Biology Reference
In-Depth Information
Fig. 18.5 Schematic illustration of a mature neuron showing an inactive (dismanted) centrosome
and the severing of microtubules. As the neuron matures, the ability of the centrosome to nucleate
microtubules diminishes. Microtubule number is increased by severing of pre-existing long
microtubules into short mobile pieces, followed by the transport of short microtubules into the
axon or the dendrites. These short microtubules can serve as seeds for assembly of longer
microtubules
The authors posited, based on these results, that microtubule-nucleating
capacity becomes de-centralized as the neuron develops such that nucleation of
microtubules can occur throughout the cell body and potentially even within axons
and dendrites themselves. We would agree, assuming that nucleation in this
context means assembly from pre-existing microtubules, even very short frag-
ments that are able to act as seeds for new assembly (Baas and Ahmad
1992
; Baas
and Black
1990
; Baas and Heidemann
1986
; Black et al.
1984
; Brady et al.
1984
;
Morris and Lasek
1982
). As neurons mature, it is virtually impossible to com-
pletely depolymerize the more stable microtubules, even with prolonged drug
treatments, so we suspect that the reassembly of microtubules observed by Steiss
and colleagues represents ''nucleation'' from stable microtubule fragments, but not
bona fide nucleation in the de novo sense. Even so, the results reported by these
authors accentuate the fact that the neuron can very ably go on ''auto pilot'' once a
robust microtubule array has been constructed, such that a centralized factory for
microtubule production can be shut down. As noted earlier (and as discussed also
by Steiss and colleagues), the severing of existing microtubules in the cell body as
well as in the axon and the dendrites is presumably sufficient for increasing the
number of microtubules whenever and wherever needed (see Fig.
18.5
).
18.6 Concluding Remarks
It may be relevant to consider that different kinds of neurons go about their
business in somewhat different ways and on different timetables. For example,
central and peripheral neurons may differ with regard to the importance of the
