21
The contact between the oocyte and the nurse cell is rarely
encountered in sections; these siblings are often so tightly
appressed that both the cells and their nuclei are deformed.
In Fig. 1.7C , however, which shows an oocyte doublet in
the ovary of Callopora craticula , the leading oocyte and the
nurse cell appear at some distance from each other and the
cytoplasmic bridge can be easily seen (see also Fig. 1.13B ).
In the related species C . lineata , although the youngest oocyte
seen with a male pronucleus was 25 × 22 μm (Fig. 1.5D ),
syngamy presumably occurs immediately after the transformation of the oogonium into the oocyte doublet. This is
confi rmed by the fi nding of a very early doublet with a sperm
head inside an oocyte of about 10 μm diameter (Fig. 1.35C )
in the calloporid Tegella armifera .
It should be noted that the earliest oocyte doublets appear
in the ovaries of young zooids considerably in advance of the
fully formed polypide. In the ovary of a young zooid with a
near-complete polypide I found, besides a solitary oogonium, two previtellogenic doublets, one of them in the process of degeneration. A three-dimensional reconstruction of
the ovary showed that the oogonium was proximally situated, with the degenerating doublet at its distal end, almost
outside the gonad. The second doublet was situated in the
middle of the ovary. In their general appearance and size, the
cells of this doublet (Fig. 1.4B ) were identical to the early
previtellogenic doublets of mature ovaries, the cell diameter
being 18 × 15 μm and the nucleus 6 μm.
The occurrence of oocyte doublets in young zooids with
pre-functioning polypides indicates that the female gametes
are in these instances formed at the expense of the colony’s
resources (channelled to the developing zooid along funicular cords) – the fi rst previtellogenic oocyte doublet emerges
long before the polypide and zooid are formed. However,
vitellogenesis is not initiated and the doublet that is formed
degenerates. This is not surprising; without a functional
polypide alien sperm cannot be received. Bishop et al.
( 2000 ) have described a similar situation in the cheilostome
Celleporella hyalina in which vitellogenesis is not initiated
in non-fertilized colonies.
Synchronous growth and development of the oocyte doublet continue throughout the previtellogenic period, concurrent with enlargement of the nuclei and nucleoli. Then the
oldest doublet enters the vitellogenic phase (Figs. 1.4D and
1.5B ; see also 1.7B ), during which the growth of all other
(younger) oocyte doublets in the ovary (if present) is
typically interrupted or retarded. Early in the vitellogenic
phase, as indicated by the presence of yolk granules in the
cytoplasm, the oocyte (of 33.5 μm mean diameter following
a >28-fold increase during the previtellogenic phase) is
usually larger than the nurse cell (25 μm mean diameter).
In some cases, the siblings attain up to 50 μm diameter
synchronously. Small dark granules (apparently yolk) begin
to accumulate in the oocyte cytoplasm (typically on the
periphery and often in a certain sector opposite the intraovarian
zone). Similar rounded granules are sometimes found in the
cytoplasm of the nurse cell, which may be somewhat darker
than in the oocyte. The nuclear envelope of both cells is
deformed, particularly in the nurse cell.
At some point during the vitellogenic phase, the growth
rate of the oocyte greatly exceeds that of the nurse cell,
which almost stops growing (Fig. 1.6A ). Prior to ovulation
(the fi nal stage of oogenesis), a mature vitellogenic oocyte
achieves 102.5 μm mean diameter (with nucleus 35 μm).
Thus, the volume of the oocyte increases 28.6-fold during
the vitellogenic phase and more than 800-fold during its
development in the ovary.
The mature oocyte is macrolecithal-plasmalecithal, with
numerous yolk granules that are not segregated in the
ooplasm but evenly distributed throughout it (see Wourms
1987 for defi nitions). The nucleus is eccentric, with the animal pole distanced as far as possible from the intraovarian
space in all cases (Figs. 1.5C and 1.6C ). A perinuclear cytoplasmic zone free of yolk granules was seen in a number of
mature oocytes (Fig. 1.5C ). Inclusions (single pale vacuoles
as in Fig. 1.6C ) and rounded dark bodies (presumably RNA
aggregates) could be seen in some nucleoli. The nucleus
loses its envelope during the late vitellogenetic stage or,
more often, directly before ovulation.
In contrast, the nucleus of the nurse cell remains intact
throughout vitellogenesis. In the mature nurse cell it occupies most of the cell volume (its cytoplasm thus appearing as
a thin peripheral band), indicating intense activity. The nurse
cell is normally situated lateral to the oocyte (Fig. 1.6A ),
with the border between them barely discernible in some
cases. During ovulation, the nurse cell (30 × 27 μm mean
diameter) is separated from the oocyte and degenerates in the
zooid cavity (Fig. 1.6D ) or in the ovary. It contains a large
nucleus (25 μm mean diameter), a very large nucleolus (up to
10 μm diameter) and its cytoplasm often contains a few large
yolk granules like those in the leading oocyte. As a rule,
these are arranged in clusters, engorging the very thin cytoplasmic layer and considerably deforming the nucleus.
The mature oocyte ovulates by rupturing the follicle wall.
This process is presumably facilitated by (1) a thinning of
the ovary wall and (2) the mechanical impact of the caecum
on the follicle during polypide movements (Gerwerzhagen
1913 ; Silén 1945 ). Owing to the limited space inside the
autozooid, the large, partly ovulated oocyte retains a connection with the ovary for some time (Fig. 1.6D ), fi nally entering the coelomic cavity
Ovulation shrinks and transforms the ovary, manifested in
the degeneration of the squamous follicular epithelium that
formerly enveloped the ovulated oocyte, with the oldest previtellogenic doublet becoming situated in the upper part of
the gonad (possibly owing to the decreasing volume and surface area of the ovary after removal of the mature oocyte);
1.2 Reproductive Patterns of Bryozoa
The contact between the oocyte and the nurse cell is rarely
encountered in sections; these siblings are often so tightly
appressed that both the cells and their nuclei are deformed.
In Fig. 1.7C , however, which shows an oocyte doublet in
the ovary of Callopora craticula , the leading oocyte and the
nurse cell appear at some distance from each other and the
cytoplasmic bridge can be easily seen (see also Fig. 1.13B ).
In the related species C . lineata , although the youngest oocyte
seen with a male pronucleus was 25 × 22 μm (Fig. 1.5D ),
syngamy presumably occurs immediately after the transformation of the oogonium into the oocyte doublet. This is
confi rmed by the fi nding of a very early doublet with a sperm
head inside an oocyte of about 10 μm diameter (Fig. 1.35C )
in the calloporid Tegella armifera .
It should be noted that the earliest oocyte doublets appear
in the ovaries of young zooids considerably in advance of the
fully formed polypide. In the ovary of a young zooid with a
near-complete polypide I found, besides a solitary oogonium, two previtellogenic doublets, one of them in the process of degeneration. A three-dimensional reconstruction of
the ovary showed that the oogonium was proximally situated, with the degenerating doublet at its distal end, almost
outside the gonad. The second doublet was situated in the
middle of the ovary. In their general appearance and size, the
cells of this doublet (Fig. 1.4B ) were identical to the early
previtellogenic doublets of mature ovaries, the cell diameter
being 18 × 15 μm and the nucleus 6 μm.
The occurrence of oocyte doublets in young zooids with
pre-functioning polypides indicates that the female gametes
are in these instances formed at the expense of the colony’s
resources (channelled to the developing zooid along funicular cords) – the fi rst previtellogenic oocyte doublet emerges
long before the polypide and zooid are formed. However,
vitellogenesis is not initiated and the doublet that is formed
degenerates. This is not surprising; without a functional
polypide alien sperm cannot be received. Bishop et al.
( 2000 ) have described a similar situation in the cheilostome
Celleporella hyalina in which vitellogenesis is not initiated
in non-fertilized colonies.
Synchronous growth and development of the oocyte doublet continue throughout the previtellogenic period, concurrent with enlargement of the nuclei and nucleoli. Then the
oldest doublet enters the vitellogenic phase (Figs. 1.4D and
1.5B ; see also 1.7B ), during which the growth of all other
(younger) oocyte doublets in the ovary (if present) is
typically interrupted or retarded. Early in the vitellogenic
phase, as indicated by the presence of yolk granules in the
cytoplasm, the oocyte (of 33.5 μm mean diameter following
a >28-fold increase during the previtellogenic phase) is
usually larger than the nurse cell (25 μm mean diameter).
In some cases, the siblings attain up to 50 μm diameter
synchronously. Small dark granules (apparently yolk) begin
to accumulate in the oocyte cytoplasm (typically on the
periphery and often in a certain sector opposite the intraovarian
zone). Similar rounded granules are sometimes found in the
cytoplasm of the nurse cell, which may be somewhat darker
than in the oocyte. The nuclear envelope of both cells is
deformed, particularly in the nurse cell.
At some point during the vitellogenic phase, the growth
rate of the oocyte greatly exceeds that of the nurse cell,
which almost stops growing (Fig. 1.6A ). Prior to ovulation
(the fi nal stage of oogenesis), a mature vitellogenic oocyte
achieves 102.5 μm mean diameter (with nucleus 35 μm).
Thus, the volume of the oocyte increases 28.6-fold during
the vitellogenic phase and more than 800-fold during its
development in the ovary.
The mature oocyte is macrolecithal-plasmalecithal, with
numerous yolk granules that are not segregated in the
ooplasm but evenly distributed throughout it (see Wourms
1987 for defi nitions). The nucleus is eccentric, with the animal pole distanced as far as possible from the intraovarian
space in all cases (Figs. 1.5C and 1.6C ). A perinuclear cytoplasmic zone free of yolk granules was seen in a number of
mature oocytes (Fig. 1.5C ). Inclusions (single pale vacuoles
as in Fig. 1.6C ) and rounded dark bodies (presumably RNA
aggregates) could be seen in some nucleoli. The nucleus
loses its envelope during the late vitellogenetic stage or,
more often, directly before ovulation.
In contrast, the nucleus of the nurse cell remains intact
throughout vitellogenesis. In the mature nurse cell it occupies most of the cell volume (its cytoplasm thus appearing as
a thin peripheral band), indicating intense activity. The nurse
cell is normally situated lateral to the oocyte (Fig. 1.6A ),
with the border between them barely discernible in some
cases. During ovulation, the nurse cell (30 × 27 μm mean
diameter) is separated from the oocyte and degenerates in the
zooid cavity (Fig. 1.6D ) or in the ovary. It contains a large
nucleus (25 μm mean diameter), a very large nucleolus (up to
10 μm diameter) and its cytoplasm often contains a few large
yolk granules like those in the leading oocyte. As a rule,
these are arranged in clusters, engorging the very thin cytoplasmic layer and considerably deforming the nucleus.
The mature oocyte ovulates by rupturing the follicle wall.
This process is presumably facilitated by (1) a thinning of
the ovary wall and (2) the mechanical impact of the caecum
on the follicle during polypide movements (Gerwerzhagen
1913 ; Silén 1945 ). Owing to the limited space inside the
autozooid, the large, partly ovulated oocyte retains a connection with the ovary for some time (Fig. 1.6D ), fi nally entering the coelomic cavity
Ovulation shrinks and transforms the ovary, manifested in
the degeneration of the squamous follicular epithelium that
formerly enveloped the ovulated oocyte, with the oldest previtellogenic doublet becoming situated in the upper part of
the gonad (possibly owing to the decreasing volume and surface area of the ovary after removal of the mature oocyte);
1.2 Reproductive Patterns of Bryozoa