Murine pre-embryo oxygen consumption and developmental competence

Abstract

In search for a new marker of preimplantation embryo viability the present study investigated oxygen consumption of individual cleavage stage murine embryos, and evaluated the predictive value regarding subsequent development to expanded blastocysts.
In all, 248 embryos were investigated from 2 cell stage until blastocyst stage with individual measurement of oxygen consumption and recording of developmental stage. Cleavage stage embryos and morula were divided in groups according to their oxygen consumption, and odds ratios (OR) for subsequent development to expanded blastocyst were calculated.
Cleavage stage (2-8 cell) individual oxygen consumption was 0.16-0.20 nl O(2) h(-1), with a significant increase to 0.21-0.23 nl O(2) h(-1) at the morula stage followed by a more than twofold increase for the expanded blastocyst 0.47 nl O(2) h(-1). A significantly higher chance of reaching the expanded blastocyst stage was found in 4-cell embryos with high oxygen consumption, than embryos with low consumption (OR 2.25, 95% CI 1.04-4.90). Among 2-cell embryos the chance of low and high consumers was not significantly different. The method used in the present study somewhat compromised embryo development (51% blastocyst rate) compared to controls (80% blastocystrate) which could make our results less robust.
Preliminary data from the present study suggest that oxygen consumption in cleavage stage embryos may be an indicator, but a not a strong predictor, of subsequent development to expanded blastocysts.

Full text

ANIMAL EXPERIMENTATION
Murine pre-embryo oxygen consumption
and developmental competence
Lars D M Ottosen & Johnny Hindkjær &
Svend Lindenberg & Hans Jakob Ingerslev
Received: 1 February 2007 /Accepted: 23 April 2007 / Published online: 17 July 2007
#
Springer Science + Business Media, LLC 2007
Abstract
Purpose In search for a new marker of preimplantation
embryo viability the present study investigated oxygen
consumption of individual cleavage stage murine embryos,
and evaluated the predictive value regarding subsequent
development to expanded blastocy sts.
Methods In all, 248 embryos were investigated from 2 cell
stage until blastocyst stage with individual measurement of
oxygen consumption and recording of developmental stage.
Cleavage stage embryos and morula were divided in groups
according to their oxygen consumption, and odds ratios
(OR) for subsequent development to expanded blastocyst
were calculated.
Results Cleavage stage (2–8 cell) individual oxygen con-
sumption was 0.16–0.20 nl O
2
h
−1
, with a significant increase
to 0.21–0.23 nl O
2
h
−1
at the morula stage followed by a
more than twofold increase for the expanded blastocyst
0.47 nl O
2
h
−1
. A significantly higher chance of reaching
the expanded blastocyst stage was found in 4-cell embryos
with high oxygen consumption, than embryos with low
consumption (OR 2.25, 95% CI 1.04–4.90). Among 2-cell
embryos the chance of low and high consumers was not
significantly different. The method used in the present
study somewhat compromised embryo development (51%
blastocyst rate) compared to controls (80% blastocystrate)
which could make our results less robust.
Conclusion Preliminary data from the present study suggest
that oxygen consumption in cleavage stage embryos may
be an indicator, but a not a strong predictor, of subsequent
development to expanded blastocy sts.
Keywords Murine embryos
.
Preimplantation
development
.
Oxygen consumption
Introduction
In vitro fertilisation (IV F) and culture of preimplantation
embryos is an established part of assi sted reproduction, and
has permitted over a million couples world wide to
conceive. Yet, the method is limited by a rather low success
rate with an overall average implantation rate of cleavage
stage embryos in the order of 20–30% [1]. Characteristics
of embryo morphology observable in the light microscope,
in concert with kinetics of development, are still today the
prime measures of embryo quality. Despite advances in the
development of morphological criteria for embryo selec-
tion, and its undoubtedly strong association with develop-
mental competence [2], additional indicators to assist
selection of embryos with a particularly high implantation
potential are still needed to improve the IVF succes s rate,
especially in connection with elective single embryo trans-
fers (eSET). Elective SET is performed more and more
frequently, particularly in the Nordic countries, and will
most likely increase the number of treatment cycles where
a specific embryo must be chosen among morphologically
equal peers. Elective SET also increases the frequency
of frozen embryo replacement (FER) cycles. As the preg-
nancy rate per frozen-thawed embryo typically is reduced
J Assist Reprod Genet (2007) 24:359–365
DOI 10.1007/s10815-007-9138-5
L. D. M. Ottosen (*)
:
J. Hindkjær
:
H. J. Ingerslev
The Fertility Clinic, Department of Obstetrics and Gynaecology,
Aarhus University Hospital Skejby Sygehus,
Brendstrupgaardsvej, 8200 Århus N, Denmark
e-mail: lars.ottosen@ki.au.dk
S. Lindenberg
The Fertility Clinic, Herlev University Hospital,
Amtssygehuset i Herlev, 2730 Herlev, Denmark

compared to fresh transfers, there may be a particular need
for embryo viability assessment in these cycles also.
It is a common practice in IVF settings to transfer
embryos on day 2 or day 3 following oocyte pick-up.
Culture to the blastocyst stage demands significantly more
laboratory resources, and requires surplus cleavage stage
embryos to be meaningful in a clinical setting. Culture to
the blastocyst stage is sometim es performed as the
blastocyst development itself is a very strong viability
indicator of the embryo [ 3]. However, although the
individual blastocyst in general has a higher implantation
rate than the cleavage stage (2–8 cells) embryo, it remains
unclear whether a blastocyst transfer policy results in higher
pregnancy rates per treatment cycle, than routine transfer of
cleavage stage embryos [4]. This may in part be due to the
fact that culture conditions still are not optimal. Hence,
human preimplantation embryo viability remains compro-
mised in vitro relative to in vivo, which supports a strategy
of day 2 transfer [5]. Early viability indicators applicable at
the cleavage stage are therefore of particular interest for the
embryologist in a clinical setting. However, it should be
considered that assessment of the cleavage stage embryo
primarily is an assessment of the oocyte and the maternal
competence, as the paternal effect mainly is evident after
the 8 cell stage [6, 7], with a shift from maternal to embryo
genomic control.
Non-invasive measures of embryo metabolism, such as
amino acid profiles [8, 9] and utilisation of substrates for
energy production [10–12] have been suggested as objec-
tive embryo quality criteria. Generation of energy is a
prerequisite for development of the preimplantation em-
bryo. The energy metabolism of preimplantation embryos
significantly changes through development from fertilised
oocyte to expanded blastocyst. The first embryonic cleav-
ages are completely dependent on oxidative met abolism [8,
13], with about 95% of ATP production derived from
oxidative phosphorylation, decreasing to 82% with com-
paction [12, 14]. From morula to blastocyst stage the
metabolism changes towards an increasing contribution of
ATP from aerobi c glycolysis. Oxygen is the sole substrate
of the oxidative energy metabolism where the embryo
cannot fa ll back on its own reserves, so changes in
mitochondrial activity should be directly reflected in altered
fluxes of oxygen to the embryo. Early phases of cell death
are associated with such changes in mitochondria [15–17]
and alte red energy metabolism may be reflected in the
oxygen consumption before morphological changes
becomes detectable. Therefore, individual embryo oxygen
consumption appears the best available indicator of the
overall energy met abolism of the cleavage stage embryo
[5]. Yet, direct measures of oxygen consumption as a
measure of ATP production may have to be interpreted with
care. In a single study on mice it was found that as much as
70% of the cleavage stage oxygen consumption may be
OXPHOS (mitochondrial oxidative phosphorylation) inde-
pendent, decreasing to 30% at the blastocyst stage, due to a
substantially increased mitochondrial OXPHOS dependent
oxygen consumption between cleavage stage e mbryos and
blastocysts [18]. Whether this is a general picture remains
uncertain, and the nature and kinetics of the OXPHOS
independent oxygen consumption as well as its implications
for the embryo development needs to be studied in further
detail.
In this study we applied a recent ly developed strong
modification of existing technology to test whether oxygen
consumption of the individual murine cleavage stage
embryo (as an expression of a healthy oxidative energy
metabolism) could predict the competence of the embryo to
develop to the expanded blast ocyst. For the first time, a
coherent dataset of individual oxygen consumption and
developmental stage throughout the preimplantation devel-
opment was generated
Materials and methods
The oxygen consumpti on measurement principle
Individual embryo oxygen consumption was measured
using an Embryo Respirometer prototype (www.unisense.
com), capable of recording linear oxygen concentration
gradients towards single embryos placed in a microwell.
The basic measuring principle was substantially different
from a previous method based on self referencing electro-
des [18], as it overcomes the difficulty associ ated with
spherical diffusion, is indepe ndent on exact knowledge of
embryo sensor dist ance, and produce easily calculable
absolute values of oxygen consumption. The method is
validated and described in detail in [19]. Although based on
exactly the same principle, the apparatus used for this
experiment was technically different from the system used
by [19], consisting of an automated unit capable of repeated
oxygen consumption measurements by means of an oxygen
microsensor and a specially devised culture dish. In brief,
the embryo culture dish consisted of a series of small gas
impermeable glass wells (depth 3 mm, diameter 0.7 mm).
Each well was filled with culture medium (ISM1, Medicult
A/S, Denmark) and the embryo was placed on the bottom
of the well (Fig. 1). Oxygen supply to the embryo was
maintained through molecular diffusion from the overl ay-
ing culture medium down through the well. As ox ygen was
consumed by the embryo at the bottom of the well, a linear
360 J Assist Reprod Genet (2007 ) 24:359–365

oxygen concentration gradient was establis hed. The oxygen
flux towards the embryo was then calculated by measuring
the oxygen concentration gradient from the top towards the
bottom well. This was accomplished using an oxygen
microsensor guided by microcontrollers ca pable of software
controlled movements within and between the individual
wells. Under steady state conditions the flux equals the
oxygen consumption rate of the embryo at the bottom of
the well as illustrated in Fig. 1 (Adapted from www.
unisense.com with permission). Although the calculation of
oxygen consumption was based on an oxygen gradient, the
absolute change in oxygen partial pressure from the top
towards the embryo at the bottom of the well was less than
1%, hence practically unchanged compared to normal
culture conditions under an atmosphere of 5% CO
2
in air.
Each glass dish was designed with 36 wells, with four
subunits of 3*3 wells with a common overlaying reservoir
for oil. The respirometer was placed insi de a Galaxy
incubator.
The reliability of the method was tested by continuous
measurements in wells with culture media but without
embryos. Average calculated oxygen consumption (95%
CI) in w ells filled with culture medium but without
embryos was 0.01 (0.00–0.02) nl/h. Oxygen consumption
estimates of individual embryos were obtained by subtract-
ing actual blanks from actual measurements in each
measurement session.
Morphology scoring
Individual embryo developmental stage and morphology
was registered manually, and supported by advanced digital
image recording (Fertimorph by IH-Medical).
Biological material
Embryo production in vivo
Three week old virgin mice (C57/Black) were superovu-
lated by intraperitonal injection of 5 IU folligon (100 μl)
pregnant mare serum gonadotropin (Intervet International
B.V), followed by 5 IU (100 μl) Suigonan (serumgonado-
tropin (PMSG) and choriongonadotropin (HCG), Intervet
International B.V) 48 h later. The mice were transferred to
F1 C57/Black males, left overnight, and inspected for
vaginal plug the following morning as indication of mating.
The mice were killed by cervical dislocation and the
oviduct, plus 2–3 mm of the uterine arm, was immediately
removed and placed in pre-warmed (37°C) M2 flushing
medium (Sigma-Aldridge). Embryos were retrieved by
flushing the oviduct with M2 medium using a thin cannula,
and washed twice in M2 medium before placed in an
appropriate culture medium and incubated at 37°C under a
5% CO
2
atmosphere (Galaxy incubator). The morning
following mating was designated day 1 for the embryos.
Fertilisation occurred from 24 h to about 3 h before day 1.
Therefore, synchronized development of the embryos from
batch to batch was not expected.
Embryo selection and loading procedure
Embryos (2PN if detectable) were selec ted for oxygen
consumption measurement and loaded individually into the
glass dishes, one in each well, using a denudation pipette
(Swemed). Loading was done under a stereo microscope at
10–15 times magnification, followed by inspection from
below using an inverted microscope, to ensure that the
embryo actually reached the bottom of the well. Embryos in
the glass dishes were cultured in a separate incubator, and
only placed in the respirometer incubator during measure-
ments, approximately 90 min daily. Continuous culture of
embryos inside the respirometer was associated with >90%
developmental arrest between 1 cell and expanded blasto-
cyst and was therefore avoided.
Experimental design and data analysis
Oxygen consumption measuremen ts and developmen tal
stage identification was performed daily from day 2 until
day 5. The first reliable steady state measurement of the
oxygen consumption was performed on the morning of day 2
where the embryos were at the 2, 3 or 4 cell developmental
stages. An embryo was considered arrested if there was no
progress in developmental stage or increase in cell number,
from one day to the next. In case of doubt, digital recorded
Depth
Oxygen
concentration
Δx
ΔC
Oxygen
microsensor
Glass well
Embryo
Respiration = - D
ΔC
Δx
A
Fig. 1 Drawing of respirometer measuring principle
J Assist Reprod Genet (2007) 24:359–365 361

images were used for evaluation of developmental progress.
Oxygen consumption rates of embryos with detected arrest
were not used in the data analysis.
Mean individual oxygen consumption according to devel-
opmental stage was determined. Individual cleavage stage
oxygen consumption was determined and compared among
embryos with subseq uent arrested development, and embry-
os developing to expanded blastocysts. Individual cleavage
stage embryos were grouped according to their oxygen
consumption. OR for development to expanded blastocyst,
among the groups, were determined by logistic regression.
The effect of different cut points between low and high
consumption groups was tested using ROC curve analysis for
a range of cut points. Hence, the cut point resulting in the
highest difference in Odds among low and high consumers,
for development to expanded blastocysts, was used. Statis-
tical calculations were performed using the statist ical
programme package Stata for windows (www.stata.com).
Results
A total 248 embryos were investigated. Of these, 200
embryos had their oxygen consumption recorded from day
2, and 48 embryos from day 3. Individual oxygen consump-
tion rates are shown in Table 1. Oxygen consumption was
low and relatively constant at the cleavage stage, and then
rose slightly at the morale stage, followed by a significant
increase at the expanded blastocyst stage. However once an
embryo stopped developing (no increase in cell number, or no
sign of blastulation for morulas within 24 h) it was excluded
from the analysis thereafter. So values reported in Table 1,
were based on rates from embryos, with ongoing develop-
ment at the time of oxygen consumption measurement.
Among embryos subsequently reaching the expanded
blastocyst stage , there was a significant increase in oxygen
consumption from the 4 cell stage (0.17 nl O
2
h
−1
) to the 7–
8 cell stage (0.22 nl O
2
h
−1
)(p<0.05) and to the morula
stage (0.26 nl O
2
h
−1
embryo
−1
)(p<0.05). A total increase
of 53% from the 4 cell to the morula stage ( p<0.05) was
observed. Embryos with arrested development had a
smaller (30%) increase in oxygen consumption from 4 cell
(0.16 nl O
2
h
−1
) to the morula (0.21 nl O
2
h
−1
embryo
−1
)
yet still significant. However, among embryos arresting
before reaching the expanded blastocyst stage the increase
in mean oxygen consumption from 4 cell to 7–8 cell, and
then to morula was not significant for each step ( p>0.05).
Cleavage stage mean oxygen consumption at each devel-
opmental stage (2, 4 and 7–8 cell) was not significantly
different among embryos with subsequent arrest compared
to embryos developing to expanded blastocysts.
Two and 4 cell embryos
Embryos at the 2 and 4 cell stage respectively were grouped
into a high oxygen consumption group (>0.145 nl O
2
h
−1
embryo
−1
) and a low oxygen consumption group (>0.145 nl
O
2
h
−1
embryo
−1
). The calculated OR for subsequent blas-
tocyst development and its confidence interval (95% CI)
was quite sensitive to which cut off point for high and low
consumers that was used. Sensitivity analysis (data not
shown) showed that using a cut point of 0.145 nl O
2
h
−1
embryo
−1
for high and low consumption resulted in the
most significant OR for the 2 and 4 cell group. For 2 cell
embryos Odds Ratio for development into expanded
blastocyst was 1.925 (CI 0.67–5.57, p=0.227) in the high
consumption group (n=54) relative to the low consumption
group (n=39).
Odds Ratio for development into expanded blastocyst
was for the 4 cell embryos in the high consumption (n=85)
group was 2.25 relative to the low consumption group (n=
43), with a 95% confidence interval 1.04–4.90, hence just
significant ( p=0.040) Table 2.
7–8 cells
The b est OR f or subsequent blastocyst developme nt
(according to sensitivity analysis) for a high consumption
Table 1 Mean individual oxygen consumption rates ± standard error of the estimate
Developmental
stage
Mean oxygen consumption
(nl O
2
h
−1
embryo
−1
±SE)
All embryos
Mean oxygen consumption (nl O
2
h
−1
embryo
−1
±SE) embryos reaching exp
blast
Mean oxygen consumption (nl O
2
h
−1
embryo
−1
±SE) embryos arrested before exp
blast
2 cell 0.162±0.0043 (n=93) 0.162±0.0073 (n=20) 0.162±0.0052 (n=73)
4 cell 0.166±0.0038 (n=128) 0.172±0.0053 (n=55) 0.161±0.0053 (n=73)
7–8 cell 0.197±0.0089 (n=33) 0.218±0.0115 (n=15) 0.179±0.0120 (n=18)
Morula 0.234±0.0056 (n=142) 0.261±0.0070 (n=71) 0.207±0.0074 (n=71)
Expanded
blastocyst
0.464±0.0185 (n=69) ––
N number of embryos with a recorded oxygen consumption.
362 J Assist Reprod Genet (2007 ) 24:359–365

group (>0.19 nl O
2
h
−1
embryo
−1
)(n=19) was 4.07 relative
to the low consumption group (n=14), (95% CI: 0.85–
19.43), hence not significant. The number of embryos with
7–8 clearly individual cells was quite low as the embryos of
the actual mouse strain, cultured under the given culture
circumstances, often turned to a morula-like morphology,
where the embryo appeared like a solid mass of indistin-
guishable cells, already immediately following the 5–6 cell
stage.
Morula
Mean oxygen consumption for all morula was 0.23 nl
O
2
h
−1
embryo
−1
. When divided into a low and high
oxygen consumption group, best OR for development into
expanded blastocyst in the high consumption g roup
(>0.23 nl O
2
h
−1
embryo
−1
)(n=70) was 3.18 relative to
the low consumption group (<0.23 nl O
2
h
−1
embryo
−1
)
(n=72) (95% CI: 1.60–6.32, p=0.001). Hence oxygen
consumption above mean clearly indicated that the morula
would continue development into expanded blastocyst.
Ordering embryos in three or more groups covering
different consumption intervals, or using other cut off
points for division into two groups , did not result in the
identification of other oxygen consumption groups with a
better prediction of development to expanded blastocyst
(Data for less informative grouping of high and low
consumers not shown). Additional analysis taking degree
of fragmentation at each cleavage stage into account was
conducted, but without significant chan ge in results (data
not shown).
The blastocyst rate during culture in the glass wells, with
one daily oxygen consumption measurement, was 51%
from 2 cell blastocyst. Blastocyst rate in control embryos in
standard Nunc 4 well dishes was significantly higher 82%,
although cultured in identical medium from the same batch.
Discussion
This study shows individual oxygen consumption rates
from freshly in vivo produced murine embryos from day 2
following mating until the expanded blastocyst stage on
day 5. Day 2 embryos were typically 2–4 cells, day 3 were
7–8 cells or morula, day 4 were morula or early blastocysts,
and day 5 expanded or hatched blastocysts. Individual
recording of developmental stage was performed daily;
hence a coherent datase t of individual oxygen consumption
and developmental stage throughout the preimplantation
development, not previously reported, was generated. In
general, oxygen c onsumption remained low until the
blastocyst stage where a significant ( p<0.001) more than
twofold increase was observed. This overall pattern is in
accordance with several previous studies of preimplantation
embryo oxygen consumption [12, 14, 18–20]. Trimarchi
et al. [18] reported 0.1 nl O
2
h
−1
for cleavage stage murine
embryos and 0.3 nl O
2
h
−1
for blastocysts using a self
referencing oxygen electrode. Murine oxygen consumption
rates reported in this study were remarkably similar to
values reported by Houghton et al. [12] using closed
respirometry, except for the moru la stage where the present
study indicate somewhat higher oxygen consumption rates
(0.23 nl O
2
h
−1
embryo
−1
) compared to approxi mately
0.15 nl O
2
h
−1
embryo
−1
as reported by Houghton et al.
[12]. An early study by Mills and Brinster [20], using a
Cartesian diver technique, actually observed an increase in
oxygen consum ption from the 2–8 cell stage to the morula
stage quite similar to the observations in the present study.
Differences in murine oxygen consumption rates reported,
may relate to the different measuring techniques, different
mouse strains as well as the use of varying culture media
which may influence embryo metabolism [21].
Among embryos with a subsequent development to
expanded blastocyst, the increase in oxygen consumption
from 4 cell embryos (0.17 nl O
2
h
−1
)to7–8 cell embryos
(0.22 nl O
2
h
−1
), and again from 7–8 cell to morula (0.26 nl
O
2
h
−1
) was significant ( p<0.05). The significant increase
may be associated with a successful rise in energy
production required for the compactation and subsequent
blastulation process as discussed by Trimarchi et al. [18]
and Houghton et al. [12]. The observed increase in oxygen
consumption among embryos which did not develop into
expanded blastocysts was not significant from the 4 cell to
the 7–8 cell stage or from the 7–8 cell to morula.
Table 2 Grouping of embryos according to their oxygen consumption, and Odds Ratios (with 95% confidence intervals) for development to
expanded blastocysts among various consumption groups
Developmental
stage
Low consumption group
nl O
2
h
−1
embryo
−1
High consumption group
nl O
2
h
−1
embryo
−1
, (n)
OR (95% CI) High consumers for
development to expanded blastocyst
2 cell < 0.145 (n = 39) > 0.145 (n = 54) 1.93 (0.67 –5.57)
4 cell < 0.145 (n = 43) > 0.145 (n = 85) 2.25 (1.04 –4.90)
7–8 cell < 0.190 (n = 14) > 0.190 (n = 19) 4.07 (0.85 –19.4)
Morula < 0.230 (n = 72) > 0.230 (n = 70) 3.18 (1.60 –6.32)
J Assist Reprod Genet (2007) 24:359–365 363

Investigation of association between oxygen consump-
tion and developmental competence was done by grouping
embryos in different consumption groups and calculating
odds ratios for development to expanded blastocysts among
the different consumption groups at a given developmental
stage. The groupi ng of embryos reported in the present
study yielded the best possible immediate prediction of
development to expanded blastocyst, based on individual
oxygen consumption. Taking degree of fragmentation at the
2, 4 and 7–8 cell stage into account did not add information
about subsequent developmental compe tence. This was not
surprising as the in vivo produced mouse embryos used in
the present study were virtually unfragmented at the
cleavage stage.
Several combinations of grouping embryos according to
oxygen consumption at a given developmental stage were
tested (data not shown), and it was empirically found that a
simple division into a high and low consumption group
yielded the best prediction of preimplantation developmen-
tal competence. The general trend was that embryos in the
high consumption group had a higher probability of
developing into expanded blastocysts compared to embryos
grouped as low consumers, although not signi ficantly so at
the 2 cell stage and the 7–8 cell stage, and just significant at
the 4 cell stag e. Hence, ox ygen consumption at the
cleavage stage was a relatively poor predictor of preim-
plantation developmental competence. In contrast, oxygen
consumption at the morula stage was a fair predictor with
OR among high consumers being 3.18 relative to the low
consumers, and highly significant ( p<0.001).
We acknowledge, in accordance with the manufacturer
(www.unisense.com), that the current technology used in
the present study was not truly non-disturbing, and resulted
in reduced blastocyst rates compared to controls. However,
it is hard to believe that embryos which would have
arrested under optimal conditions should not also have
arrested during the culture conditions in the present study,
and still it was not possible clearly at the 2 or 4 cell stages
to identify a specific group of embryos with subsequent
preimplantation arrest, which it should have been if a
specific oxygen consumption pattern predicts developmen-
tal arrest, or ongoing development. We therefore suggest
that although the results possibly could have been clearer with
optimal culture conditions, our conclusions remain valid.
Our main purpose of the study was to investigate if
oxygen consumption among morphologically like embryos,
at the early preimplantation stage (2 and 4 cells) could
predict subseq uent development into expanded blastocysts.
As a selection tool in the clinical setting, we suggest that
oxygen consumption measurements would be particularly
useful to select among morphologically similar high quality
embryos, equally well suited for transfer at a first glance.
The murine embryos used in the present study were
morphologically quite similar at each developmental stage,
thus a suitable model reflecting a clinical situation, even if
the general morphological variance among in vitro fertilised
human embryos is higher than among in vivo produced
murine embryos. High oxygen consumption relative to
developmental stage indicated subsequent development to
expanded blastocyst, although the results do not quite
indicate that oxygen consumption of murine cleavage stage
embryos is actually a strong predictor of development to
expanded blastocyst. The actual usefulness of oxygen
consumption at the cle ava ge stage, as a predictor of
developmental competence, remains to be investigated in
further detail, preferably with truly non disturbing equip-
ment currently under development. Compared to the 2–
8 cell stage, where the embryos exhibit a quiet metabolism
with no growth in dry mass or protein content [5], and before
onset of embryonic genome transcription [6, 7], it may be
more promising to apply oxygen consumption measure-
ments as a viability indicator at the blastocyst level, although
it conflicts with the general idea of a very early marker.
In the present study we saw a significant predictive value
of oxygen consumption at the morula stage. The expanded
blastocyst is normally the latest developmental stage to be
evaluated in vitro before transfer to the uterus. A key
question there is whether a single measurement of oxygen
consumption among morphologically like blastocysts could
predict subsequent implantation and pregnancy. A truly non
disturbing version of the technology used in the present
study is currently under development (www.unisense.com).
In general we suggest implantation and pregnancy as a
better and clinically more relevant end point than blastocyst
rate, and we recommend this to be applied in future studies.
An important future study would therefore, with an
improved method, be to investig ate associ ation between
implantation potential of blastocysts and their oxygen
consumption.
Acknowledgements This wo rk was sup ported by t he Danish
Medical Research Council, The Beckett Foundation, The Foundation
of 17/12–1981, The Toyota Foundation, the A.P. Møller Foundation
for the advancement of Medical Science, K aren Elise Jensens
Foundation, Clinical Institute Aarhus University and Organon Den-
mark. Thanks to Morten Raaschou Image House Medical A/S for
providing the Fertimorph equipment used in this study. Thanks to Puk
Sandager for statistical advice.
Ethics The use of experimental animals is approved and controlled
by the Faculty Veterinary and follows the regulations of the National
Committee of Experimental Animals.
References
1. Andersen AN, Gianaroli L, Felberbaum R , et al. Assisted
reproductive technology in Europe, 2002. Results generated from
European registers by ESHRE. Hum Reprod 2006;21:1680– 97.
364 J Assist Reprod Genet (2007 ) 24:359–365

2. Ziebe S, Petersen K, Lindenberg S, et al. Embryo morphology or
cleavage stage: how to select the best embryos for transfer after
in-vitro fertilization. Hum Reprod 1997;12:1545–9.
3. Gardner DK, Lane M. Culture and selection of viable blastocysts:
a feasible proposition for h uman IVF? Hum Reprod Updat
1997;3:367–82.
4. Blake D, Proctor M, Johnson N, Olive D. Cleavage stage versus
blastocyst stage embryo transfer in assisted conception 2005
Cochrane Database of Systematic Reviews, Issue 4. Art. No.:
CD002118. doi:10.1002/14651858.CD002118.pub2.
5. Leese HJ. What does an embryo need? Human Fertil 2003;6:
180–5.
6. Janny L, Menezo YJ. Evidence for a strong effect on human
preimplantation embryo development and blastocystformation.
Mol Reprod Dev 1994;38(1):36–42.
7. Miller JE, Smith TT. The effect of intracytoplasmic sperm
injection and semen parameters on blastocyst development in
vitro. Hum Reprod 2001;16:918–24.
8. Brison DR, Leese HJ. Energy metabolism in late preimplantation
rat embryos. J Reprod Fertil 1991;93(1):245–51.
9. Houghton FD, Hawkhead JA, Humpherson PG, et al. Non-
invasive amino acid turnover predicts human embryo develop-
mental capacity. Hum Reprod 2002;17:999–1005.
10. Gardner DK, Leese HJ. Assessment of embryo viability prior to
transfer by the noninvasive measurement of glucose uptake. J Exp
Zool 1987;242:103–5.
11. Overstrom EW. Manipulation of early embryonic development.
Anim Reprod Sci 1992;28:277 – 85.
12. Ho ughton FD, Thompson JG, Kennedy CJ, et al. Oxygen
consumption and energy metabolism of the early mouse embryo.
Mol Reprod Dev 1996;44:476–85.
13. Kane MT, Buckley NJ. The effects of inhibitors of energy
metabolism on the growth of one-cell rabbit ova to blastocysts
in vitro. J Reprod Fertil 1977;49(2):261–6.
14. Thompson JG, Partridge RJ, Houghton FD, Cox CI, Leese HJ.
Oxygen uptake and carbohydrate metabolism by in vitro derived
bovine embryos. J Reprod Fertil 1996;106(2):299–306.
15. Liu L, Trimarchi JR , Keefe DL. Thiol oxidation-induced
embryonic cell death in mice is prevented by the antioxidant
dithiothreitol. Biol Reprod 1999;61:1162–9.
16. Crompton M. The mitochondrial permeability transition pore and
its role in cell death. Biochem J 1999;341:233–49.
17. Gross A, McDonnell JM, Korsmeyer SJ. BCL-2 family members
and the mitochondria in apoptosis. Genes Dev 1999;13:1899– 911.
18. Trimarchi JR, Lui L, Porterfield DM, et al. Oxidative phosphoryla-
tion dependent and independent oxygen consumption by individual
preimplantation mouse embryos. Biol Reprod 2000;62:1866–74.
19. Lopes AS, Larsen LH, Rasming N, et al. Respiration rate of
individual bovine in vitro produced embryos measured with a
novel, non invasive and highly sensitive microsensor system.
Reproduction 2005;130:669–79.
20. Mills RM, Brinster RL. Oxygen consumption of preimplantation
mouse embryos. Exp Cell Res 1967;47:337–44.
21. Barnett DK, Bavister BD. What is the relationship between the
metabolism of preimplantation embryos and their developmental
competence? Mol Reprod Dev 1996;43(1):105–33.
J Assist Reprod Genet (2007) 24:359–365 365