19
1.2.4.1 Ovary Structure and Oogenesis
in the Family Calloporidae
Reproductive pattern II will be described fi rst using the
family Calloporidae as an example. Calloporids demonstrate
the typical traits of this pattern and have been studied in
more detail in this respect. They lack sexual dimorphism,
sterile and sexual zooids being morphologically indistinguishable; fertile zooids (those with female gonads) initiate
the formation of brood chambers. Below, a detailed portrait
of oogenesis in the type species of Callopora , C . lineata , is
presented. In several instances, examples of other calloporids (genera Callopora , Tegella and Cauloramphus ) are
given and their histology illustrated.
Ovary Structure in Callopora lineata
Oogenesis is localized, i.e. it takes place in a well-defi ned ovary
(Wourms 1987 , but see Schmidt-Rhaesa 2007 ). A non- paired
female gonad (ovary) is located in the distal half of the cystid,
on the basal wall and generally also apposed to one of the lateral
walls (Figs. 1.4 , 1.5 , and 1.6 ). In young zooids with an almost
fully formed (but yet not functioning) polypide, the ovary has a
loose structure consisting of rounded and oval somatic cells
4.5–5.5 μm in diameter. The early female cells (oogonia) can
be distinguished in the central part of the gonad by their vesicular shape and large size (Fig. 1.4A ; see also below).
Ovarian structure changes during the course of growth of
the fi rst oocyte doublet. Cells of the ovary increase in number, as fi rst reported in a fl ustrid (Vigelius 1884b ), and are
rearranged. The fully formed ovary consists of (1) an ovary
wall, (2) the oocyte doublet(s) and (3) the so-called “subovarian space” that was fi rst described by Hageman ( 1983 ).
The leading oocyte occupies the central (often apical) position (Figs. 1.4D and 1.5A, B ) and its animal pole and sides
are surrounded by follicle cells. The oocyte is underlain by the
basal cells and lacunae of the subovarian space that laterally
is bounded by the ovary wall (see Figs. 1.5A–C , 1.6A–C ; for
other cheilostomes see Figs. 1.7A, C , 1.8C, D , 1.9 , 1.10C, D ,
1.11 , 1.12C , 1.13D , 1.14C, D , 1.5 , 1.16C , 1.30A , 1.34C , and
1.35B, C ). At the site where the ovary adjoins the zooid wall,
some basal cells are in direct contact with the epithelial cells
of the cystid (Figs. 1.6C , 1.7C, D and 1.9A, B, D ). Only those
cells of the ovary wall that surround the leading oocyte doublet and directly contact its surface are referred to as follicle
cells in this book. In other words, a follicle is incomplete (see
also Wourms 1987 ), and its cells constitute the “upper” part
of the ovary wall (Fig. 1.5A–C ). This defi nition, however,
may not always strictly apply, especially in species with
ovaries made up of a few cells and a small subovarian space
(see, for instance, Fig. 1.11A ).
It should be noted that, since the position of the subovarian
space varies according to the extent of the contact (or its
absence, see below) between an ovary and the cystid wall, it
would be better to term it “intraovarian space” or “intraovarian
zone”; these three terms are synonymous in this book.
The basal cells of the intraovarian space are visually different from the other cells of the ovary in all respects. Under the
light microscope they appear swollen, are oval, polygonal or
irregular in shape and have pale cytoplasm (transparent,
poorly stained or not stained at all) and dark nuclei
(Figs. 1.5A and 1.9A ). Some of these cells are apposed to the
lower surface of the leading oocyte, which itself directly
fronts the slit-like lacuna between the oolemma and basal
cells. This lacuna is a part of the intercellular lacunar system
of the intraovarian space, which obviously communicates
with the lacunae of funicular strands approaching the ovary
(as in M . serrilamella ; see Hageman 1983 ) but is isolated
from the coelomic cavity of the zooid. In early female gonad
the intraovarian zone is not expressed and the basal cells are
not recognizable under the light microscope. [They are also
hardly visible in species with fully formed ovaries made up
of a few cells (see Sect. 1.2.5 ).] As the oocyte grows, the
intraovarian zone enlarges, occupying from a third (lower) to
half of the gonad (Figs. 1.5A and 1.9A ). Further oocyte
growth is accompanied by considerable fl attening of this
zone (Fig. 1.5C ), so that, prior to ovulation, it is often hardly
discernible (Fig. 1.6C ).
Depending on the degree of free space within the coelom
the mature ovary is oval, ellipsoidal or rounded in
cross- section or, more rarely, irregular. The size of the contact zone between the ovary and the epithelial lining of the
cystid varies. As a rule, the ovary lies on the cystid wall
(Figs. 1.5 and 1.6C ). At the same time the contact zone is
sometimes small as compared to the diameter of the ovary
(Fig. 1.6A ) and can be pedunculate, tapering considerably
towards the base (Fig. 1.7D ).
As synthesis and transport in the ovary are enhanced, the
cells of the gonad wall are progressively enlarged and more
intensely stained, becoming dark at the peak of vitellogenesis. Large pale vacuoles appear in many of these cells
(Fig. 1.5A ). As the oocyte grows, the follicle cells covering
its animal pole fl atten considerably (i.e. become “squamous”) (Figs. 1.5B, C , 1.6C and 1.9 ) while the follicle cells
limiting the oocyte from the sides and the cells of the ovary
wall enveloping the intraovarian zone remain cubic or prismatic (in the wall of some gonads they may be arranged in
two to three layers). For this reason, the ovary containing a
late vitellogenic doublet often acquires a specifi c shape that
is characteristic of many cheilostomes with macrolecithal
eggs. Because of the considerable fl attening of the follicle
cells on the apical pole, oocytes appear to be encased in a
muff of cubic and high prismatic cells (Figs. 1.5C and 1.6C ).
The basal cells adjoining the lower surface of the large leading oocyte fl atten, sometimes wedging themselves between
the oocyte and the ovary-wall cells (Figs. 1.5B and 1.9B ).
Owing to fl attening of the intraovarian space, its zone of contact with the oocyte becomes more extensive.
1.2 Reproductive Patterns of Bryozoa
1.2.4.1 Ovary Structure and Oogenesis
in the Family Calloporidae
Reproductive pattern II will be described fi rst using the
family Calloporidae as an example. Calloporids demonstrate
the typical traits of this pattern and have been studied in
more detail in this respect. They lack sexual dimorphism,
sterile and sexual zooids being morphologically indistinguishable; fertile zooids (those with female gonads) initiate
the formation of brood chambers. Below, a detailed portrait
of oogenesis in the type species of Callopora , C . lineata , is
presented. In several instances, examples of other calloporids (genera Callopora , Tegella and Cauloramphus ) are
given and their histology illustrated.
Ovary Structure in Callopora lineata
Oogenesis is localized, i.e. it takes place in a well-defi ned ovary
(Wourms 1987 , but see Schmidt-Rhaesa 2007 ). A non- paired
female gonad (ovary) is located in the distal half of the cystid,
on the basal wall and generally also apposed to one of the lateral
walls (Figs. 1.4 , 1.5 , and 1.6 ). In young zooids with an almost
fully formed (but yet not functioning) polypide, the ovary has a
loose structure consisting of rounded and oval somatic cells
4.5–5.5 μm in diameter. The early female cells (oogonia) can
be distinguished in the central part of the gonad by their vesicular shape and large size (Fig. 1.4A ; see also below).
Ovarian structure changes during the course of growth of
the fi rst oocyte doublet. Cells of the ovary increase in number, as fi rst reported in a fl ustrid (Vigelius 1884b ), and are
rearranged. The fully formed ovary consists of (1) an ovary
wall, (2) the oocyte doublet(s) and (3) the so-called “subovarian space” that was fi rst described by Hageman ( 1983 ).
The leading oocyte occupies the central (often apical) position (Figs. 1.4D and 1.5A, B ) and its animal pole and sides
are surrounded by follicle cells. The oocyte is underlain by the
basal cells and lacunae of the subovarian space that laterally
is bounded by the ovary wall (see Figs. 1.5A–C , 1.6A–C ; for
other cheilostomes see Figs. 1.7A, C , 1.8C, D , 1.9 , 1.10C, D ,
1.11 , 1.12C , 1.13D , 1.14C, D , 1.5 , 1.16C , 1.30A , 1.34C , and
1.35B, C ). At the site where the ovary adjoins the zooid wall,
some basal cells are in direct contact with the epithelial cells
of the cystid (Figs. 1.6C , 1.7C, D and 1.9A, B, D ). Only those
cells of the ovary wall that surround the leading oocyte doublet and directly contact its surface are referred to as follicle
cells in this book. In other words, a follicle is incomplete (see
also Wourms 1987 ), and its cells constitute the “upper” part
of the ovary wall (Fig. 1.5A–C ). This defi nition, however,
may not always strictly apply, especially in species with
ovaries made up of a few cells and a small subovarian space
(see, for instance, Fig. 1.11A ).
It should be noted that, since the position of the subovarian
space varies according to the extent of the contact (or its
absence, see below) between an ovary and the cystid wall, it
would be better to term it “intraovarian space” or “intraovarian
zone”; these three terms are synonymous in this book.
The basal cells of the intraovarian space are visually different from the other cells of the ovary in all respects. Under the
light microscope they appear swollen, are oval, polygonal or
irregular in shape and have pale cytoplasm (transparent,
poorly stained or not stained at all) and dark nuclei
(Figs. 1.5A and 1.9A ). Some of these cells are apposed to the
lower surface of the leading oocyte, which itself directly
fronts the slit-like lacuna between the oolemma and basal
cells. This lacuna is a part of the intercellular lacunar system
of the intraovarian space, which obviously communicates
with the lacunae of funicular strands approaching the ovary
(as in M . serrilamella ; see Hageman 1983 ) but is isolated
from the coelomic cavity of the zooid. In early female gonad
the intraovarian zone is not expressed and the basal cells are
not recognizable under the light microscope. [They are also
hardly visible in species with fully formed ovaries made up
of a few cells (see Sect. 1.2.5 ).] As the oocyte grows, the
intraovarian zone enlarges, occupying from a third (lower) to
half of the gonad (Figs. 1.5A and 1.9A ). Further oocyte
growth is accompanied by considerable fl attening of this
zone (Fig. 1.5C ), so that, prior to ovulation, it is often hardly
discernible (Fig. 1.6C ).
Depending on the degree of free space within the coelom
the mature ovary is oval, ellipsoidal or rounded in
cross- section or, more rarely, irregular. The size of the contact zone between the ovary and the epithelial lining of the
cystid varies. As a rule, the ovary lies on the cystid wall
(Figs. 1.5 and 1.6C ). At the same time the contact zone is
sometimes small as compared to the diameter of the ovary
(Fig. 1.6A ) and can be pedunculate, tapering considerably
towards the base (Fig. 1.7D ).
As synthesis and transport in the ovary are enhanced, the
cells of the gonad wall are progressively enlarged and more
intensely stained, becoming dark at the peak of vitellogenesis. Large pale vacuoles appear in many of these cells
(Fig. 1.5A ). As the oocyte grows, the follicle cells covering
its animal pole fl atten considerably (i.e. become “squamous”) (Figs. 1.5B, C , 1.6C and 1.9 ) while the follicle cells
limiting the oocyte from the sides and the cells of the ovary
wall enveloping the intraovarian zone remain cubic or prismatic (in the wall of some gonads they may be arranged in
two to three layers). For this reason, the ovary containing a
late vitellogenic doublet often acquires a specifi c shape that
is characteristic of many cheilostomes with macrolecithal
eggs. Because of the considerable fl attening of the follicle
cells on the apical pole, oocytes appear to be encased in a
muff of cubic and high prismatic cells (Figs. 1.5C and 1.6C ).
The basal cells adjoining the lower surface of the large leading oocyte fl atten, sometimes wedging themselves between
the oocyte and the ovary-wall cells (Figs. 1.5B and 1.9B ).
Owing to fl attening of the intraovarian space, its zone of contact with the oocyte becomes more extensive.
1.2 Reproductive Patterns of Bryozoa