Plasmodesmata are narrow channels that act
as intercellular cytoplasmic bridges to facilitate
communication and transport of materials between
plant cells.
The plasmodesmata serve to connect the symplastic
space in the plant and are extremely specialized
channels that allow for intercellular movement
of water, various nutrients, and other molecules
(including signalling molecules) (Epel, 1994).
Plasmodesmata are located in narrow areas
of cell walls called primary pit fields, and
they are so dense in these areas (up to one
million per square millimeter) that they make
up one percent of the entire area of the cell
wall.
Since the function of plasmodesmata are so
closely associated with their rather complex
structure, it is necessary to delinate the
make-up of plasmodesmata before we discuss
their functions in plants.
It has been demonstrated that the plasma membrane
is continuous between cells, the outer leaflet
contiguous with the cell wall and the inner
leaflet contiguous with the plasmodesmal pore
(general structure reviewed by Epel, 1994;
Overall and Blackman, 1996; and Oparka, 1993).
Within the plasmodesmal pore, a tightly wound
cylinder of membrane termed the desmotubule
runs the length of the plasmodesma.
Thus, the desmotubule is essentially a tube
within a larger tube bordered by the plasma
membrane.
The structure of the desmotubule and how it
relates to the overall structure of plasmodesmata
was studied by Tilney, et al. (1991) by using
plasmolysis, Triton X detergent extraction,
and protease digestion.
This investigation utilized fern (Onoclea
sensibilis) gametophytes by cutting them in
half, exposing the cut surfaces to Triton
X 100, and then fixing the gametophytes.
This detergent solubilized the plasma membrane
component of the plasmodesmata of the gametophytes,
but the desmotubule was not affected by the
detergent.
However, when the cut gametophyte surfaces
were exposed to papain, the desmotubule is
destroyed but the plasma membrane remained
intact (yet swollen).
Finally, the gametophytes were plasmolyzed,
and it was found that plasmodesmata remained
intact as long as the desmotubule stayed in
its normal, fixed position as the cells detached
from the cell walls.
Thus, Tilney, et al. (1991) suggested that
the desmotubule provides a rigid stability
to plasmodesmata and confers a fixed diameter
and pore size to the plasmodesmal canal, much
like a cytoskeletal structure.
However, one should 
be cautious in mistaking the desmotubule as
a completely rigid strcuture, since the desmotubule
is linked to the endoplasmic reticulum in
each of the adjacent cells, forming a dynamic
endomembrane continuum in the symplastic space,
a topic to be discussed in more detail later.
The space between the plasmalemma and the
desmotubule is the cytoplasmic sleeve or cytoplasmic
annulus, and transport through plasmodesmata
has been proposed to occur either through
the lipid portions of 
the desmotubule or this cytoplasmic sleeve
(or both).
In some plasmodesmata, there is 
a region at each end of the plasmodesmal channel
that is constricted and termed the neck region,
where the plasma membrane component of the
plasmodesmata 
closely associates 
with 
the central desmotubule.
The neck region is proposed to 
contain several, spoke-like protein subunits
that are located both extracellularly and
between the desmotubule 
and 
the plasmalemma (linking these two structures),
and these proteins can 
then 
act 
as 
a sphincter to regulate the passage of materials
through the plasmodesmata, much like gap junctions
in animal cells.
