20
Thus, the mature ovary of Callopora lineata is represented
by the leading oocyte doublet that is enveloped apically and
laterally by the single-layered follicle epithelium and from
below by the intraovarian zone (containing oogonia and one
or several (up to four) young oocyte doublets) (Fig. 1.5C, D ;
see also 1.7A , 1.9C ). This zone itself is limited laterally by
the cells of the ovary wall (continuous with the follicle epithelium). In the walls of several ovaries, groups of small,
rounded, presumably dividing cells were found between the
ovarian cells (Fig. 1.6B ).
Polypide recycling does not result in degeneration of the
ovary, at least during the reproductive period. Mature ovaries
with vitellogenic oocytes occur in zooids containing regenerating polypides and brown bodies. Moreover, the appearance
of these oocytes indicates that vitellogenesis does not stop
even during polypide regeneration. This is reminiscent of
Chartella papyracea (Flustridae), in which vitellogenesis
starts during polypide recycling (Dyrynda and Ryland 1982 )
(see Sect. 1.2.1 ). Nevertheless, it is quite possible that, in
some species, degeneration of the polypide may induce an
interruption or at least a deceleration of ovarian function,
which is resumed after the new polypide begins to function.
The brown body is not removed from the polypide in
Callopora lineata . The remains of the degenerated polypide
were often seen to abut the ovary (Fig. 1.4C ). This may indicate that substances formed during its resorption are directly
channelled into vitellogenesis. Hastings ( 1932 ) was the fi rst
to suggest this when describing polypide degeneration
accompanied by oocyte increase in Stylopoma .
Oogenesis in Callopora lineata
Early gonado- and gametogenesis in bryozoans is extremely
poorly studied, and the descriptions given below are the fi rst
attempt to systematize available information on the succession of oocyte developmental stages in brooding cheilostomes. Based on anatomical data, the general sequence of
events during oogenesis is supposedly as follows. Primordial
germ cells (PGC) could not be detected by at light-microscopy level (see also Hageman 1983 ). In the early ovary at
least one such cell should be formed from a totipotent cell. Its
further division should result in a couple of oogonia. One of
them grows and divides to form the fi rst oocyte doublet. The
second oogonium divides to form a pair of oogonia, one of
which gives rise to the second oocyte doublet and the other
the next pair of oogonia, and so on. If this scheme is correct,
the ovary should contain at least one oogonium maintaining
the oogonial line, and one or several oogonia preparing for
division and transformation into an oocyte doublet.
Dedifferentiation of the cells of the mature ovary into PGC
seems unlikely but the possibility cannot be discounted.
In brooding cheilostomes including calloporids, early
female cells are recognizable in the developing polypide
of the young zooid mainly by their large size (Fig. 1.8A, B ).
In all cases observed by me, such cells were paired and their
cytoplasm was intensely stained (see also Calvet 1900 ). In
Cauloramphus spinifer a pair of oogonia was recognized
within the mesolethial layer in the early polypide bud
(“bilayered vesicle” stage) (Fig. 1.8A ). They were larger
than somatic cells, attaining 11 × 7 μm diameter (nucleus
7.5 × 6 μm; nucleolus 3 × 2.5 μm). They also had darker cytoplasm and were oval.
In Callopora lineata , a similar but unpaired cell was
found in a young ovary lying on the basal wall of the zooid
with a formed but not-yet-functioning polypide (Fig. 1.4A ).
Again, this cell was recognizable by its size (6–7.5 μm diameter, greater than the average size of an ovary cell, which is
4.5–5.5 μm), its vesicular shape, darker cytoplasm and large
pale nucleus.
Growing oogonia are normally solitary and often lose their
vesicularity (Fig. 1.4C, D ). The diameter of the oogonium prior
to mitosis, generally 18.0 × 15.0 μm, may reach 27 × 19 μm,
with that of the nucleus being 6 μm. In the material studied by
me, the ovary often contained one growing and one late
(premitotic) oogonium as well as the leading and previtellogenic oocyte doublets. These oogonia may, in some cases,
result from the division and subsequent differentiation of the
same precursor oogonium, their growth rates being asynchronous as they differentiate in separate directions. Separation of
oogonia from their siblings appears to be passive, possibly
resulting from displacement of dividing ovary cells and oocyte
growth. Detailed studies of early gameto- and gonadogenesis
involving TEM are necessary to confi rm the above picture.
The maximum number of oogonia and oocyte doublets
simultaneously present in the ovary in C. lineata was fi ve. In
one instance, these comprised a young vitellogenic doublet
and four oogonia, one of which was dividing, and in another,
a vitellogenic doublet, three previtellogenic doublets and an
oogonium. The arrangement of young oocyte doublets in the
ovary (between the cells of the ovary wall and, as a rule,
under the leading oocyte within the intraovarian zone)
(Fig. 1.5C, D ) indicates that oogonia concentrate in a relatively small area in the basal part of the female gonad.
In the initial phase of oogenesis, the division of the mature
oogonium results in an oocyte doublet consisting of two
sibling cells, connected by a cytoplasmic bridge, that later
differentiate into a vitellogenic oocyte and its nurse cell
(Fig. 1.4B ). The nature of the differentiation is presumably
determined by the fertilization “address”, the cell that fuses
with the sperm becoming the vitellogenic oocyte (Fig. 1.5D )
(see also below). The average diameter of newly formed sibling oocytes is 11 μm (nucleus 6.5 μm). The early stages of
their growth and development are synchronous and apparently proceed in the same manner. When they reach 18 μm in
diameter, a certain unevenness in cytoplasmic staining
becomes evident, with some areas staining more intensely
than others and small pale vacuoles appearing.
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
Thus, the mature ovary of Callopora lineata is represented
by the leading oocyte doublet that is enveloped apically and
laterally by the single-layered follicle epithelium and from
below by the intraovarian zone (containing oogonia and one
or several (up to four) young oocyte doublets) (Fig. 1.5C, D ;
see also 1.7A , 1.9C ). This zone itself is limited laterally by
the cells of the ovary wall (continuous with the follicle epithelium). In the walls of several ovaries, groups of small,
rounded, presumably dividing cells were found between the
ovarian cells (Fig. 1.6B ).
Polypide recycling does not result in degeneration of the
ovary, at least during the reproductive period. Mature ovaries
with vitellogenic oocytes occur in zooids containing regenerating polypides and brown bodies. Moreover, the appearance
of these oocytes indicates that vitellogenesis does not stop
even during polypide regeneration. This is reminiscent of
Chartella papyracea (Flustridae), in which vitellogenesis
starts during polypide recycling (Dyrynda and Ryland 1982 )
(see Sect. 1.2.1 ). Nevertheless, it is quite possible that, in
some species, degeneration of the polypide may induce an
interruption or at least a deceleration of ovarian function,
which is resumed after the new polypide begins to function.
The brown body is not removed from the polypide in
Callopora lineata . The remains of the degenerated polypide
were often seen to abut the ovary (Fig. 1.4C ). This may indicate that substances formed during its resorption are directly
channelled into vitellogenesis. Hastings ( 1932 ) was the fi rst
to suggest this when describing polypide degeneration
accompanied by oocyte increase in Stylopoma .
Oogenesis in Callopora lineata
Early gonado- and gametogenesis in bryozoans is extremely
poorly studied, and the descriptions given below are the fi rst
attempt to systematize available information on the succession of oocyte developmental stages in brooding cheilostomes. Based on anatomical data, the general sequence of
events during oogenesis is supposedly as follows. Primordial
germ cells (PGC) could not be detected by at light-microscopy level (see also Hageman 1983 ). In the early ovary at
least one such cell should be formed from a totipotent cell. Its
further division should result in a couple of oogonia. One of
them grows and divides to form the fi rst oocyte doublet. The
second oogonium divides to form a pair of oogonia, one of
which gives rise to the second oocyte doublet and the other
the next pair of oogonia, and so on. If this scheme is correct,
the ovary should contain at least one oogonium maintaining
the oogonial line, and one or several oogonia preparing for
division and transformation into an oocyte doublet.
Dedifferentiation of the cells of the mature ovary into PGC
seems unlikely but the possibility cannot be discounted.
In brooding cheilostomes including calloporids, early
female cells are recognizable in the developing polypide
of the young zooid mainly by their large size (Fig. 1.8A, B ).
In all cases observed by me, such cells were paired and their
cytoplasm was intensely stained (see also Calvet 1900 ). In
Cauloramphus spinifer a pair of oogonia was recognized
within the mesolethial layer in the early polypide bud
(“bilayered vesicle” stage) (Fig. 1.8A ). They were larger
than somatic cells, attaining 11 × 7 μm diameter (nucleus
7.5 × 6 μm; nucleolus 3 × 2.5 μm). They also had darker cytoplasm and were oval.
In Callopora lineata , a similar but unpaired cell was
found in a young ovary lying on the basal wall of the zooid
with a formed but not-yet-functioning polypide (Fig. 1.4A ).
Again, this cell was recognizable by its size (6–7.5 μm diameter, greater than the average size of an ovary cell, which is
4.5–5.5 μm), its vesicular shape, darker cytoplasm and large
pale nucleus.
Growing oogonia are normally solitary and often lose their
vesicularity (Fig. 1.4C, D ). The diameter of the oogonium prior
to mitosis, generally 18.0 × 15.0 μm, may reach 27 × 19 μm,
with that of the nucleus being 6 μm. In the material studied by
me, the ovary often contained one growing and one late
(premitotic) oogonium as well as the leading and previtellogenic oocyte doublets. These oogonia may, in some cases,
result from the division and subsequent differentiation of the
same precursor oogonium, their growth rates being asynchronous as they differentiate in separate directions. Separation of
oogonia from their siblings appears to be passive, possibly
resulting from displacement of dividing ovary cells and oocyte
growth. Detailed studies of early gameto- and gonadogenesis
involving TEM are necessary to confi rm the above picture.
The maximum number of oogonia and oocyte doublets
simultaneously present in the ovary in C. lineata was fi ve. In
one instance, these comprised a young vitellogenic doublet
and four oogonia, one of which was dividing, and in another,
a vitellogenic doublet, three previtellogenic doublets and an
oogonium. The arrangement of young oocyte doublets in the
ovary (between the cells of the ovary wall and, as a rule,
under the leading oocyte within the intraovarian zone)
(Fig. 1.5C, D ) indicates that oogonia concentrate in a relatively small area in the basal part of the female gonad.
In the initial phase of oogenesis, the division of the mature
oogonium results in an oocyte doublet consisting of two
sibling cells, connected by a cytoplasmic bridge, that later
differentiate into a vitellogenic oocyte and its nurse cell
(Fig. 1.4B ). The nature of the differentiation is presumably
determined by the fertilization “address”, the cell that fuses
with the sperm becoming the vitellogenic oocyte (Fig. 1.5D )
(see also below). The average diameter of newly formed sibling oocytes is 11 μm (nucleus 6.5 μm). The early stages of
their growth and development are synchronous and apparently proceed in the same manner. When they reach 18 μm in
diameter, a certain unevenness in cytoplasmic staining
becomes evident, with some areas staining more intensely
than others and small pale vacuoles appearing.
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis