Essential Cell Biology 5th edition

708 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
Figure 20–29 Extracellular signals
can regulate the permeability of gap
junctions. (A) A neuron in a rabbit retina
(center) was injected with a dye (dark
stain) that passes readily through gap
junctions. The dye diffuses rapidly from
the injected cell to label the surrounding
neurons, which are connected by gap
junctions. (B) Treatment of the retina with
the neurotransmitter dopamine prior to dye
injection decreases the permeability of the
gap junctions and hampers the spread of
the dye. (Courtesy of David Vaney.)
injected neuron
neurons labeled through gap junctions 20 µm
QUESTION 20–5
(A)
before dopamine
(B)
after dopamine
Gap junctions are dynamic structures
that, like conventional ion channels,
are gated: they can close by a
reversible conformational change
in response to changes in the cell.
The permeability of gap junctions
decreases within seconds, for
example, when the intracellular Ca 2+
concentration is raised. Speculate
why this form of regulation by Ca 2+
might be important for the health of
a tissue.
Figure 20–30 Several types of cell
junctions are found in epithelia in animals.
Whereas tight junctions are peculiar to
epithelia, the other types also occur, in
modified forms, in various nonepithelial
tissues.
example, reduces gap-junction communication between certain neurons
in the mammalian retina when secreted in response to an increase in
light intensity (Figure 20–29). This reduction in gap-junction permeability
alters the pattern of electrical signaling and helps the retina switch from
using rod photoreceptors, which are good detectors of low light levels,
to cone photoreceptors, which detect color and fine detail in bright light.
ECB5 e20.30/20.30
The function of gap junctions—and of the other junctions found in animal
cells—are summarized in Figure 20–30.
Plant tissues lack all the types of cell junctions we have discussed so far,
as their cells are held together by their cell walls. Importantly, however,
they have a functional counterpart of the gap junction. The cytoplasms of
adjacent plant cells are connected via minute communicating channels
called plasmodesmata, which span the intervening cell walls. In contrast
to gap-junction channels, plasmodesmata are cytoplasmic channels lined
with plasma membrane (Figure 20–31). Thus in plants, the cytoplasm is,
in principle, continuous from one cell to the next, allowing the passage of
both small and large molecules—including some proteins and regulatory
RNAs. The controlled traffic of transcription regulators and regulatory
RNAs from one cell to another is important in plant development.
actin
intermediate
filaments
name
tight
junction
adherens
junction
desmosome
function
seals neighboring cells together in an epithelial
sheet to prevent leakage of extracellular
molecules between them; helps polarize cells
joins an actin bundle in one cell
to a similar bundle in a
neighboring cell
joins the intermediate filaments
in one cell to those in a neighbor
gap
junction
forms channels that allow small, intracellular,
water-soluble molecules, including inorganic
ions and metabolites, to pass from cell to cell
basal lamina
hemidesmosome
anchors intermediate filaments
in a cell to the basal lamina