Biology Reference
In-Depth Information
FIGURE 5.14
The basic structure of a small microvillus. What cannot easily be captured in the diagram is the
fact that the filament system is not static, but is being built at the barbed end and depolymerized at the pointed end.
Even the rates of these processes are inconstant and most small microvilli last only minutes.
Small microvilli begin to form when microfilaments are nucleated by a patch of proteins
just under the plasma membrane. The polymerization of actin at the tip of the microvillus
features the same problems for the
T. briareus
sperm as discussed above. The problem of actin
transport is present without the water export mechanism to ameliorate it, and direct obser-
vations on the rate of extension of small microvilli of different lengths confirm that the
rate of elongation varies approximately inversely with microvillus length,
57
as would be
expected from the transport equation quoted above. The problem of access may also be
worse
d
presumably whatever membrane-associated nucleating factors initiate microtubule
polymerization must let go and get out of the way of the barbed ends to allow fresh mono-
mers to be added.
When the bundles of actin in microvilli are examined closely, they are seen to have fewer
microfilaments at the tip end than at the base. Since it is unlikely that new filaments would be
nucleated part of the way along a filament, it seems probable that barbed ends face a certain
probability of being capped and therefore blocked from polymerization; if this capping were
to take place randomly, it would result in the observed thinning of filament bundles towards
the microvillus' tip (
Figure 5.15
).
Small microvilli usually last a short time and then regress. This regression takes place at
the steady state rate of pointed end depolymerization, which suggests that the destruction
of a microvillus takes place simply by preventing further polymerization at the barbed
end of the microfilaments. The control of polymerization at the barbed end must be highly
localized to one part of the cell membrane because some small microvilli regress at the
same time that others are growing. The general pattern will be under global controls too,
though
d
an obvious example of a global control would be the availability of free actin.
