMP2 fascicle, the posterior growth of the axon is stalled and the axon often explores other axon fascicles, such as the transverse commissures (Harrelson and Goodman, 1988). 

These results show that fasciclin II is indeed involved in axon fasciculation but is not responsible for the direction of axon outgrowth or the polarity of axon extension along the nerve cord. Furthermore, fasciclin II is clearly not the sole determinant of axon fasciculation for all neurons, as other central neurons which express Fasciclin II protein strongly in wild-type embryos, such as the mo-tomeurons aCC and the three U neurons, do not fasciculate with the MPl fascicle. 

A subsequent monoclonal screen in the Drosophila embryo uncovered another glycoprotein, Fasciclin III, whose pattern of expression is also suggestive of a role in axon fasciculation (Patel et al., 1987). Fasciclin III is expressed on a small subset of neurons, including the growth cones, axons and cell bodies of the RP neurons, and some axons in the anterior and posterior commissures. The Fasciclin III protein is composed of three immunoglobulin domains which are much more divergent than those found in fasciclin II (Fig. 17). 

Tract formation and axon fasciculation of molecu-larly distinct peripheral neuron subpopulations during leech embryogenesis. Neuron 8 559-572. 

Johansen, K.M., Kopp, D.M., Jellies, J. and Johansen, J. (1992) Tract formation and axon fasciculation of molecu-larly distinct peripheral neuron subpopulations during leech embryogenesis. Neuron 8 559-572. 

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