Human Reproduction vol.14 no.2 pp.465–469, 1999
Effect of oxygen concentration on human in-vitro
fertilization and embryo culture*
John C.M.Dumoulin1,3, Chantal J.J.Meijers1,
Marijke Bras1, Edith Coonen1, Joep P.M.Geraedts2
and Johannes L.H.Evers1
1Department
of Obstetrics & Gynaecology and 2Department of
Molecular Cell Biology & Genetics, Academic Hospital, University
of Maastricht, Maastricht, The Netherlands
3To
In this prospective randomized study on 1380 consecutive
in-vitro fertilization (IVF) treatments, the results were
compared of culture of human oocytes and embryos for
the first 2 or 3 days of development in microdroplets of
medium under oil using a gas phase containing either
atmospheric (~20%) or reduced (5%) O2 concentrations.
No significant differences were found between the two
groups cultured under either 5% or 20% O2 in rates
of fertilization (60 versus 61%, respectively), embryo
development at day 2 or 3, pregnancy (26.6 versus
25.4%, respectively), and implantation (13.4 versus 14.0%,
respectively). Culture of surplus embryos under 5% O2
resulted in a significantly higher mean incidence of
blastocyst formation per cycle as compared to the 20%
O2 group (25.8 K 2.0 versus 20.4 K 1.9, respectively).
The mean number of cells of embryos classified as
blastocysts by microscopic observation of a blastocoel
was significantly higher in the 5% O2 group as compared
to the 20% O2 group, both in blastocysts fixed on day
5 (39.8 K 1.7 versus 31.9 K 1.9, respectively), as well
as those fixed on day 6 (45.6 K 2.6 versus 33.7 K 3.4,
respectively). This difference was due to the fact that
significantly more blastocysts of the 20% O2 group had
an abnormal low cell number of < 25 as compared to
the 5% O2 group, both in blastocysts fixed on day 5 (39
versus 22%, respectively), as well as those fixed on day
6 (43 versus 22%, respectively). To conclude, although
culture under 5% O2 leads to slightly improved preimplantation embryonic viability, this effect is either too
marginal to result in higher pregnancy rates, or low O2
concentrations exert an effect during the later stages of
preimplantation development only.
Key words: embryo culture/oxygen/preimplantation development
*Data presented in part at the 14th Annual Meeting of the European
Society for Human Reproduction and Embryology (ESHRE),
Göteborg, Sweden, June 21–24, 1998.
© European Society of Human Reproduction and Embryology
Mammalian embryos can be cultured successfully using a gas
phase containing either atmospheric (~20%) or reduced (5%)
O2 concentrations. For human in-vitro fertilization (IVF) treatments, both O2 concentrations are widely used and similar
success rates have been reported. In a previous study, we
evaluated the effect of both of these O2 concentrations on IVF
and embryo culture in the human and found no beneficial
effect of culturing under 5% O2 as compared to culturing
under atmospheric O2 concentrations (Dumoulin et al., 1995).
It has been demonstrated, however, that embryos from several
species (mouse, sheep, goat, cattle) show improved development when cultured under reduced O2 tension (Bavister, 1995).
The use of small, static droplets of medium covered with
an oil overlay has been speculated to result in marginally
hypoxic culture conditions at lower O2 concentrations (ByattSmith et al., 1991). Furthermore, it has been shown in several
animal studies that different culture systems may require
different O2 concentrations for optimal results (Fukui et al.,
1991; Voelkel and Hu, 1992; Ali et al., 1993). Therefore, the
results from our previous study, in which human embryos
were cultured in 1 ml of medium without an oil overlay, may
not be applicable to a culture system of droplets of medium
under oil. In the present study, we compared the results of
human IVF and embryo culture under both of these gas phases,
using 20 µl droplets under oil.
Materials and methods
Patients
1
During a period of 32 years, 1380 consecutive IVF and intracytoplasmic sperm injection (ICSI) treatment cycles were included in this
study. Oocytes and embryos were alternately allocated per set of two
treatment cycles to fertilization and culture either under ambient
1
(~20%) or reduced (5%) O2. During the last 22 years of the present
study, a second study was performed, in which oocytes and embryos
were alternately allocated to the use of either of two culture media:
either IVF-50™ or human tubal fluid (HTF). By this allocation
procedure, a random distribution of treatments over the different
culture techniques was ensured and the two studies were independent.
The results of this second study will be published separately.
ICSI treatment was performed in cases of male subfertility (621
cycles), and when no fertilization had occurred in two previous IVF
cycles (93 cycles). Male subfertility was defined as a progressive
motile sperm concentration of ,33106/ml in combination with ,5%
morphologically normal spermatozoa, evaluated using strict criteria
(Enginsu et al., 1992). For all other indications (666 cycles), a
conventional in-vitro insemination procedure with 50 000 motile
spermatozoa/ml was used.
The stimulation protocol used has been described previously (Land
465
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whom correspondence should be addressed at: IVF Laboratory,
Department of Obstetrics and Gynaecology, Academic Hospital
Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands
Introduction
J.C.M.Dumoulin et al.
et al., 1996). In summary, the gonadotrophin-releasing hormone
(GnRH) agonist nafarelin (Synarel; Searle BV, Maarssen, The
Netherlands) was used in combination with human menopausal
gonadotrophin (HMG, Pergonal; Serono, Amsterdam, The
Netherlands; or Humegon, Organon, Oss, The Netherlands) to stimulate multiple follicular development. Follicle growth was monitored
by ultrasound and 5000 IU of human chorionic gonadotrophin (HCG,
Pregnyl; Organon) was given as soon as the dominant follicle was
judged to be mature (.18 mm), to induce final follicular and oocyte
maturation. Ultrasound-guided oocyte retrieval was performed 34–35
h after HCG administration. Insemination or ICSI was performed ~5
h after oocyte retrieval.
Culture of human surplus embryos
If cryopreservation was deemed unfeasible, surplus embryos were
used in one of the studies running in our centre. These studies have
been approved by the local Ethics Committee. To avoid selection
bias, surplus embryos of all treatment cycles during prearranged
periods were used in only one study. In the present study, surplus
embryos were left in their original culture medium for another 2 or
3 days. Developmental stages were recorded at each day of in-vitro
development. On the morning of day 5 after ovum retrieval, surplus
embryos that cavitated to form blastocyst-like structures (defined as
466
Gas phase
5% O2
20% O2
Number of cycles
Number of patients
Age (years)a
690
400
33.0 6 0.1
690
402
33.0 6 0.1
Indicationb
Tubal infertility
Idiopathic infertility
Male subfertility
Other
101
89
193
17
84
96
197
25
First attemptsc
352 (51)
Duration of infertilitya
(25)
(22)
(48)
(4)
5.4 6 0.1
(21)
(24)
(49)
(6)
338 (49)
5.5 6 0.1
Type of infertilityb
Primary
Secondary
Standard IVFc
ICSI
272
128
324
366
(68)
(32)
(47)
(53)
285 (71)
117 (29)
342 (50)
348 (50)
Culture mediumc
IVF-50™
HTF
262 (38)
428 (62)
260 (38)
430 (62)
Day of embryo transferd
Day 2
Day 3
519 (80)
130 (20)
513 (81)
121 (19)
aMean 6 SEM with regard to the female partner at the time of ovum
retrieval.
bValues in parentheses are percentages of total number of patients.
cValues in parentheses are percentages of total number of treatment cycles.
dValues in parentheses are percentages of total number of embryo transfers.
a rim of cells surrounding a large cavity of extracellular fluid
accumulated within the embryo) were fixed and stained with 49,6diamidino-2-phenylindole (DAPI) as described earlier (Coonen et al.,
1994), provided that the patients had given consent. The number of
nuclei stained with DAPI was taken as the number of cells of the
embryo. All other embryos, including those that had only just started
to form a small blastocoelic cavity, were cultured for another day
and were subsequently fixed on day 6 when they had developed to
the full blastocyst stage.
Analysis of results
Data were analysed by χ2-test or unpaired Student’s t-test, where
appropriate.
Results
The patient’s characteristics and other factors are summarized
in Table I. No significant differences for any of the parameters
studied were found between the two groups.
In Tables II and III, the results in terms of fertilization,
embryo development, implantation rates and pregnancy rates
of the culture under either 5% or 20% O2 are summarized.
None of the investigated parameters was significantly different
between the two groups.
In 501 of the 1380 cycles, at least one surplus embryo was
cultured to day 5 or 6. In the other 889 cycles, either no
fertilization took place, available embryos were all transferred,
surplus embryos were cryopreserved, or they were used in
another study. In Table IV, development of surplus embryos
to the blastocyst stage is summarized. The mean incidence of
blastocyst formation per cycle in which surplus embryos were
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Culture procedures
Oocytes and embryos were cultured in 50 and 20 µl droplets,
respectively, under mineral oil (Sigma, cat. no. M-8410). HTF medium
was ‘in-house’ prepared according to Quinn et al. (1985), and
supplemented with 8% (v/v) of a pasteurized human plasma protein
solution (PPS) obtained from the Central Laboratory of the Blood
Transfusion Service (Amsterdam, The Netherlands) as described by
Huisman et al. (1992). IVF-50™ is a ready-to-use commercially
available medium (Scandinavian IVF Science AB, Göteborg, Sweden).
The incubator used was a Napco double-chamber 7300 model with
separate gas controlling systems for both chambers (Boom BV,
Meppel, The Netherlands). In one chamber, an atmosphere of 5%
CO2 in air (~20% O2) was used, while in the other chamber a gas
mixture of 5% CO2, 5% O2 and 90% N2 was used. The CO2 and O2
concentrations of both incubator chambers were regularly checked
using a Servomex-570A Oxygen Analyzer (Servomex, Zoetermeer,
The Netherlands) and a Normocap 200 CO2-Analyzer (Datex Medical
Electronics, Hoevelaken, The Netherlands), and, if necessary, the
incubator chambers were recalibrated. After incubation for 18–20 h,
the oocytes were checked for the presence of pronuclei as proof of
fertilization, washed once and, after transfer to fresh medium, cultured
for another day. At the second and third day after oocyte recovery,
the developmental stage and morphological aspect of all embryos
were assessed under an inverted microscope at 3200 magnification
according to the criteria of Bolton et al. (1989). For each embryo,
an embryo score was calculated by multiplying the morphological
grade by the number of blastomeres (Steer et al., 1992). For each
treatment cycle, the score of all embryos was averaged to obtain a
mean embryo score (MES) (Steer et al., 1992). Embryo transfer was
routinely performed on day 2 after ovum retrieval, or, in a minority
of the cases, on day 3 for reasons of convenience to avoid transfers
on Sundays. If available, two or three embryos, depending on the
developmental stage and morphological appearance of the embryos,
as well as on the age of the patient, were transferred. After transfer,
any supernumerary embryos were cultured until the third day after
ovum retrieval. Cryopreservation of supernumerary embryos was
performed on the morning of the third day after insemination if one
or more embryos had reached the 8-cell stage, and if they were of
good morphological quality (grades 3 and 4; Bolton et al., 1989).
Table I. Characteristics of the two study groups cultured under either 5% or
20% O2
Effect of oxygen on human IVF results
Table II. Summary of results of the culture under either 5% or 20% O2:
fertilization and embryo development at day 2 and day 3 of in-vitro
development
Table IV. Development to the blastocyst stage of surplus embryos cultured
under either 5% or 20% O2
Gas phase
Gas phase
5% O2
20% O2a
Cycles
No. of oocytes per cycleb
Fertilization rate per cycleb,c
690
10.0 6 0.2
0.60 6 0.01
690
9.6 6 0.2
0.61 6 0.01
Embryonic development at day 2
Cleavage to ù4-cell stageb,d
Morphologically normal embryosb,e
Embryo score per cycle (MES)b
49 6 2
56 6 1
8.3 6 0.2
45 6 2
56 6 2
8.1 6 0.1
Embryonic development at day 3f
Cleavage to ù6-cell stageb,g
Morphologically normal embryosb,h
Embryo score per cycle (MES)b
42 6 3
52 6 3
15.4 6 0.6
36 6 3
56 6 4
14.7 6 0.6
aNo
Table III. Summary of results of the culture under either 5% or 20% O2:
pregnancy and implantation rates
Gas phase
Cycles
Embryo transfers
Pregnancies (%)b,c
Ongoing (ù12 weeks) pregnancies
(%)b,d
Multiple pregnancies (%)d,e
Mean no. of embryos transferred per
cycle (6 SEM)
Total no. of transferred embryos
Implantation sites (%)d,f
Viable fetuses (ù12 weeks) (%)d,f
5% O2
690
649 (94.1)
184 (26.7)
155 (22.5)
41 (26.5)
2.62 6 0.04
1762
236 (13.4)
196 (11.1)
20% O2a
690
634 (91.9)
175 (25.4)
147 (21.3)
50 (34.0)
2.55 6 0.04
1757
246 (14.0)
204 (11.6)
aNo
significant differences were noted between the two groups.
bValues in parentheses are percentages of total number of treatment cycles.
cPositive urinary pregnancy test (sensitivity 50 IU/l HCG) at 16–18 days
after ovum retrieval.
dAs determined by ultrasound at 5 and ù12 weeks after ovum retrieval,
respectively.
eValues in parentheses are percentages of total number of ongoing
pregnancies.
fValues in parentheses are percentages of total number of embryos replaced.
cultured was significantly higher in the group cultured under
5% O2. Of the embryos that developed a blastocoel, 236 of
310 (76%) embryos cultured under 5% O2, and 157 of 231
(68%) embryos cultured under 20% O2 were fixed. Reasons
for not fixing were either that no consent was obtained from
the patients, or embryos that were classified as early blastocysts
on day 5 and subsequently cultured for another day, were
found to be degenerated on day 6. The mean number of cells
per blastocyst (as determined by the observation of a clear
blastocoel under the microscope) was significantly higher in
Fixation of blastocysts on day 5
Blastocysts fixed
Fixation successfulc
Cells per blastocystd
Blastocysts consisting of at least
25 cellse
Cells per confirmed blastocystf
Fixation of blastocysts on day 6
Blastocysts fixed
Fixation successfulc
Cells per embryod
Blastocysts consisting of at least
25 cellse
Cells per confirmed blastocystf
247
1034
310 (30.0)
20% O2
254
1006
231 (23.0)h
25.8 6 2.0
20.4 6 1.9i
112
103 (92)
39.8 6 1.7
80 (78)
103
89 (86)
31.9 6 1.9j
54 (61)g
46.2 6 1.6
42.2 6 2.1
124
117 (94)
45.6 6 2.6
91 (78)
54
44 (81)
33.7 6 3.4j
25 (57)g
54.9. 6 2.6
49.1 6 4.6
aValues
in parentheses are percentages of total number of surplus embryos
cultured.
bExpressed as mean percentage 6 SEM of number of embryos developing
to the blastocyst stage on day 5 or 6/total number of surplus embryos
cultured per cycle.
cValues in parentheses are percentages of total number of blastocysts fixed.
dExpressed as mean number 6 SEM of cells per successfully fixed embryo.
eValues in parentheses are percentages of successfully fixed blastocysts.
fOnly blastocysts with a minimum of 25 cells are taken into account.
g,h,i,jSignificant difference when compared with culture under 5% O : χ2-test
2
(gP , 0.05; hP , 0.01), or unpaired Student’s t-test (iP , 0.05; jP ,
0.001).
the group cultured under 5% O2, both in blastocysts fixed on
day 5, as well as those fixed on day 6. This difference was
for the largest part due to the fact that significantly more
blastocysts of the 20% O2 group consisted of ,25 cells.
Discussion
This prospective randomized study on 1380 consecutive IVF
treatments shows that culture of human oocytes and embryos
for the first 2 or 3 days of development under a gas phase of
either 5% CO2/95% air (20% O2) or 5% CO2/90% N2/5% O2
does not result in significant differences between the two
groups, neither in pregnancy rates, nor in implantation rates.
Rates of fertilization and cleavage of the embryos also did not
differ significantly between the two groups. These results are
comparable to those reported in an earlier study in which
oocytes and embryos were cultured in 1 ml medium instead of
in microdroplets under an oil overlay (Dumoulin et al., 1995).
It has been speculated that the use of small medium droplets
under an oil phase could result in O2 levels in the medium close
to detrimentally low levels, especially when large numbers of
embryos or other cells (e.g. cumulus cells) are cultured together
(Baltz and Biggers, 1991; Byatt-Smith et al., 1991). From
these studies employing mathematical models it was calculated
that human embryos possibly could become marginally hypoxic
at lower O2 concentrations (Byatt-Smith et al., 1991). In our
467
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significant differences were noted between the two groups.
6 SEM.
oocytes/total number inseminated oocytes
per cycle.
dPercentage of embryos developed to the 4-cell stage or more at 39–42 h
after insemination or injection per cycle.
ePercentage of embryos of good morphological quality (grades 3 and 4;
Bolton et al., 1989) at 39–42 h after insemination or injection per cycle.
fOnly cycles of which the embryo transfer was performed on day 3 (i.e. all
embryos were cultured until day 3) were taken into account.
gPercentage of embryos developed to the 6-cell stage or more at 63–66 h
after insemination or injection per cycle.
hPercentage of embryos of good morphological quality (grades 3 and 4;
Bolton et al., 1989) at 63–66 h after insemination or injection per cycle.
bResults are expressed as means
cPercentage of number fertilized
Cycles with at least one surplus embryo
cultured
Total no. of surplus embryos cultured
Total no. of blastocysts observed on
day 5 or 6a
Incidence of blastocyst formation per
cycleb
5% O2
J.C.M.Dumoulin et al.
468
O2 concentrations exert an effect during the later stages of
preimplantation development only.
To conclude, in a culture system of microdroplets of medium
under oil, no beneficial effect on fertilization, embryonic
development and pregnancy rates of culturing human oocytes
and embryos for 2 or 3 days under 5% O2 as compared to
atmospheric O2 concentrations was found. Only when embryos
were cultured in vitro during the total preimplantation period,
was a slight but significant improvement of development to
the blastocyst stage found when 5% O2 was used.
Acknowledgements
The authors thank the following members of the IVF team, Academic
Hospital Maastricht, Maastricht, The Netherlands, for their clinical
and technical assistance: J.Marij Bergers-Jansen, MT; Gerard
A.J.Dunselman, MD; Germaine Kengen, RN; Renate Kuijper, RN;
Jacques Maas, MD; Nienke Muntjewerff, MD; Jolande A.Land, MD;
Anja Winkens-Tersteeg, RN; Lucie C.P.van Wissen, MT.
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study, however, no evidence was found of any adverse effect
when embryos were cultured under oil at 5% O2.
In the present study we show that culture under 5% O2
results in significantly more human surplus embryos reaching
the blastocyst stage, as well as a higher proportion of blastocysts
consisting of a normal number of cells. In a study from Hardy
et al. (1989) it was shown that the minimum number of
cells in normally fertilized human blastocysts on day 5 of
development was ~25 cells. The mean number of cells per
blastocyst in their study was 58 on day 5, which is somewhat
higher than in the present study (42–46 cells, Table IV). A
possible reason for this discrepancy is the fact that all surplus
embryos used in our study were of poor morphological quality.
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The use of a low O2 concentration has been shown to be
beneficial for embryo development in vitro to the blastocyst
stage in various animal species, e.g. mouse (Quinn and Harlow,
1978; Pabon et al., 1989; Umaoka et al., 1992; Gardner and
Lane, 1996); hamster (McKiernan and Bavister, 1990); rabbit
(Li and Foote, 1993); rat (Kishi et al., 1991); pig (Berthelot
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et al., 1991; Liu and Foote, 1995), and goat (Batt et al., 1991).
Also in the human, indirect evidence exists that a low O2
tension of 5% appears to enhance the blastulation rate of
surplus embryos (Noda et al., 1994). However, other studies
have failed to demonstrate the beneficial effect of low O2 on
the development to the blastocyst stage in the mouse (NasrEsfahani et al., 1992; Ali et al., 1993), cat (Johnston et al.,
1991), and sheep (Betterbed and Wright, 1985). Also it
appears that different culture systems may require different O2
concentrations for optimal results. When cow or sheep embryos
are co-cultured with other cells, 20% O2 is found to give better
results as compared to 5% O2 in several studies (Fukui et al.,
1991; Voelkel and Hu, 1992; Watson et al., 1994).
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O2 concentrations are more suitable culture conditions for
preimplantation embryos. Embryos develop in vivo under low
O2 levels, as in the oviduct and uterus of various mammalian
species O2 concentrations have been reported to be ~11–60
mmHg, which corresponds to ~1.5–9% O2 (Fischer and
Bavister, 1993). Furthermore, a reduced O2 concentration of
5% has been shown to result in a slightly decreased formation
of reactive oxygen species in mouse embryos as compared to
20% O2 (Goto et al., 1993). Reactive oxygen species have
been implicated in the retardation of early embryo development
in vitro (Johnson and Nasr-Esfahani, 1994; Tarı́n, 1996).
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development in vitro. However, little is known about the effect
of a lower O2 concentration during the first 2 or 3 days of invitro development on postimplantation embryonic development
and pregnancy rates. The present study shows that, although
culture under 5% O2 indeed leads to slightly improved preimplantation embryonic viability in the human, this effect is
either too marginal to result in higher pregnancy rates, or low
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Received on June 26, 1998; accepted on October 15, 1998
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