HELGOLANDER MEERESUNTERSUCHUNGEN
HelgolSnder Meeresunters. 43, 157-i69 (1989)
An ultrastructural study of oogenesis in the polychaete
Nephtys hombergi Savigny
M. G. Bentley
Gatty Marine Laboratory, University of St. Andrews; St. Andrews, Fife KY16 8LB,
Scotland, U.K.
ABSTRACT: The polychaete Nephtys hombergi has an annual cycle of reproduction. Ovaries were
fixed for electron microscopy during the gametogenic phase from September to March, and during
the breeding and post-breeding period. Oogenesis takes place entirely within the ovary, the
integrity of which is maintained by a network of simple follicle cells. Previtellogenic oocytes have
close contacts with the peri-vasal cells which surround the genital blood capillaries. These contacts
are lost as the oocytes enter vitellogenesis. The vitellogenic oocytes have a cytology typical of
oocytes which are thought to undergo autosynthetic production of protein yolk. Biochemical studies
would be required to establish whether heterosynthesis of yolk also occurs. As the oocytes proceed
through vitellogenesis, cortical material is laid down near the periphery of the oocyte and a
microvillous surface is developed. When the microvillous surface is complete the oocytes, by then
hormone independent, are ovulated from the ovary and are ready to be spawned.
INTRODUCTION
The ovaries of Nephtys hombergi are paired structures which are p r e s e n t in each of
the post-pharyngeal segments (Olive, 1978). D e v e l o p m e n t of oocytes within the ovaries
commences in September and continues throughout the winter months (Olive, 1978). The
oocytes are retained within the ovaries until vitellogenesis is complete. They are then
released from the ovaries (Olive & Bentley, 1980) a n d float freely in the coelomic fluid
until they are s p a w n e d in the spring (Bentley et al., 1984). The ovaries develop around
b l i n d - e n d i n g blood capillaries at the base of the parapodia. A light microscopical study of
the ovaries a n d oocyte d e v e l o p m e n t was carried out by Olive (1978) and this showed that
the ovary consists of a network of fine peritoneal cells, which m a k e up a simple follicle
cell system. The term follicle cell is used to describe these cells b e c a u s e of the very close
relationship they have to the developing oocytes a n d b e c a u s e of the essential role they
play in allowing normal oogenesis to proceed (Olive & Bentley, 1980). The follicle cells
surround the oocyte throughout their d e v e l o p m e n t a n d the integrity of the ovary is
m a i n t a i n e d by contact areas b e t w e e n the follicle cells a n d developing oocytes.
Vitellogenic oocytes of IV. hombergi require the support of a gonadotrophic hormone
for normal oogenesis to proceed (Olive & Bentley, 1980). This h o r m o n e m a i n t a i n s follicle
cell contacts thus p r e v e n t i n g premature ovulation a n d resorption of oocytes which have
not completed the formation of a microvillous surface. Once vitellogenic oocytes have
completed the microvillous surface, contacts b e t w e e n oocytes a n d follicle ceils are lost
and the oocytes are released from the ovaries prior to spawning.
9 Biologische Anstalt Helgoland, Hamburg
158
M . G . Bentley
O o g e n e s i s in ?4. hombergi b e g i n s with the production of p r i m a r y o o c y t e s a r o u n d the
capillaries of the genital blood vessels. Usually, the ovary at this time, c o n t a i n s a n u m b e r
of oocytes well into oogenesis a n d at almost full size, as a result of i n c o m p l e t e s p a w n i n g
of the previous y e a r ' s cohort of oocytes in the spring (Olive et al., 1981a; Olive et al.,
1981b; Olive et al., 1985). Whilst most u n s p a w n e d oocytes are resorbed, i t m a y b e that
s o m e of these oocytes will r e m a i n in the ovary t h r o u g h o u t the n e x t p h a s e of
g a m e t o g e n e s i s a n d b e s p a w n e d in the following April or May. This p h e n o m e n o n of
i n c o m p l e t e s p a w n i n g seems to b e a c o m m o n feature of N. hornbergi in t h e River Tyne
e s t u a r y b u t is v a r i a b l e in extent from y e a r to y e a r (Olive et al., 1985).
MATERIALS AND METHODS
S p e c i m e n s of Nephtys hombergi w e r e collected, by d i g g i n g in s a n d a t low w a t e r of
s p r i n g tides, at a site in the River Tyne estuary, N o r t h - e a s t E n g l a n d k n o w n locally as the
"Black M i d d e n s " . Animals w e r e a n a e s t h e t i z e d in 0.07 % MS222 (Sandoz) in sea water.
T h e ovaries w e r e r e m o v e d b y m a k i n g a m i d - d o r s a l incision t h r o u g h the b o d y wall a n d
lifting the ovaries out using w a t c h m a k e r s forceps. Excised ovaries w e r e i m m e r s e d
i m m e d i a t e l y in 2.5 % g l u t a r a l d e h y d e in 0.2 M p h o s p h a t e buffer (Millonig, 1961) containing 0.14 M NaC1 at p H 7;4 a n d fixed for l h at room temperature. T h e o v a r i e s w e r e then
r i n s e d in p h o s p h a t e buffer containing 0.14 M NaC1 for 10 rain. a n d p o s t - f i x e d in 2 %
o s m i u m tetroxide in 0.2 M p h o s p h a t e buffer at p H 7.4 containing 1.25 % N a H C O 3 for l h
(Fischer, 1975). The tissue was then rinsed in several c h a n g e s of tap w a t e r a n d d e h y d r a t e d t h r o u g h an alcohol series a n d p r o p y l e n e oxide before b e i n g e m b e d d e d in araldite.
Sections w e r e cut using an LKB Ultratome, m o u n t e d on c o p p e r grids a n d s t a i n e d with
Reynold's l e a d citrate and uranyl acetate. S t a i n e d sections w e r e e x a m i n e d on either an
EM6B or Kratos Cora electron microscope.
RESULTS
Protogonia and pre-meiotic stages
In this study no special a t t e m p t was m a d e to e x a m i n e g e r m cell proliferation a n d
s t a g e s of meiotic prophase. Light microscopical e x a m i n a t i o n h o w e v e r r e v e a l s the pres e n c e of protogonia in i n t e r p h a s e on the g e n i t a i blood vesseIs (Olive, 1978). Mitotic
figures have b e e n o b s e r v e d in acetic orcein s q u a s h p r e p a r a t i o n s (Bentley, u n p u b l i s h e d )
a n d stages of meiotic p r o p h a s e are c o m m o n l y s e e n in the ovary in S e p t e m b e r . Oocytes
r e m a i n arrested at this stage until after s p a w n i n g a n d g e r m i n a l vesicle b r e a k d o w n occurs
following liberation of oocytes into the s u r r o u n d i n g sea water.
Previtellogenic oocytes
Previtellogenic oocytes can b e s e e n in the ovary of Nephtys hombergi from Sept e m b e r to April b u t are p r e s e n t in l a r g e s t n u m b e r s at the b e g i n n i n g of the p e r i o d of active
g a m e t o g e n e s i s in September. T h e y a r e found in the ovary in a r e a s n e a r t h e b l i n d - e n d i n g
b l o o d capillaries with which t h e y m a y d e v e l o p close associations (Fig. 1).
Previtellogenic oocytes have a large, p r o m i n e n t nucleus w h i c h has a d i a m e t e r of
Oogenesis in Nephtys hombergi
159
10-20 ~tm. The nucleus is a fairly e m p t y looking structure which m a y h a v e a nucleolus
situated n e a r the n u c l e a r envelope. The nucleolus is a h o m o g e n o u s e l e c t r o n - d e n s e
structure. In some previtellogenic oocytes t h e nucleolus a p p e a r s a b s e n t a n d the nucleus
contains d i s p e r s e d chromatin lumps, some of which are c o n c e n t r a t e d close to t h e n u c l e a r
e n v e l o p e (Fig. 2). T h e r e is evidence for transfer of chromatin-hke m a t e r i a l across the
nuclear e n v e l o p e to the cytoplasm (Fig. 3). The cytoplasm of previtellogenic oocytes is
relatively featureless. Mitochondria are the most a b u n d a n t cell inclusion a n d these are
ovoid in shape. The mitochondria m a y b e locahsed in areas n e a r the n u c l e a r m e m b r a n e ,
w h e r e t h e y are found in association with accumulations of chromatin-like material. T h e y
also occur further a w a y from the nucleus a n d n e a r the cell surface. Previtellogenic
oocytes also contain small amounts of e n d o p l a s m i c reticulum which often h a v e connections with the n u c l e a r envelope. A few lipid droplets m a y also be s e e n in oocytes at this
stage. Golgi b o d i e s are virtually a b s e n t in early previtellogenic oocytes.
The surface of previtellogenic oocytes is characterised b y the p r e s e n c e of a smooth
o o l e m m a a n d an a b s e n c e of microvilli. T h e oocytes are completely s u r r o u n d e d b y fo!licle
cells (Fig. 4) a n d there is close contact b e t w e e n the p l a s m a m e m b r a n e of the follicle cells
a n d the oolemma. No specialised junctions or cytoplasmic continuity occurs b e t w e e n
a d j a c e n t cells as has b e e n r e p o r t e d in some p o l y c h a e t e species (Fischer, 1975) a n d which
occurs b e t w e e n early spermatocytes in Nephtys (Bentley, u n p u b l i s h e d observations). It is
at this stage h o w e v e r that the oocytes h a v e their most intimate contact with other cells.
A r o u n d the blood capillaries in the centre of the ovaries, peri-vasal cells are e n v e l o p e d b y
the cytoplasm of the oocytes (Fig. 5) a n d these peri-vasal cells m a y pass t h r o u g h "holes"
in the oocyte cytoplasm. Similar a r r a n g e m e n t s h a v e b e e n r e p o r t e d in s o m e other
p o l y c h a e t e s Phragmatopoma lapidosa (Eckelbarger, 1979), Kefersteinia cirrata (Olive &
Pillai, 1983) and Diplocirrus glaucus (see Olive, 1983) but these contacts are r a t h e r more
complex than those d e s c r i b e d here in Nephtys. Figure 6 shows a d i a g r a m m a t i c r e p r e s e n tation of the ovarian structure in Nephtys a n d shows the c h a n g e s w h i c h occur in the
oocyte surface a n d the c h a n g i n g relationship b e t w e e n the,oocyte, follicle cells a n d perivasal cells as oogenesis progresses.
Vitellogenic oocytes
Oocytes enter vitellogenesis w h e n t h e y attain a d i a m e t e r of a b o u t 50 ~ n a n d h a v e a
n u c l e a r d i a m e t e r of about 20 ~tm. This p h a s e of d e v e l o p m e n t continues within the ovary
until the oocytes h a v e c o m p l e t e d vitellogenesis and h a v e a d i a m e t e r of 180-200 btm with
a n u c l e a r d i a m e t e r of a b o u t 40 ~tm. The nucleus during vitellogenesis r e m a i n s electronlucent with a g r a n u l a r a p p e a r a n c e a n d the nucleolus either b e c o m e s b i p a r t i t e a n d is
e l e c t r o n - d e n s e with a less d e n s e l y staining medulla, or it m a y b e c o m e f r a g m e n t e d .
The vitellogenic p h a s e is c h a r a c t e r i s e d b y the a p p e a r a n c e of active golgi c o m p l e x e s
which p r o d u c e e l e t r o n - d e n s e m a t e r i a l (Fig. 7) a n d which m a y a s s u m e a spherical
configuration during later vitellogenesis (Fig. 8). The yolk granules are up to 2.5 ~tm in
d i a m e t e r and a c c u m u l a t e throughout the cytoplasm during this growth p h a s e . The y o l k
granules are h e t e r o g e n o u s in a p p e a r a n c e , h a v i n g irregularly s h a p e d d e n s e l y staining
areas (Fig. 9) a n d t h e y m a y also h a v e a r e g u l a r striated p a t t e r n over part of t h e m (Fig. 10).
Lipid droplets also a c c u m u l a t e in the c y t o p l a s m during vitellogenesis. Rough e n d o p l a s mic reticulum is p r e s e n t in the c y t o p l a s m a n d has mitochondria in close proximity to its
160
M. G. Bentley
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Oogenesis in Nephtys
hombergi
161
cisternae. Connections b e t w e e n r o u g h e n d o p l asm i c reticulum and annulate l a m e l l a e can
b e s e e n close to the nuclear e n v e l o p e (Fig. 11).
Early during the vitellogenic phase, cortical vesicles b e g i n to b e d ep o si t ed n e a r the
oocyte surface. T h e s e are about 1 ~m in diameter and are el ect r o n - l u cen t e x c e p t for
fibrillar inclusions in some (Fig. 12). A few electron-dense, m e m b r a n e - b o u n d cortical
granules which m a y be spherical or c i g a r - s h a p e d and are also about 1 ~m in d i a m e t e r are
p r e s e n t n e a r the surface of the oocyte at this stage (Fig. 12). As vitellogenesis proceeds,
large bodies of a n n u l a t e lamellae (Fig. 13) a p p e a r in the cytoplasm and these give rise to
clear vesicles w h i c h contain small granular inclusions (Fig. 14). T h e s e vesicles m a y
r e p r e s e n t e x p a n d e d cisternae of r o u g h e n d o p l a s m i c reticulum.
In mature oocytes the cytoplasm is full of protein yolk granules and lipid droplets.
T h e mitochondria h a v e d e v e l o p e d "hollow" vesicles (Fig. 15), an d golgi c o m p l e x e s are
mostly absent, although a few n o n - a c t i v e profiles remain. Th e cortical r e g i o n of the
oocyte has large n u m b e r s of cortical vesicles and cortical granules w h i ch contain rods h a p e d inclusions.
D e v e l o p m e n t of t h e e g g e n v e l o p e a n d m i c r o v i l l i
Previtellogenic oocytes, as d e s c r i b e d above, h a v e a smooth o o l e m m a w h i c h is
i n v e s t e d in a covering of follicle cells. T h e plasma m e m b r a n e of the follicle cell is closely
ap p l i ed to the oolemma. There are no specialised junctions b e t w e e n the o o l e m m a and
follicle cell m e m b r a n e although close junctions b e t w e e n adjacent follice cells do occur
(Fig. 16). At these points there is a t h i c k e n i n g of the plasma m e m b r a n e s of a d j a c e n t
follicle cells. Similar d e s m o s o m e s h a v e b e e n reported b e t w e e n a d j a c e n t follicle cells in
Phragmatopoma lapidosa (Eckelbarger, 1979).
Early during vitellogenesis, the egg envelope or chorion and microvilli begin to
develop. This occurs in small patches on the surface of the oocyte. In these areas, the
close contact of the o o l e m m a and follicle cell m e m b r a n e is lost. At other points on the
oocyte surface, there r e m a i n l a r g e areas w h e r e close contacts are retained. T h e s e folliclecell contact areas b e c o m e smaller with the progressive d e v e l o p m e n t of the microvillous
surface of the oocyte. This is illustrated diagrammatically in F i g u r e 6. Finally, as the
Fig. 1. A previtellogenic oocyte in close association with the peri-vasal cells which surround a blood
capillary in an ovary. Scale bar = I ~m (Bv = blood vessel, N = nucleus, Pvc = peri-vasal cell, Pvo =
previtellogenic oocyte)
Fig. 2. Chromatin situated in close proximity to the nuclear envelope. Scale bar = 1 ~m {Ch =
chromatin, N = nucleus)
Fig. 3. Transfer of chromatin-like material across the nuclear envelope of a previteUogenic oocyte.
Scale bar = 0.5 Bm (N = nucleus)
Fig. 4. Part of a previtellogenic oocyte and its associated follicle cell covering. Follicle ceils
completely cover the surface of previtellogenic oocytes. Scale bar = 1 ~m (Fc = follicle cell, M =
mitochondria, N - nucleus, Pvo = previtellogenic oocyte)
Fig. 5. Previtellogenic oocytes may almost completely surround the perivasal cell. Scale bar = 1 ~m
(Pvc = peri-vasal cell, Pvo = previtellogenic oocyte)
162
M. G. B e n t l e y
Ivo
6
Fig. 6. A diagrammatic representation of ovarian development in Nephtys hombergi. The previtellogenic oocytes are found close to the genital blood vessel and are completely surrounded by, and in
close contact with the follicle cells. At the oocytes progress through vitellogenesis they are found
further away from the blood vessel, and follicle cell contacts become reduced until they remain as
simple stalks (arrowed) (evo = early vitellogenic oocyte, gbv = genital blood vessel, lvo = late
vitellogenic oocyte, pc = perivasal cell, pvo = previtellogenic oocyte)
Fig. 7. Active golgi complexes in the cytoplasm of a vitellogenic oocyte. Electron-dense material is
being produced by the golgi, and large numbers of mitochondria are associated with the active golgi
profiles. Scale bar = 1 ~m (G = golgi complex, M = mitochondria)
Fig. 8. A spherical golgi profile and associated electron-dense material. Scale bar = I ~m (G = golgi
complex)
Fig. 9. A general view of the cytoplasm of a vitellogenic oocyte showing a number of yolk granules
which have irregular d e n s e staining patches in them. Scale bar = 1 ~m (G = golgi complex, Yg =
yolk granule)
Fig. 10. A yolk granule showing characteristic striations within the body of the granule. Scale bar -0.5 ~tm (Yg = yolk granule)
Fig. 11. Rough endoplasmic reticulum which may be associated with annulate lamellae near the
nuclear membrane. Scale bar = 0.5 ~m (M = mitochondrion, Rer = rough endoplasmic reticuhim)
Oogenesis in Nephtys hombergi
rt
163
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164
M . G . Bentley
oocytes mature, the ovary assumes a structure rather hke a b u n c h of g r a p e s , in which
oocytes are attached to the follicle cell network by stalks.
The surface of the early viteliogenic oocytes consists of simple b u l b o u s microvilh
(Fig. 17) 0.5 ~m high and about 0.2 Bm across the tips which p r o t r u d e through the
vitelline envelope. The vitelline envelope is a n electron-dense l a y e r outside the
oolemma. It has a bi-Iaminar structure about 0.27 ~tm thick with a thick i n n e r layer a n d a
thin, more electron-dense outer layer. Microvilli in late viteUogenic oocytes have stellate
tips (Figs 18, 19). The d e v e l o p m e n t a n d the structure of the surface of the oocyte a n d its
relationship to the follicle cells has imphcations regarding the h o r m o n a l support of
oogenesis (Ohve & Bentley, 1980), since the completion of a microvfllous surface must be
attained before the oocytes are h o r m o n e i n d e p e n d e n t a n d can b e o v u l a t e d a n d spawned.
P o s t - s p a w n i n g r e s o r p t i o n of u n s p a w n e d o o c y t e s
Oosorption has b e e n reported following incomplete s p a w n i n g or s p a w n i n g failure in
a n u m b e r of polychaetes i n c l u d i n g Nephtys hombergi (Olive et aL, 1981a, b) w h e r e it
seems to play an important role in the co-ordination of the reproductive cycle. Premature
oosorption in IV. hombergi occurs following gonadotrophic hormone d e p r i v a t i o n during
the vitellogenic phase. The ultrastructural c h a n g e s which occur in the o v a r y a n d oocytes
following this h o r m o n e deprivation have b e e n described in detail (Olive & Bentley, 1980).
The ultrastructure of ovaries in which natural oosorption is taking place is essentially the
same as that of ovaries in which oosorption has b e e n e x p e r i m e n t a l l y i n d u c e d by
decerebration (hormone deprivation) a n d so will not be described here.
DISCUSSION
Oogenesis in Nephtys hombergi is of an ovarian type in which t h e oocytes are
retained in ovaries by a simple folhcle cell system.-This kind of o o g e n e s i s could be
regarded as of a type intermediate b e t w e e n solitary [or extraovarian (Eckelbarger, 1984)]
oogenesis, in which oocytes are ovulated prior to the c o m m e n c e m e n t of vitellogenesis,
a n d follicular [or intraovarian, (Eckelbarger, 1984)] oogenesis w h e r e n u t r i e n t s m a y be
supplied to the developing oocytes b y the follicle cells or from the g e n i t a l blood vessel.
The ultrastructure of solitary oogenesis has b e e n described in a n u m b e r of polychaete
Fig. 12. Electron-lucent cortical vesicles and electron-dense cortical granules near the periphery of a
vitellogenic oocyte. Scale bar = 1 ~tm. (Cg = cortical granule, Cv = cortical vesicle)
Fig. 13. A large body of annulate lamellae in the cytoplasm of a vitellogenic oocyte. The presence of
these is characteristic of oocytes in late vitellogenesis. Scale bar = 1 ~tm (A1 = annulate lamellae)
Fig. 14. Clear vesicles associated with a body of annulate lamellae. These vesicles may represent
expanded cisternae of rough endoplasmic reticulum. Scale bar = 1 ~tm (A1 = annulate lamellae, V =
vesicle)
Fig. 15. Part of an oocyte which has completed vitellogenesis. It has large numbers of cortical
granules at the surface and characteristic hollow mitochondria. Scale bar = 1 ~tm (Cg = cortical
granule, M = mitochondrion)
Oogenesis
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Nephtys hombergi
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M.G. Bentley
species including Nereis diversicolor (Dhainaut, 1967, 1969a, b), Nereis pelagica
(Dhainaut, 1970; Dhainaut & Porchet, 1977a, b), Nereis virens (Fischer & Dhainaut, 1985),
Platynereis dumeri]ii (Fischer, 1975), a n d Arenicola marina (Rashan & Howie, 1982). In
some cases, such as in Pla~nereis dumerilii (Fischer, 1975), there may be intimate contact
of early oocyte stages with the follicle cells. These appear to play no part in the supply of
nutrients to the developing oocytes but rather have a mechanical, supportive role (see
Olive, 1983; Eckelbarger, 1984 for review). Intraovarian oogenesis may involve nutritive
support for the oocyte from folhcle cells or from the genital blood vessel, or the folhcle cell
system may simply serve to contain the oocytes throughout vitellogenesis. In Phragmatopoma lapidosa (Eckelbarger, 1979) and Streblospio benedicti (Eckelbarger, 1980)
complex contacts between oocytes and genital blood vessels are maintained throughout
oogenesis, and endocytotic profiles (coated pits) provide evidence that the oocytes obtain
part of their protein yolk material (a heterosynthetic component) directly from the blood
vessels. In a number of other polychaetes whose ultrastructure has been described such
as Kefersteinia cirrata (Ohve & Pillai, 1983) and Harmothoe imbricata (Oarwood, 1981)
and in Nephtys hombergi, reported here, oocytes are retained within the ovaries throughout vitellogenesis and whilst complex contacts between follicle cells, blood vessels and
oocytes may exist at the previtellogenic stage, these contacts are lost as vitellogenesis
proceeds and there is no ultrastructural evidence to suggest any heterosynthetic yolk
synthesis. The simple follicle ceil system which exists in Nephtys hornbergi during
vitellogenesis maintains the integrity of the ovary by simple contacts between the
oolemma and follicle cell membrane. It has been established that a gonadotrophic
hormone maintains these contacts until the oocytes have completed the formation of a
microvillous surface (Olive & Bentley, 1980). The cytology of the developing oocytes in N.
hombergi is typical of oocytes which are believed to have autosynthetic yolk production.
There is the presence of large numbers of mitochondria, rough endoplasmic reticulum
and active golgi profiles producing material which condenses to form yolk granules.
Annulate lamellae are also present but although they ~re commonly observed in oocytes
their precise function remains unknown (see Eckelbarger, 1984). Whilst there may be no
ultrastructural evidence to suggest that heterosynthetic yolk formation occurs (coated pits
at the surface of the oocyte or synthetic apparatus within the follicle cells) this cannot be
ruled out and biochemical studies would be required to establish this. The use of radiolabelled precursors as probes for the investigation of oocyte yolk synthetic pathways have
now been carried out, in conjunction with electron microscopy, in a number of polychaete
species, most notably in the Nereidae. The identification of a high molecular weight yolk
protein precursor "Nereis vitellin" in Nereis virens (Fischer, 1979; Fischer & Schmitz,
1981; Fischer et ai., 1984) and Perinereis cultrifera (Baert, 1985, 1986a, b) has enabled the
heterosynthetic pathway and incorporation into the oocyte of one type of yolk protein to
be demonstrated (Fischer et al., 1984; Fischer & Dhainaut, 1985). A second, autosynthetic
pathway of yolk protein formation has also been demonstrated in the Nereidae by the
uptake of radio-labelled amino acids (and sugars for the synthesis of complex polysaccharides) (Dhainaut, 1967, 1969, 1970; Dhainaut & Porchet, 1977a, 1977b; Dhainaut,
1984). An investigation into the biochemistry of yolk synthesis in Nephtys would clearly
tell us a great deal, especially in the fight of the information we have available on the
gonadotrophic hormone in this species (Olive & Bentley, 1980; Bentley, 1982; Bentley &
Olive, 1982).
Oogenesis in
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Nephtys hombergi
167
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Fig. 16. A complex junction b e t w e e n two adjacent follicle cells. These junctions do not occur
b e t w e e n oocytes and follicle cells. Scale bar = 0.5 ~m
Fig. 17. Bulbous microvilli of an early vitellogenic oocyte. Scale bar = 0.1 ~m
Fig. 18. A tangential section through the surface of a late vitellogenic oocyte. The microvilli at this
stage have narrow stalks and hexagonal tips. Scale bar = 0.5 ~m
Fig. 19. The microvilli of a late vitellogenic oocyte have narrow stalks and spiky tips. Scale bar = 0.5
~m
168
M.G. Bentley
Acknowledgements. Part of this work was carried out in the Department of Zoology, University of
Newcastle upon Tyne under SERC grant GR/B/99583 to P. J. W. Olive. The author is grateful to Dr.
P, J. W. Olive for commenting on the manuscript and to R. McD. Hewit for technical advice,
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