* Unanswered questions: debates *

Richard Kondo rkondo at ephys.ucla.edu
Wed Jun 26 11:26:37 EST 1996

Margaret Fowler wrote:

	comments to other posters deleted

> From: Richard Kondo <rkondo at ephys.ucla.edu>
> Date: Mon, 10 Jun 1996 13:35:21 -0700
> Organization: UCLA Cardiovascular Research Lab
> >Harold Hillman wrote:
> >>

> >       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'
> >42:39-50
> >
> >       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.

	Comments to Anthony Pelletier deleted.

	Vesicle movement is not my area of expertise, though it 
interests me.  Two comments, though.

	You stated to Dr. Pelletier that since the resolution of 
light microscopy is 200-250nm, while that of electron microscopy is 
<25nm, then the microtubules seen with light microscopy can not be 
the same as the ones seen with EM.  Video enhanced light microscopy 
does not 'see' microtubules similarly as EM does.  This technique 
allows the investigator to see the diffractional pattern created by 
the microtubule, after the contrast of the image has been enhanced.  
The size of the microtubule seen in these images are larger than 
the real McCoy.  In that regard, the Rayleigh criteria (200-250nm 
for light) prevents one from resolving two objects within 200-250nm 
of each other, not from 'seeing' very small objects, given 
sufficient gain and contrast.  Perhaps, you would argue that this 
strengths your argument, because investigators can not state with 
assurance that the microtubule created patterns seen in Sheetz and 
colleagues' beautiful experiments are the same as the microtubules 
seen with EM.  It is a problem, but one that I hold in abeyance till 
I see stronger evidence for another mechanism.

	The 'simpler' mechanisms that you cite, are, in my opinion, 
not simpler, but require more complicated explanation for 
directional movement.

	Perhaps, you were thinking of spindles when you write of 
actin pulling mitochondria.  However, vesicles are proposed to move 
along microtubules using kinesin and dynein as motors.  Consider the 
microtubules to be highways.

Richard Kondo

More information about the Cellbiol mailing list