Essential Cell Biology 5th edition

Microtubules
585
nerve cell body
–
–
microtubule
become more dynamic, switching between growing and shrinking much
more frequently than cytoplasmic microtubules normally do. This change
enables microtubules to disassemble rapidly and then reassemble into
the mitotic spindle. On the other
ECB5
hand,
E17.16/17.17
when a cell has differentiated into
a specialized cell type, the dynamic instability of its microtubules is often
suppressed by proteins that bind to the ends or the sides of the microtubules
and protect them against disassembly. The stabilized microtubules
then serve to maintain the organization of the differentiated cell.
Most differentiated animal cells are polarized; that is, one end of the cell
is structurally or functionally different from the other. Nerve cells, for
example, put out an axon from one end of the cell and dendrites from
the other (see Figure 12−30). Cells specialized for secretion have their
Golgi apparatus positioned toward the site of secretion, and so on. The
cell’s polarity is a reflection of the polarized systems of microtubules in
its interior, which help to position organelles in their required location
within the cell and to guide the streams of vesicular and macromolecular
traffic moving between one part of the cell and another. In the nerve cell,
for example, all the microtubules in the axon point in the same direction,
with their plus ends toward the axon terminals; along these oriented
tracks, the cell is able to transport organelles, membrane vesicles, and
macromolecules—either from the cell body to the axon terminals or in
the opposite direction (Figure 17–17).
Although some of the traffic along axons travels at speeds in excess of 10
cm per day (Figure 17–18), it could still take a week or more for materials
to reach the end of a long axon in larger animals. Nonetheless, movement
guided by microtubules is immeasurably faster and more efficient
than movement driven by free diffusion. A protein molecule traveling
by free diffusion could take years to reach the end of a long axon—if it
arrived at all (see Question 17−12).
The microtubules in living cells do not act alone. Their activity, like those
of other cytoskeletal filaments, depends on a large variety of accessory
proteins that bind to them. Some of these microtubule-associated proteins
stabilize microtubules against disassembly, for example, while
mitochondrion
backward
transport
(to cell body)
axon
outward
transport
(to axon
terminal)
axon
terminal
+
+
Figure 17–17 Microtubules guide the
transport of organelles, vesicles, and
macromolecules in both directions along
a nerve cell axon. All of the microtubules
in the axon point in the same direction, with
their plus ends toward the axon terminal.
The oriented microtubules serve as tracks
for the directional transport of materials
synthesized in the cell body but required
at the axon terminal. For an axon passing
from your spinal cord to a muscle in your
shoulder, the journey takes about two
days. In addition to this outward traffic
(red circles), which is driven by one set
of motor proteins, there is traffic in the
reverse direction (blue circles), which is
driven by another set of motor proteins.
The backward traffic includes worn-out
mitochondria and materials ingested by
the axon terminals.
Figure 17–18 Organelles can move
rapidly and unidirectionally in a nerve
cell axon. In this series of video-enhanced
images of a flattened area of an invertebrate
nerve axon, numerous membrane vesicles
and mitochondria are present, many of
which can be seen to move. The white circle
provides a fixed frame of reference. These
images were recorded at intervals of 400
milliseconds. The two vesicles in the circle
are moving along microtubules toward the
axon terminal. (Courtesy of P. Forscher.)
vesicles
5 µm