44
the ovary are represented by oval (in ovaries with immature
oocytes) or fl attened (when containing ripe oocyte doublets)
follicle cells while the cells of the lower part are mostly oval
or polygonal. Sometimes they are arranged in two layers,
with narrow intercellular spaces becoming visible between
them and the oolemma (Figs. 1.26B, D and 1.27A, B ).
Nevertheless, as in species with pattern III, the occurrence of
basal cells and an intraovarian space cannot be determined
precisely by light microscopy.
In one instance, a young ovary at an early stage of differentiation of the polypide bud was found to contain a presumed oogonial pair (Fig. 1.26A ). The cells were 19 × 15 μm,
rounded, with darkly staining cytoplasm and a paler nucleus
that was 18 × 13 μm.
One or two rounded solitary oogonia were found in several mature ovaries (Fig. 1.26C ), sometimes containing no
oocytes. The diameter of the smallest solitary oogonia was
6–7 μm (nucleus 5 μm) and that of the largest 11–12 μm
(nucleus 6–8 μm).
As many as three gamete doublets can co-exist in a mature
ovary (Fig. 1.26B ) but one or two are usual. Since oogonial
doublets cannot be distinguished from early oocyte doublets at
the level of light microscopy, the exact number of sibling pairs
of both types is diffi cult to determine, as is the case in reproductive pattern II. Nevertheless, insofar as the total number
of doublets is rather low, it is unlikely that there is more than
one oogonial doublet in the ovary. Cell diameter in the earliest
doublet found was 12 μm (nucleus 8 μm), corresponding to
the size of solitary oogonia; mid-stage previtellogenic doublets are 14.5–15 × 10–12 μm (nucleus 6–7 × 5–6 μm). Their
cytoplasm stains darkly while the nuclei are paler. The
younger the cell, the less difference there is in staining of
cytoplasm and nucleoplasm. As oocytes grow, their cytoplasm becomes increasing paler when stained, with the
nucleus typically remaining even paler than the cytoplasm.
Vitellogenesis starts when oocytes attain a diameter of ca.
18 μm (cell diameter 19.8 × 16.5 μm, nucleus diameter
8.45 × 8.25 μm). The two largest mature doublets encountered had the following dimensions: (1) oocyte 80 × 70 μm
(nucleus 24 × 17 μm), nurse cell 19 × 15 μm (nucleus
18 × 13 μm); (2) oocyte 75 × 70 μm (nucleus 23 × 20 μm),
nurse cell 22 × 12 μm (nucleus 20 × 10 μm). No other doublets were found in these ovaries. Mature macrolecithalplasmalecithal oocytes occupy most of the cavity of the
maternal zooid; in stained sections they have pale, fi nely
granulated cytoplasm densely and evenly fi lled with mediumsized yolk granules (Fig. 1.27A, B ). Hughes ( 1987 , р. 703,
pl.Vb-с), who reported the same-sized mature oocytes in this
species, called them “telolecithal” and “yolk-fi lled”.
The nucleoplasm of the oocytes is homogeneous with rare
tiny inclusions (Fig. 1.26E, F ). In histological preparations it
stains to the same degree or somewhat lighter than the cytoplasm (Fig. 1.26C ), being darker than the latter only in the
oldest oocytes (Fig. 1.27A ). Nuclei of mature oocytes have
two or more (up to fi ve) nucleoli 2–10 μm diameter
(Fig. 1.27A ), indicating enhanced synthesizing activity in
these cells. The volume of the oocyte increases 316-fold during oogenesis and 70.5-fold during vitellogenesis.
Adult nurse cells contain a very large nucleus, occupying
almost all the cell, with a single nucleolus (Fig. 1.27B ). The
staining of nurse-cell nuclei and cytoplasm is almost identical to that in oocytes.
The oocyte that is transferred to the brood chamber (ovicell) starts to cleave as soon as meiosis has been completed
and pronuclei have merged. It is surrounded by a barely discernible fertilization envelope that, judging by the TEM photos in Hughes ( 1987 ), disappears completely in adult larvae.
The young embryo is suspended inside the brood cavity; as it
grows it occupies all the available space (Fig. 1.27C ), with
part of its surface tightly pressed against the non-calcifi ed
distal wall (ooecial vesicle) of the maternal zooid
(Fig. 1.27D ). The cells of its wall form the large embryophore denoted by Hughes ( 1987 , p. 691) as a “placental system” and also referred to as “nutrient-storage cells” (p. 703).
The embryophore is limited by a two-layered cuticle and
consists of the underlying layer of epidermal cells and associated funicular cells. During brooding, the size and number
of embryophore cells increase abruptly and their cytoplasm
stains more intensely (Fig. 1.27D ) (Ostrovsky 1998 , 2013 ).
In ovicells lacking embryos, embryophore cells are much
less developed (Fig. 1.26E ).
The size of late embryos in this species varies within the
range of 170–180 × 115–140 μm. During development in the
ovicell their volume increases 8.8-fold, which is less than the
15.6-fold value reported by Hughes ( 1987 ). This may indicate that the populations studied belong in fact to different
(sibling) species.
1.2.6.2 Ovary Structure and Oogenesis in Other
Cheilostomes with Reproductive
Pattern IV
Ovary structure and oogenesis in most cheilostomes with
reproductive pattern IV conform to that associated with pattern II. On the other hand, ovary structure in Celleporella
hyalinа , Cellaria tenuirostris (Cellariidae) and Bicellariella
ciliata (Bugulidae) (see Moosbrugger et al. 2012 ) accords
with pattern III, and these species have the smallest oocytes.
In Beania bilaminata (Beaniidae), Costaticella bicuspis and
C. solida (Catenicellidae), the ovary consists of only a few
cells (as in species with pattern III), but the general structure
of the ovary corresponds to that of pattern II.
Ovaries of B . bilaminata are located in the distal half of
the fertile autozooid on the basal cystid wall (Fig. 1.21 , inset)
and comprise only a few relatively small cells with pale cytoplasm and dark nuclei. The sides and the lower wall of the
ovary consist of oval cells, whereas the upper half of the
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
the ovary are represented by oval (in ovaries with immature
oocytes) or fl attened (when containing ripe oocyte doublets)
follicle cells while the cells of the lower part are mostly oval
or polygonal. Sometimes they are arranged in two layers,
with narrow intercellular spaces becoming visible between
them and the oolemma (Figs. 1.26B, D and 1.27A, B ).
Nevertheless, as in species with pattern III, the occurrence of
basal cells and an intraovarian space cannot be determined
precisely by light microscopy.
In one instance, a young ovary at an early stage of differentiation of the polypide bud was found to contain a presumed oogonial pair (Fig. 1.26A ). The cells were 19 × 15 μm,
rounded, with darkly staining cytoplasm and a paler nucleus
that was 18 × 13 μm.
One or two rounded solitary oogonia were found in several mature ovaries (Fig. 1.26C ), sometimes containing no
oocytes. The diameter of the smallest solitary oogonia was
6–7 μm (nucleus 5 μm) and that of the largest 11–12 μm
(nucleus 6–8 μm).
As many as three gamete doublets can co-exist in a mature
ovary (Fig. 1.26B ) but one or two are usual. Since oogonial
doublets cannot be distinguished from early oocyte doublets at
the level of light microscopy, the exact number of sibling pairs
of both types is diffi cult to determine, as is the case in reproductive pattern II. Nevertheless, insofar as the total number
of doublets is rather low, it is unlikely that there is more than
one oogonial doublet in the ovary. Cell diameter in the earliest
doublet found was 12 μm (nucleus 8 μm), corresponding to
the size of solitary oogonia; mid-stage previtellogenic doublets are 14.5–15 × 10–12 μm (nucleus 6–7 × 5–6 μm). Their
cytoplasm stains darkly while the nuclei are paler. The
younger the cell, the less difference there is in staining of
cytoplasm and nucleoplasm. As oocytes grow, their cytoplasm becomes increasing paler when stained, with the
nucleus typically remaining even paler than the cytoplasm.
Vitellogenesis starts when oocytes attain a diameter of ca.
18 μm (cell diameter 19.8 × 16.5 μm, nucleus diameter
8.45 × 8.25 μm). The two largest mature doublets encountered had the following dimensions: (1) oocyte 80 × 70 μm
(nucleus 24 × 17 μm), nurse cell 19 × 15 μm (nucleus
18 × 13 μm); (2) oocyte 75 × 70 μm (nucleus 23 × 20 μm),
nurse cell 22 × 12 μm (nucleus 20 × 10 μm). No other doublets were found in these ovaries. Mature macrolecithalplasmalecithal oocytes occupy most of the cavity of the
maternal zooid; in stained sections they have pale, fi nely
granulated cytoplasm densely and evenly fi lled with mediumsized yolk granules (Fig. 1.27A, B ). Hughes ( 1987 , р. 703,
pl.Vb-с), who reported the same-sized mature oocytes in this
species, called them “telolecithal” and “yolk-fi lled”.
The nucleoplasm of the oocytes is homogeneous with rare
tiny inclusions (Fig. 1.26E, F ). In histological preparations it
stains to the same degree or somewhat lighter than the cytoplasm (Fig. 1.26C ), being darker than the latter only in the
oldest oocytes (Fig. 1.27A ). Nuclei of mature oocytes have
two or more (up to fi ve) nucleoli 2–10 μm diameter
(Fig. 1.27A ), indicating enhanced synthesizing activity in
these cells. The volume of the oocyte increases 316-fold during oogenesis and 70.5-fold during vitellogenesis.
Adult nurse cells contain a very large nucleus, occupying
almost all the cell, with a single nucleolus (Fig. 1.27B ). The
staining of nurse-cell nuclei and cytoplasm is almost identical to that in oocytes.
The oocyte that is transferred to the brood chamber (ovicell) starts to cleave as soon as meiosis has been completed
and pronuclei have merged. It is surrounded by a barely discernible fertilization envelope that, judging by the TEM photos in Hughes ( 1987 ), disappears completely in adult larvae.
The young embryo is suspended inside the brood cavity; as it
grows it occupies all the available space (Fig. 1.27C ), with
part of its surface tightly pressed against the non-calcifi ed
distal wall (ooecial vesicle) of the maternal zooid
(Fig. 1.27D ). The cells of its wall form the large embryophore denoted by Hughes ( 1987 , p. 691) as a “placental system” and also referred to as “nutrient-storage cells” (p. 703).
The embryophore is limited by a two-layered cuticle and
consists of the underlying layer of epidermal cells and associated funicular cells. During brooding, the size and number
of embryophore cells increase abruptly and their cytoplasm
stains more intensely (Fig. 1.27D ) (Ostrovsky 1998 , 2013 ).
In ovicells lacking embryos, embryophore cells are much
less developed (Fig. 1.26E ).
The size of late embryos in this species varies within the
range of 170–180 × 115–140 μm. During development in the
ovicell their volume increases 8.8-fold, which is less than the
15.6-fold value reported by Hughes ( 1987 ). This may indicate that the populations studied belong in fact to different
(sibling) species.
1.2.6.2 Ovary Structure and Oogenesis in Other
Cheilostomes with Reproductive
Pattern IV
Ovary structure and oogenesis in most cheilostomes with
reproductive pattern IV conform to that associated with pattern II. On the other hand, ovary structure in Celleporella
hyalinа , Cellaria tenuirostris (Cellariidae) and Bicellariella
ciliata (Bugulidae) (see Moosbrugger et al. 2012 ) accords
with pattern III, and these species have the smallest oocytes.
In Beania bilaminata (Beaniidae), Costaticella bicuspis and
C. solida (Catenicellidae), the ovary consists of only a few
cells (as in species with pattern III), but the general structure
of the ovary corresponds to that of pattern II.
Ovaries of B . bilaminata are located in the distal half of
the fertile autozooid on the basal cystid wall (Fig. 1.21 , inset)
and comprise only a few relatively small cells with pale cytoplasm and dark nuclei. The sides and the lower wall of the
ovary consist of oval cells, whereas the upper half of the
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis