Human Reproduction vol.5 no.7 pp.847-852, 1990
Significance of cumulus oophorus in in-vitro fertilization
and oocyte viability and fertility
S.Magier1, H.H.van der Ven, K.Diedrich and
D.Krebs
Department of Obstetrics and Gynaecology, University of Bonn,
5300 Bonn 1, FRG
Fertilization and cleavage rates of human cumulus-intact
oocytes incubated in vitro for 36—48 h with normal spermatozoa tended to be higher than those which were cumulusdenuded (73 versus 68%; 68 versus 56%, respectively);
however, the difference was not significant. Nor were these
differences significant when using sperm samples of various
qualities (normozoospermic samples: 75 versus 70% fertilized
oocytes; asthenozoospermic: 66 versus 64%; oligozoospermk:
64 versus 56%; oligoasthenozoospermic: 35 versus 33%). The
beneficial effect of the human cumulus oophorus on the
binding of human spermatozoa to denuded hamster oocytes
and on head decondensatkm of human spermatozoa observed
after 2 h of incubation (9.3 versus 7.0 bound spermatozoa
per oocyte, P < 0.05; 0.5 versus 0.3 decondensed sperm heads
per oocyte, P < 0.02) disappeared after 6 h. A protective
effect of the cumulus oophorus on hamster oocytes preincubated in medium containing 50% human preovulatory
follicular fluid was observed in the sperm penetration assay
(fertilization rate of cumulus-intact:cumulus-denuded
oocytes, 26 versus 13%, P < 0.05) and confirmed using
fluorescein diacetate stain (cumulus-intact oocytes: 86 versus
100% vitality, non-significant; cumulus-denuded oocytes: 64
versus 100%, P < 0.01). These data suggest the accelerating
effect of the human cumulus oophorus on fertilization in its
early stages. Furthermore, the cumulus plays an important
part in protecting the oocyte against adverse environmental
influences.
Key words: cumulus oophorus/IVF/oocyte viability/fertility
Materials and methods
Introduction
At the time of ovulation and fertilization, the oocyte is surrounded
by a wide layer of granulosa cells, which form the cumulus
oophorus and corona radiata and remains on the oocyte surface
long after fertilization. This indicates that the cumulus may be
important for fertilization, maintenance of oocyte viability and
fertility, or for early embryo development.
Such assumptions have already been substantially confirmed.
© Oxford University Press
Sperm preparation
Sperm samples were obtained by volunteers under sterile
conditions through masturbation following 3 - 5 days of sexual
abstinence. After 30 min liquefaction, the quality of the ejaculate
was assessed employing the following criteria for normozoospermia; ejaculate volume 2—6 ml, morphology >60% normal
forms, motility >40% motile, concentration >20 X lC^/ml;
oligozoospermia: concentration <20 x lOVml, all other
parameters as for normozoospermia; asthenozoospermic condi847
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'Present address: Department of Obstetrics and Gynecology,
Lincoln Medical and Mental Health Center, 234 East 149th Street,
New York, NY 10451, USA
In a number of studies, it has been shown that the cumulus has
an important role in fertilization, making it possible for the
acrosome reaction to take place, or at least initiating this process
(Bavister, 1982; Tesarik, 1985), which is essential for
sperm—zona pellucida interaction (Soupart and Morgenstern,
1973; Overstreet and Hembree, 1976) and fusion with the oocyte
membrane (Yanagimachi and Noda, 1970; Barros and Berrios,
1977; Barros and Herrera, 1977; Shalgi and Phillips, 1980;
Talbot and Chacon, 1982). Since the oocytes can be fertilized
in the absence of the cumulus using spermatozoa capacitated in
vitro, the cumulus is not absolutely necessary for fertilization to
occur. Removal of the cumulus before insemination, at least in
certain animals, results in a decrease in sperm—oocyte interaction
and the fertilization rate (Mattioli et al., 1988), the cleavage rate
of fertilized ova (Cross and Brinster, 1970) and early embryo
development (Eyestone et al., 1987; Goto et al., 1988).
The significance of the cumulus in humans has not yet been
elucidated in all respects. Since its assessment might be very
important, for instance in possibly improving in-vitro fertilization (TVF) results, we chose to study the importance of the human
cumulus.
To estimate the significance of the human cumulus in fertilization and early embryo development, both cumulus-intact and
cumulus-denuded oocytes were used. To assess exactly the
possible effect of the cumulus on the individual steps of fertilization, i.e. on sperm binding to the oocyte membrane, sperm head
decondensation and pronucleus development, and to examine its
time dependency, a sperm penetration assay (SPA) was performed
in the presence of the human cumulus. In addition, the possibility
of a protective effect exerted on the oocyte by the cumulus was
investigated using incubation medium containing human follicular
fluid (FF). This was especially important because FF enters the
Fallopian tube at the time of ovulation (Bedford, 1970) and it
has been found to exert a cytotoxic effect on cumulus-denuded
oocytes (S.Magier et al., unpublished data).
S.Magler et al.
tions: motility <40% motile spermatozoa, all other parameters
as for normozoospermia; oligoasthenozoospermia: concentration
<20 X lOfyml, motility <40% motile spermatozoa, all other
parameters as for normozoospermia. A part of the sample was
mixed with culture medium and washed twice for 10 min at
200 g. Such a method was sufficient to remove 97% of seminal
fluid (Makler and Jakobi, 1981) enabling capacitation to occur.
The sperm concentration was estimated using a Makler chamber
(Makler, 1980).
Human oocyte preparation
Preparation of human cumulus oophorus and follicular
fluid
Human cumuli used in this study were removed using thin needles
from oocyte obtained in an FVF programme. Since it seemed
unlikely that under physiological conditions an immature follicle
can rupture, or that the oocyte originating from such a follicle
could undergo fertilization, only mature cumuli were used.
848
Hamster oocyte preparation and sperm penetration assay
As a model of fertilization under physiological conditions, a
sperm penetration assay (SPA) was chosen (Rogers, 1985;
Yanagimachi et al., 1976). Results obtained with this test
correlated significantly with the fertilizing potential of
spermatozoa, which has already been confirmed in numerous
studies (Rogers, 1985).
For recruitment of ova, 8- to 12-week old hamsters were used.
This was the best age for collecting oocytes of high fertility
(Rogers, 1985). Cycling hamsters, selected according to the
presence of vaginal secretion, were given i.p. injections of 30
U PMS, which was followed by i.p. injection of 30 U HCG
administered 48-72 h later. Hamsters were killed 14-18 h after
the second injection. The Fallopian tubes were removed, put onto
a Petri dish and covered with paraffin oil, after which they were
cut open and the cumulus mass containing oocytes was extracted.
After hyaluronidase and trypsin treatment, the oocytes were
washed in five drops of fresh medium. For SPA, only sperm
samples considered normal according to the criteria described
above were used. The 0.2-ml insemination drops containing
spermatozoa at a concentration of 20 X 106/ml were always
covered with paraffin oil. Before insemination, spermatozoa were
preincubated in the culture medium for 2 h.
To evaluate the possible effect of human cumulus on the
fertilization process, denuded hamster oocytes were divided into
two groups. In one group, the oocytes were incubated alone,
while those in the other groups were covered with cumulus after
transfer into the insemination drop (the cumulus was absent during
preincubation to eliminate its effect on spermatozoa). The
influence of the cumulus on fertilization and its component steps
with respect to time was assessed, estimating the number of
spermatozoa bound to the oocytes, the number of decondensed
sperm heads, the number of fertilized oocytes and those showing
pronucleus formation after 2, 4 and 6 h incubation.
Assessment of a protective effect of the oocyte by the cumulus
against adverse influences was performed using medium
containing 50% human FF. Cumulus-intact and cumulus-free (but
zona pellucida-intact) hamster oocytes were preincubated in the
presence of FF for 4 h. After washing them free of FF and
removing their outer investments, the ova were incubated with
spermatozoa for 4 h in the absence of FF to avoid the effects
of FF on spermatozoa. In the control group, cumulus-free hamster
oocytes were preincubated in FF-free culture medium prior to
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Human oocytes used in this study were collected in an IVF
programme following ovarian stimulation with clomiphene
citrate/HCG or HMG/HCG via laparoscopic or sonographically
controlled follicular puncture. If immature oocytes were obtained,
they were incubated in the culture medium until they became
mature. Ovum maturity was assessed according to the appearance
of the cumulus—oocyte complex and the oocytes themselves
under the light microscope. Mature oocytes had clear cytoplasm
and the first polar body was located in the perivitellin space. They
were distinctly demarcated from the surrounding viscous, fully
expanded and homogenous cumulus.
For ethical reasons, no ova were used only for experimental
purposes. All the inseminated oocytes were included in the IVF
programme and transferred after successful fertilization. Before
insemination, ova designated to be used in this study were
allocated to one of two groups. The oocytes in one group were
left within their intact cumulus, whereas those in the other group
were deprived of it. To eliminate the possible effect of enzymes
on the oocyte, the cumulus was removed using thin needles
(leaving only very scanty fragments of the granulosa cell layer
to avoid zona pellucida damage by manipulation). Thereafter once
cumulus-derived factors had been removed by washing, the
oocytes were put into hyaluronidase solution (bovine testicular
hyaluronidase, 300 NFU/mg, Sigma Chemical Company; used
at a concentration of 1 mg/ml culture medium) for 3 - 5 min and
washed three times in a large volume of fresh medium. Following
this procedure, practically all the cumulus was removed in the
vast majority of cases. Cumulus removal was performed, as all
other procedures, in medium at 37°C. Insemination took place
usually after 3 - 5 h preincubation following collection. For
insemination, spermatozoa were resuspended in culture medium
at a concentration of 200-300 x 103 spermatozoa/ml and an
incubation volume of 0.7-1.0 ml (in every case an attempt was
made to maintain the proportion of 200 x 103 spermatozoa/
oocyte). After 3 6 - 4 8 h of insemination, the fertilization and
cleavage rates were estimated. The oocytes were classified as
fertilized if they had developed two polar bodies and two pronuclei.
Cumulus maturity was assessed according to the criteria described
above. Before use in the sperm penetration assay (performed in
BWW medium), the cumulus was washed free of Ham's F 10
medium, in which the oocytes had been placed after recovery.
In each case, for five consecutive washings in large volumes of
fresh BWW medium, the time employed was ~ 10 min.
Human follicular fluid (FF) used in this study was collected
in an IVF programme. Only clear FF, free of blood and
originating from mature follicles containing oocytes which
fertilized and completed at least the first cleavage division, was
used. Thus, to some extent, the FF selected for this study was
comparable to that produced under physiological conditions. After
collection, FF was centrifuged for 10 min at 1000 g and kept
frozen at -20°C until required.
Significance of cumulus oophorus in fertilization
the removal of the zona pellucida and insemination, applying the
same preincubation and incubation times.
Vital staining with fluorescein diacetate
Incubation conditions
For preparation and incubation of human oocytes, Ham's F 10
medium (Ham, 1963) supplemented with 10% fetal cord serum
(v/v) was used. The SPA and hamster oocyte incubation were
performed in BWW medium (Biggers et al., 1971) enriched with
human serum albumin, fraction V, 3 mg/ml. Incubation took
place in a humidified atmosphere of 5 % CO2 and 95 % air at
37°C. All experiments were carried out under sterile conditions.
Table II. Effect of cumulus oophorus on fertilization and cleavage of
human oocytes inseminated in vitro
Oocytes
No. of
oocytes
No. of
oocytes
fertilized
Fertilization
rate (%)
Cleaved
oocytes
Cleavage
rate (%)
Cumulus-intact
Cumulus-free
19
16
14
11
73.7
68.7
13
9
68.4
56.2
Chi-square test: the differences in the fertilization and cleavage rates
between cumulus-intact and cumulus-denuded oocytes were non-significant
(P > 0.05).
Results
After 36—48 h incubation, the fertilization rate of the cumulusintact human oocytes in all groups tended to be higher than that
of those which had been denuded (normozoospermic samples:
75.0 versus 70.0%; asthenozoospermic samples: 66.7 versus
64.7%; oligozoospermic samples: 64.3 versus 56.2%; oligoasthenozoospermic samples: 35.7 versus 33.3%; the mean
fertilization rates were 61.3 versus 58.5%, and in another
experiment using normozoospermic samples: 73.7 versus
68.4%). However, the differences were not statistically significant
(Table I, Table II). Similarly, the cleavage rates of the cumulus-
Tabte I. Effect of cumulus oophorus on human IVF
Sperm sample
Normozoospermic
Normozoospermic
Asthenozoospermic
Asthenozoospermic
Oligozoospermic
Oligozoospermic
01 igoasthenozoosperm ic
Oligoasthenozoospermic
Mean
Mean
Cumulus
status of
oocytes
No. of
oocytes
intact
free
intact
free
intact
free
intact
free
intact
free
16
20
18
17
14
16
14
12
62
65
Table HI. Effect of human cumulus oophorus on sperm binding to the
hamster oocyte membrane in the sperm penetration assay (n = 5)
Incubation
medium
Cumulus
Cumulus
Cumulus
Cumulus
Cumulus
Cumulus
present
present
present
absent
absent
absent
Incubation
time (h)
2
4
6
2
4
6
No. of
oocytes
53
54
56
44
43
45
Number of bound
spermatozoa
Total
Per oocyte
(mean ± SEM)
492
761
979
309
563
760
9.3
14.1
17.5
7.0
13.1
16.9
±
±
±
±
±
±
2.2 1
2.5
2.6
1.9*
2.0
2.4
•Statistics (paired Student's Mest), P < 0.05.
No. of
oocytes
fertilized
Fertilization
rate (%)
12
14
12
11
9
9
5
4
38
38
75.0
70.0
66.7
64.7
64.3
56.2
35.7
33.3
61.3
58.5
Table IV. Effect of human cumulus oophorus on sperm head decondensation in sperm penetration assay (n = 5)
Incubation
medium
Cumulus
Cumulus
Cumulus
Cumulus
present
present
present
absent
C^iirmihis flJKwit
Chi-square test: the differences in the fertilization rate between cumuhis-
Cumulus absent
Incubation
time (h)
2
4
6
2
4
6
No. of
oocytes
53
54
56
44
43
45
Numbei" of decondensed
sperm heads
Total
Per oocyte
(mean ± SEM)
27
59
89
14
33
67
0.5
1.1
1.6
0.3
0.8
1.5
±
±
±
±
±
±
0.1"
0.4 b
0.4
0.1"
0.3 b
0.4
Statistics (paired Student's /-test): 'P < 0.02; bP < 0.03.
849
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To estimate oocyte viability, a vital stain with fluorescein diacetate
(FDA) was used (Schilling et al., 1979; Mohr and Trounson,
1980). Based on the fact that after 5 - 1 0 min of FDA treatment,
the cytoplasm of living cells emits a greenish fluorescence, this
technique allowed a quicker and more accurate assessment of
ovum vitality than other staining methods.
To assess the possible significance of the cumulus oophorus
in protecting oocytes against adverse influences of the cumulusfree and cumulus-intact hamster oocytes were employed. They
were incubated either in the pure culture medium or in the
medium containing 50% human FF for 6 h. After incubation,
the oocytes were washed free of FF, the cumulus was removed
and vital staining was performed. The oocytes used in this part
of the study were not deprived of the zona pellucida.
intact oocytes seemed to be higher than in those which were
cumulus-denuded (68.7 versus 56.2%), but was also not
significantly different (Table II).
Presence of human cumulus in the SPA resulted in an increase
in sperm binding to the oocyte membrane during the first 2 h
of incubation (9.3 versus 7.0 bound spermatozoa per oocyte; P
< 0.05) (Table HI) and in sperm head decondensation after 2
(0.5 versus 0.3 decondensed sperm heads per oocyte; P < 0.02)
and 4 h incubation (1.1 versus 0.8; P < 0.03) (Table IV).
However, this beneficial effect was observed only at the
beginning of incubation and subsequently disappeared. Sperm
binding to the oocyte membrane after 4 (14.1 versus 13.1 bound
spermatozoa per oocyte) and 6 h (17.5 versus 16.9) and sperm
head decondensation after 6 h (1.6 versus 1.5 decondensed sperm
heads per oocyte) were not significantly different. Neither the
total fertilization rate (Table V) nor the pronuclear development
(Table VI) was significantly influenced by the cumulus, although
S.Magier et al.
these parameters also tended to be higher in the oocytes incubated
with cumulus.
Exposure of cumulus-denuded hamster oocytes to 50% human
FF led to significant reductions in both fertilization rate (13.1
versus 36.0% fertilized oocytes, P < 0.001; Table VII) and
vitality (64.4 versus 100% viable oocytes, P < 0.01; Table VIE)
compared with the control. Contrary to these findings, the
differences in fertilization rate (26.1 versus 36.0% fertilized
oocytes) and vitality (86.9 versus 100% viable oocytes) between
cumulus-intact oocytes incubated in the presence of FF and
control proved not to be statistically significant. The fertilization rate of the cumulus-intact oocytes preincubated in medium
containing 50% FF was significantly higher than in those which
had been cumulus denuded and preincubated in the same medium
(26.1 versus 13.1% fertilized oocytes, P < 0.05).
Although the fertilization and cleavage rates of the cumulus-intact
human oocytes inseminated in vitro tended to be higher than in
those denuded of cumulus, the differences were not statistically
significant. Even though this observation may suggest that the
cumulus is of no importance to the fertilization process, the lack
of evidence of a significant effect might have been caused by
several factors.
During incubation of human oocytes, all culture conditions
were kept optimal and all possible adverse factors were
Table V. Effect of human cumulus oophorus on fertilization rate in sperm
penetration assay of hamster oocytes (n = 5)
Incubation
medium
Incubation
time (h)
No. of
oocytes
No of
oocytes
fertilized
Fertilization
rate (%)
Cumulus
Cumulus
Cumulus
Cumulus
Cumulus
Cumulus
2
4
6
2
4
6
53
54
56
44
43
45
18
33
42
13
21
35
33.9
61.1
75.0
29.5
4S.8
77.8
present
present
present
absent
absent
absent
Statistics (Chi-square test): the differences in the fertilization rate between
the oocytes in both groups were not significant at every time point (P >
0.05).
Table VI. Effect of human cumulus oophorus on pronucleus development in
sperm penetration assay of hamster oocytes (n = 5)
Incubation
medium
Cumulus
Cumulus
Cumulus
Cumulus
Cumulus
Cumulus
present
present
present
absent
absent
absent
Incubation
time (h)
No. of
oocytes
No. of
oocytes with
pronuclei
Percentage
53
54
56
44
43
45
—
8
16
3
11
0
14.8
28.6
0
7.0
24.4
Oocyte
Preincubation
medium
No. of
oocytes
No. of
oocytes
fertilized
Fertilization
rate (%)
Cumulus-denuded
Cumulus-denuded
Cumulus-intact
BWW (control)
BWW + 50% FF
BWW + 50% FF
100
99
142
36
13
37
36 0 1
13 l l b
26. l b
Statistics (Chi-square test): 'P < 0.001; bP < 0.05
Table VOT. Protective effect of cumulus oophorus on hamster oocytes;
estimated by fluorescein diacetate stain (n = 3)
Chi-square test: the differences in the proportions of ova showing
pronucleus formation between the oocytes in both groups were notsignificant at every time point (P > 0.05).
850
Table VII. Protective effect of cumulus oophorus on hamster oocytes;
estimated by sperm penetration assay (n = 5)
Oocyte
Incubation
medium
No. of
oocytes
No. of
viable
oocytes
Longevity
(%)
Cumulus-intact
Cumulus-intact
Cumulus-free
Cumulus-free
BWW
BWW
BWW
BWW
25
23
15
45
25
20
15
29
100
86.9
100"
64.41
(control)
+ 50% FF
(control)
+ 50% FF
•Chi-square test. P < 0.01.
Downloaded from http://humrep.oxfordjournals.org/ at Pennsylvania State University on March 5, 2016
Discussion
eliminated. That may be why the potential protective effect of
the cumulus (described later) was apparently inessential under
such conditions. In addition, to avoid a possible negative effect
of temperature and atmosphere changes on the IVF process, the
oocyte condition was checked only once, at the end of incubation,
i.e. after 36-48 h. Therefore, it is likely that the cumulus might
have exerted some influence on the fertilization process, perhaps
acting only in the first hours of insemination, the effect declining
with increasing incubation time. This idea is supported by
observations made in a study employing a homologous rabbit
system (Harper, 1970) in which the influence of the cumulus on
fertilization declined with increasing preincubation time.
The cumulus was not always completely removed from human
oocytes by our technique and in a few cases, some scanty
fragments were left. The vast majority of the oocytes, however,
were completely denuded of granulosa cells. Thus it seems rather
unlikely that a few tiny fragments left on some oocytes could
make a significant contribution. More importantly there is
evidence for the impregnation of the outer part of the zona
pellucida by cumulus intercellular matrix (Tesarik et al., 1988),
which might not have been removed completely during incubation
with hyaluronidase. The purpose of this study, however, was
not to estimate the absolute effect of cumulus-derived substances
on fertilization, but rather to ascertain whether the macroscopically visible (and thus easily removable) granulosa cell mass has
any influence on IVF and therefore, whether it would be more
beneficial to remove it than to leave it intact in the IVF system.
An important observation in this study, which has been noted
previously (Plachot et al., 1986), is that the cumulus does not
represent a physical barrier to fertilization. This confirms the
findings of other workers using both animal (Bedford and Chang,
1962) and human systems (Mahadevan and Trounson, 1985). In
the present study, however, we found that, contrary to expectation based mainly on animal experiments where cumulus-denuded
ova were successfully fertilized with a very small number of
Significance of cumulus oophorus in fertilization
Furthermore, these data suggest that the effect described above
does not depend on the integrity of intercellular coupling between
the cumulus and oocytes but that it is exerted through secreted,
soluble factors.
According to current knowledge of fertilization physiology,
better SPA results in the presence of the cumulus should be
largely attributable to the positive effect on the acrosome reaction
in spermatozoa, because the cumulus is well-known to promote
this process (Bavister, 1982; Tesarik, 1985). Since only
spermatozoa undergoing the acrosome reaction are able to fuse
with the oocyte membrane (Yanagimachi and Noda, 1970; Barros
and Berrios, 1977; Barros and Herrera, 1977; Shalgi and Phillips,
1980; Talbot and Chacon, 1982), the number of spermatozoa
bound to the oocytes and the number undergoing decondensation
reflect the proportion of sperm cells having undergone the
acrosome reaction (Talbot and Chacon, 1982).
It is also possible, however, that molecules secreted by
granulosa cells might be capable of exerting a beneficial effect
on the oocytes themselves, or that other complex mechanisms
of granulosa—oocyte interaction may exist. Recent studies
indicate a more active role for the cumulus in fertilization
physiology than was assumed previously. It has already been
suggested, for instance, that granulosa cells can prevent
spontaneous hardening of the zona pellucida, which is caused
by the products of cortical granules and which decreases oocyte
fertility (De Felici and Siracusa, 1982; De Felici et al., 1985).
Using incubation medium containing 50% human FF, it has
been shown that the cumulus can protect oocytes from adverse
factors in the milieu. However, if optimal conditions are
maintained throughout, this action of the cumulus seems
inessential. Similar results were obtained in another study (Chang
and Bedford, 1962), showing the protective effect of granulosa
cells on oocyte fertility.
The above results indicate a positive effect of the cumulus on
fertilization during the first hours of insemination and a protective
effect exerted on the oocytes if the milieu conditions are not
optimal. This is also of great importance under physiological
conditions, since in the follicle and even after ovulation in the
Fallopian tube, the cumulus protects the oocyte from the cytotoxic
effect of FF. However, when incubation conditions are optimal
and the period is long enough, the cumulus does not seem to
have any significant influence on fertilization.
In spite of this, the cumulus should not be considered
unimportant for human FVF, especially since the cumulus may
have great practical significance in this area. Insemination of
cumulus-intact oocytes may cause fertilization to occur more
quickly (although this was not proven in this study), thus enabling
embryo transfer to be performed earlier. This would decrease
the asynchrony which arises during FVF owing to embryo
development proceeding slower in vitro than in the Fallopian tube.
This might have a positive effect on the chances of implantation
and gestation. Using an animal model, Vanderhyden and
Armstrong (1988) showed that the developmental potential of rat
oocytes transferred into the uterus after in-vitro fertilization is
limited by asynchrony between the embryo and uterine development. In addition, although the cumulus is not critically important
for fertilization to occur under optimal conditions, its presence
at the time of fertilization may have a beneficial effect on further
embryo development and gestation. Such a correlation seems
likely, especially since it has been shown that the development
rate of mouse embryos is significantly lower if oocytes are
cumulus-free at the time of fertilization (Cross and Brinster, 1970)
and bovine embryo development is arrested in the absence of
granulosa cells (Eyestone et al., 1987; Goto et al., 1988).
For these reasons it seems justifiable not to remove the cumulus
from oocytes used in human FVF programmes, especially since
there is no known reason for doing so, since the cumulus does
not hinder fertilization, even in cases of low-quality semen.
Further studies are required in order to elucidate exactly the
possible effects of the cumulus on very early stages of fertilization in the human, on embryo development and, especially, on
resulting development and the course of pregnancy.
References
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Bedford,.!.M. (1970) Sperm capacitation and fertilization in mammals.
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