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Journal of Neuroscience, Vol 14, 2818-2829, Copyright © 1994 by Society for Neuroscience
Changes in microtubule number and length during axon differentiation
W Yu and PW Baas
Department of Anatomy, University of Wisconsin Medical School, Madison 53706.
Hippocampal neurons in culture initially extend several minor processes
that are approximately 20 microns in length. The first minor process to
grow approximately 10 microns longer than the others will continue to grow
rapidly and become the axon (Goslin and Banker, 1989). We sought to define
changes in the microtubule (MT) array that occur during axon
differentiation. In theory, axon differentiation could involve an increase
in MT number, MT length, or some combination of both. To address this
issue, we first serially reconstructed the entire MT array of a minor
process from a cell whose axon had not yet differentiated. This minor
process contained 182 MTs that ranged in length from 0.14 to 20.09 microns.
The average MT length was 3.87 +/- 3.83 microns, and the total MT length
was 704 microns. We then reconstructed the MT arrays of a minor process and
the 56 microns axon from a cell that had undergone axon differentiation.
The minor process contained 157 MTs that ranged in length from 0.24 to
17.95 microns. The average MT length was 3.91 +/- 4.84 microns, and the
total MT length was 600 microns. The axon contained 1430 MTs that ranged in
length from 0.05 to 40.14 microns. The average MT length was 4.02 +/- 5.28
microns, and the total MT length was 5750 microns. These data indicate that
a shift occurs toward shorter as well as longer MTs, but that virtually no
change in average MT length occurs during axon differentiation. Thus,
elongation of existing MTs cannot account for the major expansion of the MT
array that occurs as a minor process becomes an axon. In contrast, the
number of MTs increases by approximately 10-fold as a minor process
differentiates and grows into an axon of the length we analyzed. Based on
these data, we conclude that the MT array of a minor process is
substantially expanded as it differentiates into an axon, and that the
principal mechanism by which this expansion occurs is the copious addition
of new MTs.
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