AMER. ZOOL., 13:1119-1128 (1973).
Functional Evaluation of Low Resistance Junctions: Influence of Cell
Shape and Size
JUDSON D. SHERIDAN
Department of Zoology, University of Minnesota, Minneapolis, Minnesota 55455
SYNOPSIS. Specialized junctions, which allow small molecules to move directly between
adjacent cells in many adult and embryonic tissues, may be involved in electrical
or non-electrical forms of intercellular communication. The ability of the junctions to mediate either form of communication depends on the permeability of the
junctions and on the shape, size, and arrangement of the interconnected cells. Electrical communication depends on junctional resistance and on non-junctional resistance which is a function of cell surface area. Non-electrical communication, it
is argued, depends on junctional permeability to small molecules and on cell volume. The dual dependency of non-electrical communication in particular is discussed in detail and some of the possible implications are illustrated with specific
examples.
INTRODUCTION
Cell junctions that permit the movement
of small molecules from one cell to the next
occur in many embryonic and adult tissues
(Loewenstein, 1966; Furshpan and Patter,
19681). These junctions have been suggested to play a role in two types of intercellular communication, one electrical and
the other non-electrical. Electrical communication (Bennett, 1966) refers to the
intercellular transmission of electrical
potentials and is likely to be important for
certain "excitable" cells, e.g., cardiac and
smooth muscle and certain nerve cells.
Non-electrical communication (Loewenstein, 1966; Furshpan and Potter, 1968)
refers to the intercellular transfer of
nutrients, metabolites, or small regulatory
molecules and may be particularly important for "non-excitable" cells in developing and mature systems. Both types
Supported in part by grants from N1H (CA11114) and from the University of Minnesota
Graduate School.
The author wishes to thank colleagues, W. S.
Herman, R. G. Johnson, M. G. Hammer, and M.
Epstein for helpful discussions and comments on
the manuscript, and also L. Steer for preparing
the figures.
i The references are not meant to be exhaustive;
reviews have been used whenever possible except
where more specific references appear warranted.
of communication depend not only on the
permeability of the junctions to small
molecules, but also in different ways on
factors related to the shape, size, and arrangements of the interconnected cells.
This dual dependency has important implications for developmental biologists interested, on the one hand, in junctional
communication, and, on the other hand,
in the control of cell shape. One such implication is that the changes in junctions
and changes in cell and tissue geometry
which occur together during development
must be coordinated in a way consistent
with the requirements for junctional communication and for morphogenesis.
Others in this symposium have discussed
some of the requirements for morphogenesis. My purpose is to discuss some
suggested requirements for junctional communication. I will begin with the relationships between junctional properties and
cell and tissue geometry and then discuss
some of the possible functional consequences of these relationships. The
methods used in the study of junctions
may be unfamiliar to many developmental
biologists. Therefore, I have simplified
some of the descriptions, especially the
electrical ones.
1119
1120
JUDSON D.
Not Coupled
Coupled
FIG. In (above). Cells A & 15 without low resistance junction are uncoupled (details in text).
FIG. \b (below). Cells A & B with low resistance
junction are coupled (details in text) .
ELECTRICAL COMMUNICATION
T h e rationale behind the study of electrical communication can be illustrated with
a few simple diagrams. Figure \a shows
two cells lacking a low resistance junction.
One cell is impaled with a microelectrodc
for passing current and the other with a
microelectrode for recording voltage. When
the current microeleotrode is made negative, ions begin to flow, i.e., there is an
electric current, across the membrane of
cell A. Nearly all of this current takes path
a and little takes b due to the high resistance (low ionic permeability) of the encircled, apposed membranes. As a result,
the electrode in cell B records no significant change in voltage during the current flow. However, when cells possessing
a low resistance junction are studied in
the same way (see Fig. 16), a potential
SHERIDAN
change does occur in cell B, that is, the
cells are "electrically coupled." The potential change in cell B is caused by the
flow of current along path b which now
occurs because the junctional membranes
have a lower electrical resistance than the
corresponding membranes in Figure \a.
With a more careful look at the coupled
cells using another microelectrode and
Figure 2, we can see that the degree of
coupling depends on the ionic permeability
(resistance) of the non-junctional as well
as junctional membranes. As current flows
along path b, there is first a voltage drop
(VB) across the resistance (r,.) of the membrane of cell B. There is another voltage drop as the current crosses the junctional resistance (ij) and the sum of this
voltage drop and VB equals VA, the volttage drop across the membrane resistance
(rA) of cell A. We can represent the degree of electrical coupling by the ratio,
V],/VA (the coupling coefficient) and examine how this ratio is affected by changes
in r B and ry If the ionic permeability
of the junction increases relative to that
of the non-junctional membrane of cell
B (i.e., rj decreases relative to r,,), the
coupling coefficient will increase, signifying
that the coupling is "better." However, a
relative decrease in permeability of the
junction (i.e., increase in Tj) will have the
opposite effect, that is, the coupling coefficient will get smaller and coupling
will be "poorer." These relationships
can be stated mathematically: V,,/VA =
Coupled
FIG. 2. Electrical model of cells in Fig ]b (details
in lexl; .
JUNCTIONAL COMMUNICATION AND CELL SHAPE
v
smoll
FIG. 3. Arrangement for studying coupling between small and large cell. Microelectrodes for
passing current into each cell omitted to simplify
diagram (details in text) .
1121
ducted to the other {i.e., can electrically
couple the cells). However, the degree of
electrical coupling in the simple, two-cell
system is not a direct measure of junctional
permeability since coupling depends on
non-junctional permeability as well.
Furthermore, degree of coupling is even
less easily related to junotional permeability
in systems involving more than two coupled
cells.
.
'
!
NON-ELECTRICAL COMMUNICATION
The degree of electrical coupling is a
suitable measure of the ability of low resistance junctions to act as electrical
The discussion can be made more con- synapses between certain "excitable" cells,
crete with a specific example (Fig. 3). Cone.g., cardiac and smooth muscle cells, and
sider a large and a small cell having nonjunctional membranes with the same re- certain nerve cells, which depend on the
sistance per unit area (i.e., same specific electrotonic conduction of electrical signals
resistance). If these cells are connected by from cell to cell (Furshpan and Potter,
a low resistance junction they will be 1958; Woodbury, 1962; Burnstock et al.,
coupled, but the degree of coupling will 1963; Bennett, 1966). In most other systems,
be asymmetrical; that is, if current is however, the2 role of potential changes is
passed first into the large cell and then not obvious. Therefore, in these systems
into the small, the coupling coefficient, we are less interested in the electrical asVsmaii/Viarge, in the first direction will be pects of junctional communication and
greater than the coupling coefficient, Vlnrgc/ more interested in changes in concentraV8n,aii> *n t n e opposite direction. The dif- tion of the transferred molecules, i.e., nonference arises from the dependence of electrical communication. To evaluate the
coupling on non-junctional resistance effectiveness of junctions in non-electrical
which in turn varies inversely with surface communication, we need first a more direct
measure of junctional permeability.
area.
There is at present no single method for
Tttie relation between electrical coupling
determining
junctional permeability both
and junctional resistance is even more
quantitatively
and qualitatively. However,
complex in systems involving more than
I
will
argue
in
the next few paragraphs
two coupled cells. In simple terms, the
that
either
junctional
resistance or junceffect of adding more coupled cells, even
when each junction has the same resistance, tional area may be suitaible as an index for
is to lower the degree of coupling between
2 This statement should be made cautiously
any adjacent pair. This happens because since potential changes are associated with nerve
the other cells shunt current away from the and hormone input to some of these other tiscell being tested for coupling. Electrically, sues, e.g., liver (Haylett and Jenkinson, 1969),
this is somewhat analogous to the effect fat (Horwitz et al., 1969), salivary gland (Schneyer
al., 1972) . For these, electrical coupling per se
(illustrated with Fig. 3) of increasing the et
might be important under certain conditions. Fursurface area of the coupled cell.
thermore, some cells previously called "non-exThus, with this simplified analysis we've citable" show active membrane electrical responses
to appropriate stimuli (Dean and Matthews, 1970;
seen that a relative increase in the ionic Nelson and Peacock, 1972), suggesting that the
permeability between two cells can allow distinction between "excitable" and "non-excitpotential changes in one cell to be con- able" cells may not always be clear.
1122
JUDSON D. SHERIDAN
comparing junctional permeabilities in
many systems.
I will begin with a further discussion
of methods for estimating junctional resistance. As we have already seen, junctional resistance is not simply related to
the coupling coefficient. However, with the
simple situation shown in Figure 2, we can
obtain enough information to calculate rj
provided we can impale cell B with a
second microelectrode for passing current.
(Details of this kind of determination are
given in a number of other papers—Bennett, 1966, is particularly thorough.) This
method will be successful only if the two
cells are isolated from other cells or are
part of a linear chain of cells. Unfortunately, this arrangement is rare; cells are more
often arranged in two or three dimensions.
The electrical aspects of multidimensional
systems can be represented by simple threeresistance networks, but the resistances can
no longer be ascribed to particular membranes. Other approaches to two and three
dimensional systems can yield average
values for junGtional and non-junctional
resistances. However, it is possible that
some cells in a population might be more
affected by junctional changes than others.
If so, an average value for junctional resistance could not be used in assessing
the junctional capabilities.
There is another method for testing
junctional permeability using small dye
molecules as intracellular tracers (Loewenstein, 1966; Furshpan and Potter, 1968;
Payton et al., 1969). This method also involves micropipettes, but now used for injecting the tracer dyes into cells as well as
for making electrical measurements. The
dyes most commonly used, e.g., fluorescein
and procion yellow, are charged and are
usually injected electrically (iontophoretically). Figure 4A and B illustrate the
general procedure and some factors important in interpretation. In Figure 4A,
cells A and B lack a low resistance junction. Cell A has been impaled with a
micropipette filled with negatively charged
dye molecules (ff) and positive counterions
(not shown). When the tip of the pipette
Not Coupled
FIG. 4A (above). Cells without low resistance
junction do not pass tracer molecules (0) (details in text) .
FIG. 4B (below). Cells with low resistance junction do pass tracer molecules (details in text).
is made negative, the dye is injected into
the cell. Once out of the pipette, the dye
molecules move primarily in the direction
of concentration decrease, i.e., by simple
diffusion. In this case, provided the dye
permeates the membrane only to a limited
extent (which is true for procion yellow
[Payton et al., 1969] and probably sufficiently true for fluorescein), little dye leaves
cell A and even less gets inside cell B (e.g.,
via pathway labeled b). However, when the
cells are electrically coupled (see Fig. 4B)
the usual finding is that dye appears in
cell B within seconds or minutes, presumably by taking the direct pathway b.
When we compare the two situations in
Figure 1 b and Figure 4J5 we see that each
has advantages and disadvantages for assessing junctional permeability. In both cases
there is a movement of molecules from cell
A to cell B by way of a junction having
relatively greater permeability than the
JUNCTIONAL COMMUNICATION AND CELL SHAPE
1123
rest of the membrane to the molecules that cules. However, the relation between the
are moving. However, there are important junctional resistance and cell size or ardifferences. In Figure \b, we cannot rangement has not been fully evaluated in
identify the species of ion(s) moving across these instances.
A possible structural basis for a close
the junction. We know only that ionic
current is passing and that it is carried relationship between ionic and dye permeaprimarily by the more abundant and more bility has been suggested by studies of the
mobile ions present (e.g., K+). In Figure ultrastructure of low resistance junctions.
4B, however, we know that the substance There is not room in this brief discussion
is moving although we can't directly relate to cover all details of the ultrastructural
its movement to the movement of naturally studies but, for the sake of clarity, just
occurring substances. Also, a quantitative those features which seem most relevant.
value for the electrical resistance of the I will consider exclusively the gap juncjunction can be obtained under favorable tion, or nexus, which is the junction most
conditions, but a comparable quantitation strongly implicated in intercellular comof permeability to tracer dyes is almost im- munication. Figure 5 is a diagram of a
cross-section through a gap junction. Each
possible.
It would obviously be useful if junctional half of the junction contains particles
resistance were directly related to overall which, in this simple view, extend into
permeability of the junction. Much of the the extracellular space and attach to their
future research on junctions will be counterparts from the other cell. The
oriented toward investigating this relation- particles and their arrangements in twoship, but at present we can't say much dimensional clusters have been revealed
with certainty. Dye movement and low by freeze-fracture methods (Kreutziger,
electrical resistance usually occur together 1968; Goodenough and Revel, 1970; Mcor are both absent, but haven't been com- Nutt and Weinstein, 1970; Chalcroft and
pared quantitatively (see for example, Pay- Bullivant, 1970); their interaction across
ton et al., 1969; Johnson and Sheridan, the extracellular cleft has been inferred
1971; Oliveira-Castro and Loewenstein, mainly from studies with extracellular
1971). In certain early embryos, dye move- tracers, e.g., lanthanum hydroxide (Revel
ment between coupled cells has not been and Karnovsky, 1967); the overall width of
observed (Slack and Palmer, 1969; Bennett, the junction and the width of the extraet al., 1972; Tupper and Saunders, 1972; cellular space bridged by the particles has
but see Sheridan, 1971). Thus, in these been determined from thin sections of
cases permeability to small ions may not material stained en bloc with uranyl acereflect permeability to other small mole- tate (Revel and Karnovsky, 1967). (See
Gilula, 1973, for further description of
these methods). Certain ultrastructural feaCELL A
tures are particularly relevant to our dis1 Membrane
cussion
and are characteristic of gap juncJj Cell A
tions in a variety of vertebrate and in] [Membrane vertebrate systems. First, the subunits, or
J| Cell B
particles, have fairly uniform size and
spacing when tightly packed. Second, each
60-85i 90-IOOA
particle in one half of the junction is
CELL B
attached
to a complementary particle in
FIG. 5. Diagram of cross-section through gap junction. Junctional particles are shown to traverse the other half. Third, the number of
each membrane and make contact extracellularly. particles in each gap junction and the total
The postulated intercytoplasmic channel is shown
area of gap junctions between adjacent
only for the particle on the right, although all
cells
are quite variable from tissue to tisparticles presumably contain them according to
sue.
the model discussed in the text.
1124
JUDSON D. SHERIDAN
In order to relate the ultrastructure to
junctional permeability, it has been speculated that a channel, 10-40A in diameter,
runs through the center of each pair of
particles (see Fig. 5) (Payton et al., 1969;
McNutt and Weinstein, 1970). A channel
of this size should allow free movement of
all small inorganic ions and should accommodate any of the dye molecules which
have been shown to pass through the junctions. No conclusive ultrastructural evidence for such channels has been obtained
although they may be related to the small
depressions in junctional particles seen in
freeze-fracture replicas (McNutt and Weinstein, 1970) or to the spots in the center
of polygons seen in lanthanum studies
(Revel and Karnovsky, 1967).
It is clear that there are many discontinuities in the experimental data relating
ionic permeability, dye permeability, and
junctional structure. Nevertheless, the data
provide some justification for using the
following three statements as part of a
working hypothesis: (i) Electrical resistance
of junctions should be directly related to
the inverse of overall junctional permeability, that is, an estimate of junctional resistance should be a good index of permeability to all small molecules which can
pass through the putative junctional channels; (ii) The area of gap junction (actually
the number of pairs of particles) should be
directly related to permeability (or inversely related to resistance); (iii) Junctional permeability and area should be
theoretically interchangeable once the
appropriate conversion factor is known.
Even if we can estimate junctional permeabilities, we need more information in
order to assess relative abilities to carry out
non-electrical communication. Junctional
permeabilities tell us relative rates of
FIG. 6. Diagram of two cell pairs, A & B, and B &
C, connected with identical junctions (details in
text).
•transfer of molecules in moles/sec. However, in order to determine the effect of
a given rate of transfer on concentration
we must consider the volume of the cells
connected. This can be illustrated with a
simple intuitive explanation. Consider the
two pairs of cells, A and B, and C and D,
drawn in Figure 6. Both pairs of cells
have identical gap junctions, i.e., the junctional areas and, thus, junctional permeabilities are the same; the difference in concentration of substance S, [S]r[S]2, is
initially the same for each pair; and as
drawn, cells A and B are smaller than
cells C and D. Since the permeabilities and
concentration differences are identical, the
rates of transfer of S in moles/sec will
initially be the same for both pairs. However, the concentration in the pair of
smaller cells will change at a faster rate
than the concentration in the pair of larger
cells because of the difference in volume.
In order to have comparable rates of change
of concentration in the large cells, the rate
of transfer would have to be increased;
this could only happen with an increase
in the junctional permeability, which probably requires an increase in junctional
area. Thus, to achieve equally effective
non-electrical communication as I've defined it, it is necessary to maintain a constant relationship between junctional area
and volume.
A more quantitative way of looking at
the importance of the relationship between
junctional area and volume can be obtained if we consider in greater detail the
transfer of substance, S, between cells A
and B. If we begin with [S]i and [S]2 as
initial concentrations in A and B and
assume that transfer of S across the junction is slower than diffusion inside the
cells, we can use the following expression
for the time it takes to reach a given fraction of equilibrium (modified from Jacobs,
1935):
t = (V/Aj) (1/2P) In (1/1-FW1),
where,
t = time to reach particular fraction
of equilibrium, in sec,
1125
JUNCTIONAL COMMUNICATION AND CELL SHAPE
V = volume of each cell (both cells
the same size), in cm3,
Aj = junctional area, in cm2,
P = permeability constant, in cm/sec,
F^, = fraction of equilibrium;
This equation can be simplified to give:
60
P<
3
o
O
where kx = (1/2P) In (1— Feq)
which is constant for any particular substance, S.
Thus, the time it would take to reach
a given fraction of equilibrium is directly
proportional to the ratio of cell volume
to junctional area. Since shorter times
would indicate better non-electrical communication, we have in a more quantitative form the relationship predicted from
our intuitive discussion; i.e., that increasing the ratio of junctional area to volume
increases the capability for non-electrical
communication.
The equation we have just considered
was derived for a rather restricted situation in which other processes were not
affecting the concentration of the substance
in the two cells. However, it can be shown
that even in more complex situations, the
ratio, Aj/V, is appropriate for comparing
the ability of two different cell pairs to
carry out non-electrical communication.
Extension of the arguments to multidimensional systems has not been made, but it
is expected that the basic feature of volume
dependency will still be present.
The salient features of the discussion to
this point are summarized in Table 1. In
the rest of the paper I will discuss a few
specific ideas generated, by the junctional
area/volume concept. In developing the
ideas, I have relied on data from a variety
of sources, some less detailed, and thus
less reliable, than others. Therefore, the
quantitative treatments should be considered only rough approximations. Nevertheless, I feel these examples illustrate some
of the possible implications of the dependency of communication on cell and
tissue geometry, and I hope that the examples might suggest fruitful directions for
further study.
ri
o
81
f
=2 S
§
I
•I
3
-4-S
II
II
~
8 1
<M
C
.2 "3
•2
..,
2 -3
3
Q
2 bt>
a •« s «
•.a
o
If
•§ 8
2 5
Mil
1.LE.I
N i? i " iT
1 -s I •§
1126
JUDSON D. SHERIDAN
The first example illustrates how degree
of coupling can be misleading in assessing
the ability of a mitotic cell to maintain
equally effective non-electrical communication with its neighbors. It is known that
cells in mitosis retain junctions with their
neighbors (O'Lague et al., 1970; Merk and
McNutt, 1972). Furthermore, preliminary
evidence suggests that the coupling ratio
between mitotic and interphase cells, i.e.,
ratio of potential change in the mitotic
cell over that in interphase cell supplied
with current, may not be greatly different
from that between interphase cells. However, the simple conclusion that communication is, therefore, the same in the
two cases is not correct as we can see by
considering the expected effect on coupling
of the change in shape of the mitotic cell.
When a cell enters mitosis it rounds up,
thereby decreasing its surface to volume
ratio. If the decrease in surface to volume
ratio comes about by a decrease in surface
area while the volume remains constant,
the cell's non-junctional resistance will
then increase and the coupling will also
increase as long as the junctional area and
resistance remain constant. The evidence
that the coupling remains constant suggests in this case that the junctional resistance has increased presumably by a
decrease in junctional area. The result
would be a decrease in ratio of junctional
area/volume. If the decrease in surface to
volume takes place by an increase in volume with the surface area remaining
constant, then the coupling should stay
constant, again provided the junctional
area and resistance remain constant. For
this case, the evidence for constant coupling suggests that the junctions are unchanged, but the result is the same as
before, a decrease in the ratio of junctional
area to volume. Therefore, no matter how
the decrease in surface to volume ratio
is brought about, constant coupling means
that the junctional area to volume has
decreased, and, somewhat unexpectedly,
that the ability for the mitotic cell to communicate non-electrically with its neighbors
may be impaired.
The second example involves comparison of estimated ratios of junctional area
to cell volume for three cells, liver hepatocyte, brown fat adipocyte, and tissue
culture fibroblast, chosen on the basis of
availability of suitable quantitative data
from the literature.
We can consider a typical liver cell to
be a 15fi cube having a volume of 3375/j.3,
and a total surface area of 1350/x2. If we
take the gap junction area to be 1.5%
of the total surface area (an estimate obtained from morphometric analysis [Bolender, quoted in Goodenough and Stoeckenius, 1972]) we obtain a junctional area
of about 20/x2 and ratio of junctional
area to volume of about 1/169.
The typical brown fat cell can also be
considered a cube with 25/i sides, thus
having a volume of 15.625/*3 and a total
surface area of 3750/*2. If we take the gap
junction area to be 2% of the total area
(an estimate obtained from freeze-fracture
studies [Revel et al., 1971]), we obtain a
junctional area of 75/x2 and junctional
area/volume ratio of about 1/209.
The fibroblast (BHK 21) is best handled
in a somewhat different fashion to avoid
the difficulty of estimating its thickness.
Provided the cell has a relatively uniform
thickness and makes contact with neighbors
only around its lateral surfaces, then the
ratio of junctional area/volume is given
by:
perimeter X thickness X fraction of
contact area as gap junction
upper surface area X thickness
thickness cancels out and only the more
easily obtained measurements remain. The
BHK cell can be approximated by a
rectangle (10/t X 37.5/*) which gives a
perimeter of 95/* and upper surface area
of 375/I2. Taking 0.0005 as the fraction of
contact area occupied by gap junctions
(Revel et al., 1971), we obtain a junctional
area/volume ratio of 1/7800.
Thus, we can see that the liver and fat
cells have similar junctional area/volume
ratios whereas that of the fibroblast is
lower. This may reflect differences in the
JUNCTIONAL COMMUNICATION AND CELL SHAPE
junotional needs of the fibrofolast, but it
is of interest to note that this cell has
many characteristics which suggest it is
not normal; e.g., it exhibits poor contact
inhibition and it will produce tumors
when injected in large numbers.
A comparison of junctional area/volume
ratios in a larger number of cell types is
not possible because the appropriate experimental data are lacking. It is possible,
however, with many cells to predict the
junctional areas they should have in order
that their jumctional area/volume ratios
be the same as that of the liver cell. Such
predictions might then be tested by appropriate ultrastructural measurements. The
two cells which have been chosen for examples have been studied ultrastructurally
and some information is available concerning their junctional areas. The first cell is
a cardiac muscle cell which is essentially
a cylinder 92/x long X 10/* i n diameter having a volume of ~ 7240/A3. In order to have
a junctional area/volume ratio of 1/169,
this cell would need a junctional area of
about 43m2. This value is about 2i/£ times
greater than the junctional area estimated
to occur at the two ends of the cell (Spira,
1971). However, extensive junctions also
occur on the lateral surfaces as well; their
area is unknown, but could conceivably
make up the difference.
The second cell is an epithelial cell from
the midgut of the horseshoe crab. This
cell is essentially rectangular (7/* X 7/* X
45/t) with a volume of 2205^3. For a junctional area/volume of 1/169, the predicted
iunctional area would be 13/*.2. This figure
is not unreasonable in light of the frequent,
extensive junctions found on these cells
(Johnson et al., 1973); however, again more
quantitative data are needed.
The third example illustrates the results
of using three different parameters, degree
of coupling, junctional area, and junctional
area to volume, to assess the relative ability
of cells to carry out non-electrical communication. The cells to be compared are:
normal liver (in vivo), normal liver (in
vitro), Novikoff hepatoma cells (in vitro)
and H-35 hepatoma cells (in vitro). The
1127
cells are ranked in order of decreasing
ability to communicate non-electrically as
assessed by a particular parameter. The
quantitative values given are "representative" and are the best estimates that could
be made from the available data (combined from Penn, 1966; Borek et al., 1969;
Sheridan, 1972, and unpublished observations); the references indicate the sources
for the data used in estimating the parameters, not necessarily for the values of the
parameters themselves. On the basis of
coupling coefficient between adjacent cells,
the ranking would be: normal liver (in
vitro; 0.9) = Novikoff (0.9) > normal liver
fin vivo; 0.7) > > H-35 (0.3). On the
basis of junctional area (except for liver in
vivo, calculated for cell in contact with
neighbors on six sides) the ranking would
be: normal liver (in vivo; 20^2) > > normal liver (in vitro; 1.26^) > Novikoff
(0.6^) > H-35 (0.28/x2). On the basis of
junctional area to volume, the ranking
would be: normal liver (in vivo; 1/169)
> > normal liver (in vitro; 1/1000) >
H-35 (1/1250) > > Novikoff (1/2880). The
model I have developed would suggest that
the last ranking is the more meaningful
one. If so, it is clear that the ranking
based on coupling coefficient, and to some
extent even the one based on junctional
area alone, would be misleading.
CONCLUSION
In this paper I have discussed possible
ways that cell and tissue geometry may be
important in junctional communication.
Many of my arguments have been speculative, especially regarding the use of junctional area/volume ratios in evaluating
capability for non-electrical communication. Yet, I feel that the arguments are
sufficiently plausible to warn against interpreting junctional experiments without
taking into full account the shape, size,
and arrangements of the cells being
studied. As we learn more about the
physiological role of junctions we will be
better able to evaluate further the influence
of cell and tissue geometry. Then we
JUDSON D. SHERIDAN
1128
should have a firmer basis for considering
the integrated control of changes in cell
junctions and changes in cell and tissue
geometry during embryogenesis.
REFERENCES
Bennett, M. V. L. 1966. Physiology of electrotonic
junctions. Ann. N. Y. Acad. Sci. 137:509-539.
Bennett, M. V. L., M. E. Spira, and G. D. Pappas.
1972. Properties of electrotonic junctions between embryonic cells of Fundulus. Develop.
Biol. 29:419-435.
Borek, C, S. Higashino, and W. R. Loewenstein.
1969. Intercellular communication and tissue
growth. IV. Conductance of membrane junctions of normal and cancerous cells in culture.
J. Membrane Biol. 1:274-293.
Burnstock, G., M. E. Holman, and C. L. Prosser.
1963. Electrophysiology of smooth muscle.
Physiol. Rev. 43:482.
Chalcroft, J. P., and S. Bullivant. 1970. An interpretation of liver cell membrane and junction
structure based on observation of freeze-fracture replicas of both sides of the fracture. J.
Cell Biol. 47:49-60.
Dean, P. M., and E. K. Matthews. 1970. Glucoseinduced electrical activity in pancreatic islet
cells. J. Physiol. 210:255-264.
Furshpan. E. J., and D. D. Potter. 1958. Transmission at the giant motor synapses of the crayfish. J. Physiol. 143:289-325.
Furshpan, E. J., and D. D. Potter. 1968. Low-resistance junctions between cells in embryos and
tissue culture, p. 95-127. In A. A. Moscona and
A. Monroy [ed.], Current topics in developmental
biology. Vol. III. Academic Press, New York.
Gilula, N. B. 1973. Development of cell junctions.
Amer. Zool. 13:1109-1117.
Goodenough, D. A., and J. P. Revel. 1970. A fine
structural analysis of intercellular junctions in
the mouse liver. J. Cell Biol. 45:272-290.
Goodenough, D. A., and W. Stoeckenius. 1972.
The isolation of mouse hepatocyte gap junctions. J. Cell Biol. 54:646-656.
Haylett, D. G., and D. H. Jenkinson. 1969. Effects
of noradrenaline on the membrane potential
and ionic permeability of parenchymal cells in
the liver of the guinea pig. Nature (London)
224:80-81.
Horwitz, B. A., J. M. Horowitz, Jr., and R. Em.
Smith. 1969. Norepinephrineinduced depolarization of brown fat cells. Proc. Nat. Acad. Sci.
U.S.A. 64:113-120.
Jacobs, M. H. 1935. Diffusion processes. Ergeb.
Biol. 12:1-160.
Johnson, R. G., W. S. Herman, and D. M. Preus.
1973. Homocelhilar and heterocellular gap junctions in Limulus: a thin-section and freeze-fracture study. J. Ultrastruct. Res. 43:298-312.
Johnson, R. G., and J. D. Sheridan. 1971. Junc-
tions between cancer cells in culture: ultrastructure and permeability. Science 174:717-719.
Kreutziger, G. O. 1968. Freeze-etching of intercellular junctions of mouse liver. Proc. Electron
Microscop. Soc. Am. 26:234.
Loewenstein, W. R. 1966. Permeability of membrane junctions. Ann. N. Y. Acad. Sci. 137:441472.
Merk, F. B., and N. S. McNutt. 1972. Nexus junctions between dividing and interphase granulosa
cells of the rat ovary. J. Cell Biol. 55:511-515.
McNutt, N. S., and R. S. Weinstein. 1970. The
ultrastructure of the nexus. A correlated thinsection and freezecleave study. J. Cell Biol. 47:
666-688.
Nelson, P. G., and J. H. Peacock. 1972. Acetylcholine responses in L cells. Science 177:10051007.
O'Lague, P., H. Dalen, H. Rubin, and C. Tobias.
1970. Electrical coupling: low resistance junctions between mitotic and interphase fibroblasts
in tissue culture. Science 170:464-466.
Oliveira-Castro, G. M., and W. R. Loewenstein.
1971. Junctional membrane permeability. Effects
of divalent cations. J. Membrane Biol. 5:51-77.
Payton, B. W., Bennett. M. V. L., and G. D. Pappas. 1969. Permeability and structure of junctional membranes at an electrotonic synapse.
Science 166:1641-1643.
Penn, R. D. 1966. Ionic communication between
liver cells. J. Cell Biol. 29:171-174.
Revel, J. P., and M. J. Karnovsky. 1967. Hexagonal
array of subunits in intercellular junctions of
the mouse heart and liver. J. Cell Biol. 33:C7C12.
Revel, J. P., A. G. Yee, and A. J. Hudspeth. 1971.
Gap junctions between electrotonically coupled
cells in tissue culture and in brown fat. Proc.
Nat. Acad. Sci. U.S.A. 68:2924-2927.
Schneyer, L. H., J. A. Young, and C. A. Schneycr.
1972. Salivary secretion of electrolytes. Physiol.
Rev. 52:720-777.
Sheridan, J. D. 1971. Dye movement and low resistance junctions between reaggregated embryonic cells. Develop. Biol. 26:627-636.
Sheridan, J. D. 1972. Electrical resistance of junctions between cancer cells. J. Cell Biol. 55:236a.
Slack, C, and J. P. Palmer. 1969. The permeability of intercellular junctions in the early embryo of Xenopus laevis, studied with a fluorescent tracer. Exp. Cell Res. 55:416-419.
Spira, A. W. 1971. The nexus in the intercalated
disc of the canine heart: quantitative data for
an estimation of its resistances. J. Ultrastruct.
Res. 34:409-425.
Tupper, J. T., and J. W. Saunders, Jr. 1972. Intercellular permeability in the early Asterias embryo. Develop. Biol. 27:546-554.
Woodbury, J. W. 1962. Cellular electrophysiology
of the heart. Handb. Phvsiol. 1:237.