* Unanswered questions: debates *
rkondo at ephys.ucla.edu
Wed Jun 26 12:46:06 EST 1996
> >Harold Hillman wrote:
> >> 1. Discourtesy, assumptions of ignorance, and emotive remarks are no
> >> substitute for measured argument and evidence. Each of these quest-
> >> ions highlights a contradiction *within* current views; for example,
> >> (a) everyone agrees that intracellular movements can be seen by
> >> low power light microscopy in living cells, yet most people also
> >> believe that there is a cytoskeleton, which would not permit such
> >> movements;
> > rest deleted
> > On the contrary, the cytoskeleton is most likely essential
> >for intracellular movement of organelles. The role of microtubules,
> >associated structural proteins and the ATP driven protein motors,
> >kinesin and dynein have been elucidated over the past 15 years.
> > Brady et al., (1982) Science 'Fast axonal transport in
> >extruded axoplasm from giant squid axon' 218:1129-31.
> > Hayden et al., (1983) Cell Motility 'Cytoplasmic transport in
> >keratocytes: direct visualization of particle translocation along
> >microtubules' 3:1-19
> > Schnapp et al., (1985) Cell 'Single microtubules from single
> >axoplasm support bidirectional movement of organelles' 40:455-62
> Schnapp et al. 1986 Ann NY 4 466 509-518
> > Vale et al., (1985) Cell 'Identification of a novel force
> >generating protein, kinesin, involved in microtubule-based motility'
> > Sheetz et al., (1987) Annals of New York Academy of Sciences,
> >'Movement of vesicles on microtubules' 493:409-16
> > Schnapp and Reese (1989) PNAS 'Dynein is the motor for
> >retrograde axonal transport of organelles' 85:1548-52.
> Vale R D, Schnapp (1985) Cell 43(3) part 2 623-632
> I am sending you my paper (Hillman, 1991) 'Some microscopic considerations
> about cell structure - light versus electron microscopy' Microscopy *36*,
> 557-576, dealing with this question in detail.
> Meanwhile, a few remarks.
> I am sure that you have seen micrographs of networks of tubulin, vimentin,
> spectrin, endoplasmic reticulum, microtrabeculae and actin. In these you do
> *not* see lysosomes, Golgi bodies or mitochondria, and with *all* these
> elements together, they would not allow *enough space* for relatively large
> bodies to move round. The hypothesis that, for example, actin can pull
> mitochondria requires a mechanism and attachments all round the mitochondria
> *and to other structures* otherwise they could not pull. The maximum
> resolution of the light microscope - under which intracellular movements are
> seen in living cells - is 200-250 nm, yet the electron microscopists describe
> them as 25nm. Therefore the microtubules are not the same structures; this
> also applies to those believed to be the spindles pulling the chromosomes to
> the poles during cell division. The hypothesis that the cytoskeleton pulls,
> say mitochondria, ignores the possibility - a simpler one - that Brownian
> movement, diffusion, streaming, convection movements (none requiring
> biological mechanisms) occur in fine granules in fluid.
> Harold Hillman.
Additionally, how do you account for the ATP requirement
for axonal transport in a model using diffusion, convection,...
Adams, RJ Nature (1982) 'Organelle movement in axons
depends on ATP' 297:327-9
Moreover, the original argument (in the first or second
post by Margaret Fowler) against organelle movement suggested
that the 'density' of the cytoskeleton was such to prevent the
movement of larger particles such as organelles. Models using
diffusion, convection,... do not address this problem.
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