J. Cell Sci. 81, 207-221 (1986)
207
Printed in Great Britain © The Company of Biologists Limited 1986
INTRACELLULAR ELECTRICAL POTENTIAL
MEASUREMENTS IN DROSOPHILA FOLLICLES
JOHANNES BOHRMANN1 1, ERWIN HUEBNER2, KLAUS SANDER1
AND HERWIG GUTZEIT '*
l
Biologisches Institut I (Zoologie), Universitdt Freiburg, Albertstr. 21a, D-7800 Freiburg,
West Germany
department ofZoology, University of Manitoba, Winnipeg, Canada R3T2N2
SUMMARY
We measured the intracellular electrical potential in oocyte and nurse cells of Drosophila follicles
at different developmental stages (6-14) and determined the intrafollicular potential difference.
During stages 8—10B, when intrafollicular transport is known to occur, no significant potential
difference was found. During late vitellogenic stages the nurse cells assume a more positive
potential than the oocyte. This result contrasts with the published data on Hyalophora follicles, in
which intercellular electrophoresis of negatively charged proteins occurs from nurse cells to oocyte
as a result of an intrafollicular potential difference (nurse cells more negative than the oocyte). Such
a potential difference was not observed in Drosophila follicles at any stage, not even after
application of juvenile hormone. The extrafollicular electrical field is described with a dipole
model. The hypothetical dipole is located in the long axis of the follicle and changes its calculated
length stage-specifically.
INTRODUCTION
In meroistic insect ovaries the nurse cells are intensely engaged in biosynthesis and
contribute much to the rapid growth of the oocyte, whose nucleus is generally
inactive, or nearly so (Bier, 1963; Telfer, 1975). The molecules synthesized in the
nurse cells reach the oocyte via cytoplasmic bridges through which the cytoplasmic
continuity of these germ-line sister cells is maintained. Because of the synthetic
differences between nurse cells and oocyte in polytrophic follicles (like Drosophila)
there is apparently one-way traffic of material from the nurse cells to the oocyte.
Despite the apparent simplicity of the system the mechanism of molecular transport
is still not satisfactorily explained.
Recently, several studies on the electrophysiological properties of meroistic
ovarioles showed that electrical currents traverse the follicles in a number of analysed
species (Jaffe & Woodruff, 1979; Dittmann, Ehni & Engels, 1981; Huebner, 1984;
Sigurdson, 1984; Overall & Jaffe, 1985; Bohrmann et al. 1984; Bohrmann, Dorn,
Sander & Gutzeit, 1986). A functional role for these currents was suggested by the
intriguing observation that in Hyalophora follicles (polytrophic type) and in
Rhodnius and Oncopeltus ovarioles (telotrophic type) microinjected fluorescently
labelled proteins migrate according to their electrical charge by way of 'intercellular
• Author to whom reprint requests should be addressed.
Key words: intracellular electrical potential, intercellular electrophoresis, Drosophila oogenesis.
208
J. Bohrmann, E. Huebner, K. Sander and H. Gutzeit
1A
vfl B
"**
Ooc
Ooc&NC
10-
-20-
-30
4 min
Fig. 1. A. Photograph of a Drosophila follicle stage 10B impaled with a microelectrode
each in the oocyte (Ooc) and in a nurse cell (NC). The follicle is held in place by slightly
pressing it against the edge of a Scotch tape (arrow) with the rounded tip of a glass rod
(GR). FC, follicle cell epithelium covering the oocyte. Bar, 100 ftfn. B. Two-channel
recording from an X/Y chart recorder of the potentials in the oocyte and a nurse cell of a
stage 10B follicle. The two arrows indicate the moments at which the electrode in the
oocyte (1) and the electrode in the nurse cell (2) were withdrawn. There is no potential
difference between oocyte and nurse cell.
electrophoresis' (Woodruff & Telfer, 1973, 1980; Telfer, Woodruff & Huebner,
1981; Woodruff & Anderson, 1984). This concept has been widely accepted and
generalized, although it has not yet been shown that normal constituents of the nurse
cell cytoplasm (as opposed to large quantities of microinjected heterologous molecules) are also subjected to intercellular electrophoresis.
Since intracellular electrical potential measurements have been carried out in only
three species {Hyalophora: Woodruff & Telfer, 1973; Rhodnius: Telfer et al. 1981;
Oncopeltus: Woodruff & Anderson, 1984), we studied Drosophila follicles with the
same methods to see if the electrical properties are compatible with the notion of
intercellular electrophoresis in this species.
MATERIALS AND METHODS
Intracellular electrical potential measurements
Follicles of Drosophila melanogaster (strain Oregon R) were carefully isolated in Robb's medium
(R-14), in which they are able to develop from stage 10 up to stage 14 (Petri, Mindrinos, Lombard
& Margaritis, 1979; Bohrmann, 1981), or in Robb's balanced saline (DPBS; Robb, 1969). The
measuring chamber used was a slight modification of the one described by Kiehart (1982).
Measuring electrodes and a ground electrode (Ag/AgCl wire) passed through a thin film
of paraffin oil covering the saline-filled chamber. A thin glass rod with a rounded tip used
for holding the follicles in place was also inserted horizontally into the chamber (Fig. 1A).
Electrical potential in follicles
209
Intracellular recording pipettes were made using 1 mm Kwik-Fil filament glass capillaries pulled on
a David Kopf model 700 D pipette puller. The measurements were carried out with a pair of 3 MKCl-filled electrodes (5-15 M£2 resistance) using a W. P. Instruments S-7000 A system with two
S-7071 A electrometers and a digital oscilloscope (Nicolet Instruments Corp., model 2090 with the
207 module). Measurements were made by a cursor on the screen and representative recordings
made via the X/Y output using a Hewlett Packard 7015 B X/Y recorder.
We measured the intracellular electrical potentials and the potential differences between the
oocyte and one or more different nurse cells (e.g., see Fig. 1B) in 243 wild-type follicles (stages
6—14; for stages, see King, 1970). They were cultured in R-14 or in some cases in DPBS, with no
differences noted in the results. About half of the analysed follicles were cultured for 10—60 min in a
medium to which the juvenile hormone (JH) analogue Altosid ZR-515 (a gift from Dr Staal,
Zoecon Corp., Palo Alto, CA) was added to a final concentration of 10~7M (see Sigurdson, 1984) or
5X 10~5 M (see Giorgi, 1979). Both concentrations gave the same results. The solutions were mixed
vigorously before use.
Further measurements (without JH) were carried out on 17 dicephalic (die) follicles (LohsSchardin, 1982) with nurse cell groups at either pole of the follicle. Also 12 follicles of the mutant
bicaudal D (bicD) were investigated (kindly provided by Dr T . Schupbach, Princeton, NJ); in
this mutant no oocyte differentiates among the 16 germ-line sister cells (cystocytes).
For statistical evaluation, we used the F test (to compare variances) and the t test (to compare
mean values) according to Sachs (1978).
Resistance measurements
In 20 wild-type follicles (stages 10A-11) we measured the intercellular resistance between the
oocyte and one nurse cell with the voltage-clamp method (see Woodruff & Telfer, 1974). In 15 of
these follicles we also determined the intercellular resistance as well as the membrane resistance of
oocyte and nurse cells (plus follicular epithelium) according to another method described by
Woodruff & Telfer (1974). Current was injected either into the oocyte or into a nurse cell and the
resulting changes in steady-state potentials of the injected and the coupled cell were monitored.
Because of the balanced bridge-compensation circuitry of the W.P.I. S-7071 A electrometers used,
only two electrodes were needed in order to inject current and measure the voltage changes
simultaneously. The intercellular resistance (R{) between oocyte and a nurse cell was then
calculated as:
/,
xU2./l2)-(U2/l2)2
(1)
where U\ is the voltage change in the stimulated cell, I\ the injected current, U2 the voltage
change in the coupled 'cell, and Ur, the voltage change in the latter cell when it is stimulated
by a current I2.
In a similar way and in the same notation, the membrane resistance of the oocyte (i?ooc) or a nurse
cell (Rnc) w a s calculated as:
or
Extrafollicular current measurements
For estimation of total current, extracellular measurements were carried out using a vibrating
probe (based on the method of Jaffe & Nuccitelli, 1974; improved by Dorn & Weisenseel, 1982) as
described by Bohrmannef al. (1986).
RESULTS
Changes of intracellular potential in oocyte and nurse cells during vitellogenesis
The intracellular potential varied considerably between different follicles of the
same developmental stage. However, after we had recorded the potentials of a large
210
J. Bohrmann, E. Huebner, K. Sander and H. Gutzeit
number of follicles (n = 243), a consistent pattern of potential changes during
different phases of oogenesis emerged. For most of vitellogenesis (stages 8-1 OB) the
mean intracellular potentials recorded in nurse cells and oocyte ranged between —20
and — 22 mV (Fig. 2A). However, some significant changes were apparent both
before and after this phase. At stages 6 and 7 (just before and at the beginning of
vitellogenesis) the nurse cell potential was less negative than at mid-vitellogenic
stages 9—10. A similar observation was made during late vitellogenesis, when the
nurse cells became increasingly depolarized up to about — 9mV at stage 13. At the
same time the oocyte became hyperpolarized, reaching about — 24 mV (Fig. 2A). All
these potential changes between stages 6 and 9 as well as between stages 9 and 13 are
statistically significant ( P < 0 - 0 5 ) .
Potential differences between oocyte and nurse cells
The potential differences were recorded simultaneously with one electrode in the
oocyte and a second electrode in a nurse cell (Fig. 1A,B). Before stage 8 and after
stage 10B the potential differences are significantly different from zero (P < 0-05). At
these stages the differences are always positive, i.e. the nurse cells represent the
anode of the intrafollicular electrophoresis unit. The reverse relationship has been
found in follicles of Hyalophora (Woodruff & Telfer, 1973) as well as in ovarioles of
Rhodnius (Telfer et al. 1981) and Oncopeltus (Woodruff & Anderson, 1984).
From the Hyalophora data, the maximal potential differences in Drosophila should
be expected to occur during stages 8-10B. However, we found no significant
differences ( P > 0 - 1 0 ) between nurse cells and oocyte in follicles at these stages
(Fig. 1B, 2 A , C ) . Thus Drosophila lacks a pre-condition for efficient transport of
charged molecules and organelles by intercellular electrophoresis — a sufficient
potential difference between nurse cells and oocyte - while such differences have
been reported in Hyalophora, Rhodnius and Oncopeltus (Woodruff & Telfer, 1973;
Telfer et al. 1981; Woodruff & Anderson, 1984).
Effect ofjuvenile
hormone
Juvenile hormone (JH) was reported to increase the electrical potential difference between tropharium and oocyte in Rhodnius by depolarizing the oocyte
Fig. 2. A,B. Diagram of the mean intracellular electrical potentials and their stagespecific changes in wild-type nurse cells (•) and oocytes (•) of stage 6—14 follicles in
R-14 medium (A) and in medium with juvenile hormone analogue (B). Bars indicate
standard deviation; n, numbers of investigated follicles. At stage 6 the accurate determination of the potential of the oocyte was impossible due to its small size. Some mean
values of potential measurements in follicles of the mutants die and bicD are included in A
and c. (•) die nurse cells; (O) die oocytes; (H) bicD nurse cells. C,D. Diagram of mean
potential differences (nurse cell minus oocyte) during stages 7-13 in normal (c) and JHcontaining culture medium (D) determined by simultaneously recording in a nurse cell
and the oocyte. In many cases the potentials of several nurse cells in one follicle were
measured one after the other relative to the oocyte. (A) wild-type; (A) die.
211
Electrical potential in follicles
+JH
Stage
6
7
8
9
10A lOBi 11
12
13
14
7
8
9
1OA
1OB
D
+ 15
+ 10-1
+5
n = 4
(H
12
48
37
58
17
-5
mV
25
16
10
-JH
10
Fig. 2
+JH
11
12
13
14
212
J. Bohrmann, E. Huebner, K. Sander and H. Gutzeit
(Telfer et al. 1981). In Drosophila follicles JH produced a minor but noticeable effect
in the opposite direction. The oocyte was significantly hyperpolarized at stages 10B,
12 and 13 (P<0-05; Fig. 2B), while there was no effect on the nurse cell potential
(P>0 - 10). As a result, a small but significant positive potential difference
( P < 0-025; Fig. 2D) already became apparent at stage 10B, in contrast to non-JHtreated follicles, where it appeared at stage 11. JH also abolished significantly
(P > 0-10) the positive potential difference (P < 0-05) during stage 7 of oogenesis by
hyperpolarizing the nurse cells (P<0-05). However, at no stage did JH cause
significant negative potential differences as reported for Rhodnius (Teller et al.
1981).
Mutants dicephalic and bicaudal D
Some follicles of the mutant dicephalic (die) possess nurse cell clusters at both
poles of the follicle, so that the growing oocyte is nursed from both ends (LohsSchardin, 1982). The intracellular potential measurements (17 follicles) of the nurse
cells (at both ends) and the oocyte gave no indication of obvious differences from
wild-type follicles (Fig. 2A,c). In some cases we were able to measure the intracellular potential in individual nurse cells that did not participate in the process of
regression, from stage 10B onward, of the nurse cell chamber (Bohrmann, 1981).
Presumably, these cells lack an intact cytoplasmic bridge(s) through which cytoplasm is transported (see Frey, Sander & Gutzeit, 1984). Our measurements indicate
that these 'intact' nurse cells lose their negative potential along with the regressing
nurse cells (Fig. 2A,c).
In follicles of the mutant bicaudalD (bicD; T. Schupbach, unpublished data) all
16 cystocytes appear to differentiate into nurse cells, so that these follicles do not
possess a functional oocyte capable of entering vitellogenesis; the follicles are blocked
at stage 6. The nurse cell potential of these follicles (—12-1 ± 3-2mV (S.D.), n = 12)
did not differ significantly (P>0-10) from that of wild-type follicles at stage 6
(-16-1 ±6-4mV (s.D.), n = 7).
Determination of resistance
The intercellular resistance between oocyte and nurse cells at stages 10A—11
was determined using three different methods as described by Woodruff & Telfer
(1974). The voltage-clamp method gave consistently the highest values (Table 1A).
Alternatively, current was injected into one cell (either oocyte or nurse cell) and the
change in the steady-state potential of both cells was recorded simultaneously. The
calculated resistance (see Materials and Methods) did not differ significantly when
current was injected into either the oocyte or a nurse cell (P>0 - 05; Table 1A).
On the basis of the latter measurements the membrane resistances of oocyte and
nurse cells (including the follicular epithelium) were also calculated (see Materials
and Methods and Table IB). The resistance of the nurse cell membranes was found
to be significantly higher than that of the oocyte membrane (P<0-05; Table IB).
Stage-specific differences were not detected.
Electrical potential in follicles
213
Estimate of intrafollicular current
Although we have been unable to detect significant intercellular potential
differences between oocyte and nurse cells during mid-vitellogenic stages (see
above), the follicles still possess an extracellular electrical field that has been
characterized previously with a highly sensitive vibrating probe (Bohrmann et al.
1984, 1986; Overall & Jaffe, 1985).
The current flux through the follicle can be estimated by integrating the current
flowing through the plane that bisects the extrafollicular electrical field, as shown in
Fig. 3A (see also Weisenseel, Nuccitelli & Jaffe, 1975). If the current is considered as
originating from a dipole consisting of a point source and a point sink located inside
the follicle (which is suggested by the geometry of the extrafollicular electrical field
with its lateral maxima; see Bohrmann et al. 1986) the field (E) must fall with the
distance (r) from the dipole axis according to:
E<x (r2 + Hz)-3/1.
(3)
where H is the dipole half-length (see Weisenseel et al. 1975, and Fig. 3D).
When the actual decrease in current density (which is proportional to E) with the
distance from the follicle axis was determined for 21 wild-type and two die follicles
(stages 8-11) with the vibrating probe and the experimental data were fitted to the
curves of different dipole half-lengths (e.g., see Fig. 3A-C) a surprising result was
obtained: the estimated length of the hypothetical dipole in the follicle axis is only
about 2 jim (H — 1 /im) in wild-type follicles at stages 8 and 9 as well as in both die
follicles at stages 10A and 11 (Table 2), but in wild-type follicles it increases up to
about 140-180 /im (H about 70-90 /im) at stages 10A-11 (Table 2). Although we do
not know the actual intrafollicular location of current source and sink, these data
suggest that at stage 10A some fundamental electrophysiological properties of the
follicle change. In the small number oldie follicles analysed, this change could either
be hidden by the extreme differences in the extrafollicular electrical field compared
to wild-type follicles (see Bohrmann et al. 1986), or such changes might not occur.
By means of the best-fitting curve (for examples, see Fig. 3A—c) we extrapolated
for each follicle the current density (i8) directly at the follicle surface and then
calculated, with the corresponding half-length (H) and the follicle radius (a), the
total current (/) flowing outside the follicle through the bisecting plane (Fig. 3A):
,
(4)
(see Weisenseel et al. 1975). In individual follicles this current varies between 0-2
and 12nA and has a stage-specific maximum at stage 10A/B (Fig. 4). The currents
calculated in a different way by Overall & Jaffe (1985) lie in a similar range.
If current source and sink are located near the oocyte/nurse cell border (also
assumed in the model of Jaffe & Woodruff (1979) for Hyalophora), the voltage (U)
214
J. Bohrmann, E. Huebner, K. Sander and H. Gutzeit
Table 1A. Intercellular resistance in vitellogenic follicles
Voltage clamp
method
Intercellular resistance
x±S.D. (kQ)»
24-8 ±7-8
Method after Woodruff & Telfer (1974),
current applied to:
Oocyte
Nurse cell
9-0 ± 6-9
6-4 ±5-4
B. Membrane resistance of oocyte and nurse cell
Oocyte membrane Nurse cell membrane
Membrane resistance
2-5 ± 1 2
4-4 ±3-4
6-8 ±2-8
X±S.D.
*n = 15 Drosophila wild-type follicles stages 10A-11 (always same sample); five more follicles
were measured by the voltage clamp method only.
that would produce this extrafollicular current (/) can be calculated, since the
membrane resistances of oocyte (Rooc) and nurse cells (Rnc) a r e known (Table IB):
U = IX(ROOC+Rnc).
(5)
This calculated potential difference (—1 to — 84/iV) between oocyte and nurse cells
is compatible with our intracellular measurements, since such low voltage differences
are well below the sensitivity of glass capillary electrodes. From the voltage and the
known intercellular resistance (Table 1A) we calculated the intrafollicular current
between the oocyte and one nurse cell to amount to only 0 04—13nA in different
follicles (stages 8-11).
DISCUSSION
In this and the preceding paper (Bohrmann et al. 1986) we have shown that in
Drosophila follicles stage-specific electrical phenomena can be recorded with
intracellular and extracellular electrodes. The results reported here are qualitatively
and quantitatively different from the results obtained in other insect species,
although we applied the same methods and worked with the same underlying
assumptions. Particularly surprising to us was the absence of a statistically
Fig. 3. A, inset. The extrafollicular current is integrated over the stippled plane that
bisects the follicle and its electrical field. The long axis of the follicle is considered to be
the axis of the hypothetical dipole. With the vibrating probe ( 11 ) we measured the
current density parallel to this axis at various distances from the follicle surface. A-C.
Examples of current density decrease with the distance (r) from the follicle's axis in wildtype follicles (stages 9—11). Each follicle is shown with the same symbols at different
measuring positions. The current density (i) is given in relative units (»'/i), where (T) is the
standard current density at 10 /an from the follicle's surface. The data of different
developmental stages fit to theoretical curves calculated for different dipole half-lengths
(H) with eqn (3) (see the text and Table 2). D. Examples of the curves calculated for
different dipole half-lengths (H), shown in fim. Field strength (E) is given in arbitrary
units with a proportional factor of 106, which resulted in best-fitting curves.
Electrical potential in follicles
Direction of
current flow
Measuring
\
-
215
8
1-0
"•
position
0-8-
0-8
Bisecting plane
0-6 •
Long axis
0-6"
0-4-
0-4Stage 9
H = 1 um
0-2
0-2
I(X)
2(K)
300
100
r (/mi)
200
300
r (
D
X
Stages 10B and 11
0-6-
0-4
0-2
100
200
300
300
Fig. 3
jf. Bohrmann, E. Huebner, K. Sander and H. Gutzeit
216
Table 2. Stage-specific change in half-length of electrical dipole
Number of
follicles
Stage
Wild-type
1
1
70 ± 2 2
74 ± 1 6
85 ± 7
1
1
2
2
8
9
10A
10B
11
10A
11
dicephalic
Estimated half-length H (/im)
(x±S.D.)of
hypothetical electrical dipole
6
ft
I
1
1
significant intercellular potential difference between oocyte and nurse cells during
mid-vitellogenic stages. Could, perhaps, a small voltage difference of —1 to —84/xV
(the range suggested by our calculations), which we would not pick up with
intracellular recordings, still result in intercellular migration of charged molecules?
If the electrophoretic mobility is about lfims" 1 per Vcm" 1 (Jaffe, 1977;
Nuccitelli, 1983) the distance, for example, between an apical nurse cell and the
ooplasm bordering the nurse cells at stage 10A (approx. 150/im, which is also about
the length of the hypothetical dipole) would be traversed by a charged molecule in
20—150 h. Such slow migration of molecules appears unreasonable in view of the fact
that the oogenesis in Drosophila (stage 1-14) takes only about 80 h altogether
(Mahowald & Kambysellis, 1980). However, electrophoretic transport over short
distances (from the proximal nurse cells to the oocyte) is more likely to occur: if one
101
I
U
6
4-
Stage
X
l
)
IDA IDA IMA H 1MB
11
Fig. 4. Diagram of stage-specific differences in mean total current through 21 wild-type
follicles (stages 8-11), calculated using eqn (4) (see the text). Bars indicate 1/2 standard
deviation; n, numbers of investigated follicles.
Electrical potential in follicles
217
assumes that the calculated potential difference acts only over a small distance of, for
example, 5 fim through the cytoplasmic bridge, this distance would be traversed in
1-8 min.
The potential difference observed during late stages of oogenesis is opposite to the
expected value and the results from other insects: in Drosophila the bulk of
macromolecules and organelles (mostly negatively charged) if moved electrophoretically would migrate instead from the oocyte into the nurse cells 1 However,
during the phase of nurse cell regression the nurse cell cytoplasm streams into the
oocyte, presumably as a result of microfilament contraction (Gutzeit, 1986) and,
therefore, an electrophoretic transport mechanism would be superfluous (and
probably ineffective) at this stage of oogenesis. Yet in other species with polytrophic
ovaries (including Hyalophora) cytoplasmic streaming by contractile mechanisms
does not seem to occur (H. Gutzeit, unpublished data).
We have compiled the available data for Drosophila and Hyalophora in Table 3 to
permit a direct comparison. For Hyalophora intrafollicular potential differences of
several mV were recorded during mid-vitellogenesis, and consequently the estimated
electrophoretic velocity of molecules between an apical nurse cell and the oocyte
(approx. 500/jm) was about 50 times higher than that calculated for Drosophila
follicles at comparable stages (9-10). Also the calculated current between a single
nurse celland the oocyte was 100-200 times higher in Hyalophora than in Drosophila
follicles. Another difference between the two species is that in Drosophila the
membrane resistance of the nurse cell chamber and the oocyte is nearly the same as
the intercellular resistance between nurse cells and oocyte. Therefore, comparatively
more current should be shunted via the extrafollicular current loop in Drosophila
than in Hyalophora, in which the membrane resistance is 10 times higher than the
intercellular resistance. The differences beween both species in intrafollicular
potential difference cannot be attributed to juvenile hormone, which in Rhodnius was
reported to increase the potential difference between oocyte and tropharium (Telfer
et al. 1981). But we cannot exclude the possibility that Drosophila follicles, in order
to establish or maintain an electrical potential difference, need some unknown factors, lacking in our culture medium. However, transport processes are maintained in
this medium, as shown by time-lapse filming and in vitro autoradiography (Gutzeit
& Koppa, 1982; H. Gutzeit, unpublished data), and by the fact that the follicles
develop in vitro from stage 10 up to stage 14 (Petri et al. 1979; Bohrmann, 1981).
JH was found to affect the oocyte and the nurse cell potential independently in
different developmental stages. Whereas the oocyte was hyperpolarized significantly
only at stages 10B, 12 and 13, the nurse cells were hyperpolarized only at stage 7.
Probably, at stage 14 the oocyte is not affected by JH because the chorion forms a
barrier. The low nurse cell potential of stages 6—7 (see also bicD follicles) is raised by
JH applied during stage 7, to reach levels that are normally reached later during
oogenesis (stages 8-10).
We often found that different nurse cells within the same follicle had different
intracellular electrical potentials (also some of them may be positive while others are
negative compared to the oocyte potential). A potential gradient related to the
Calculated potential difference
based on extrafollicular
current
Calculated electrophoretic
velocity of negatively charged
molecules from nurse cells to
oocyte
Measured potential difference
between one nurse cell
and oocyte
Membrane resistance
Intrafolllcular resistance
Total extrafollicular current
Intrafollicular current between
one nurse cell and cmyte
Intracellular potential of
oocyte and nurse cells
U,
v,,,
=-21mV
1-7ph-'
Unc =-21 mV
hsophila
(stages 9- 10)
-46 mV
-50 mV
Unc -40 mV
36-468 p h-'
-30 mV
-40 mV
-47 mV
v,,,,
U,
Hyalophora
(comparable midvitellogenic stages)
Table 3. Comparison between Drosophila and Hyalophora
References (for Hyalophora)
(our calculation)
Woodruff & Telfer (1973)
Telfer et al. (1981)
Woodruff, Huebner &
Telfer, personal
communication
Woodruff & Telfer (1974)
Woodruff & Telfer (1974)
Jaffe & Woodruff (1979)
Woodruff & Telfer (1974)
(our calculation, based on
extrafollicular current)
Woodruff & Telfer (1973)
Telfer et al. (1981)
Woodruff, Huebner &
Telfer, personal
communication
(our calculation)
D
z
5?
9
Electrical potential in follicles
219
distance of the respective nurse cell from the oocyte was not found. It appears that
nurse cells can individually regulate their membrane potential.
The nurse cells become depolarized during late vitellogenesis but this is not only
due to their regression caused by cytoplasmic streaming into the oocyte, since nonregressing nurse cells, which are often found in die follicles of stages 13 and 14
(Bohrmann, 1981), also lose their negative intracellular potential in a stage-specific
manner. The depolarization of the nurse cells during late stages of vitellogenesis
(observed with JH already from stage 10B on) is not reflected in the extrafollicular
current pattern (Bohrmann et al. 1986; Overall & Jaffe, 1985). These observations
indicate that at least part of the extrafollicular current pattern is produced by the
follicular epithelium, not exclusively by the germ-line cells.
The observed increase in oocyte potential during oogenesis correlates well with
the intracellular potential of fertilized eggs (—27 mV) and early embryonic stages
(Miyazaki & Hagiwara, 1976). Extracellular electrical currents (Bohrmann et al.
1984, 1986; Overall & Jaffe, 1985) may reflect the necessity of maintaining this negative intracellular potential in the developing oocyte as well as in the young embryo.
Unfortunately, we know very little about the role of the follicular epithelium for
the intra- and extrafollicular current pattern. The follicle cells that migrate between
the oocyte and nurse cells have been thought to play a role in the intrafollicular
current loop of Hyalophora follicles (Jaffe & Woodruff, 1979). In Drosophila these
cells migrate centripetally at the time when a drastic increase in the hypothetical
dipole length has just appeared (late stage 10A to early 10B) and when the
extrafollicular current is at the maximum level. Furthermore, groups of Ca2+-rich
cells were detected amongst this population of follicle cells (Heinrich & Gutzeit,
1985) but their function remains to be elucidated.
We thank Drs M. Weisenseel and A. Dorn for helpful discussions and for making it possible to
carry out the vibrating probe work in their laboratory. The Deutsche Forschungsgemeinschaft
gavefinancialsupport. Facilities in the laboratory of E.H. are maintained with Canadian NSERC
funds.
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(Received30July
1985 -Accepted 10 September 1985)