Download New concepts in embryonic growth and

New concepts in embryonic growth and
implantation
R.G.Edwards
Moor Barns Farmhouse, Madingley Road, Coton, Cambridge, UK
Once again, we have been privileged to hear a most distinguished set of lectures
today. Each speaker has been authoritative and imaginative. My task is again to
stress some of the points in these lectures, and in the Discussions and perhaps
to embellish them here and there. There were two major topics during the day:
five lectures on embryo quality and five on implantation and I shall discuss the
papers under these headings.
Embryo quality and development
The laboratory environment
This was a somewhat rueful topic for me, reminding me of the early years of
Bourn Hall. In those days, there were temporary cabins to serve as wards and
embryology laboratories, as some members of the audience will remember. We
were often at the mercy of our agricultural surroundings, and our farmer
neighbours had no qualms about burning the straw in the fields after harvest
time. Flames, and not small ones, came to our boundary edge, and the smoke
penetrated into our carefully designed, high-effective, high-cost filters and into
the environmentally-controlled embryology laboratory. It even penetrated the
filters installed in our permanent buildings a few years later. This infiltration
gave us considerable grounds for concern. On one occasion, builder's plaster
also penetrated into our laboratories and incubators, even through paraffin
overlayers into media, and destroyed the embryos of 12 or more patients. This
was one of my worst days in in-vitro fertilization (IVF), explaining to the patients
what had happened and offering them compensation.
Light has also been one of my major concerns ever since IVF began. We were
aware of the many papers on mammals published by embryologists on the
evolution of reactive oxygen species in response to light exposure, and its
deleterious effects on embryo growth. We could not afford any risks with human
embryos to be replaced into the mother, so we used green filters routinely to
remove some of the light radiation, lower the light intensity and produce a more
acceptable colour for the eye by modifying the harsh artificial light from the
microscope. The potential effects of light concerned me in another way. During
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transfer, gynaecologists often used an intense operating theatre light to shine on
the cervical os. Yet this was where the embryo is passed during the transfer
process. At the last moment, after hormone stimulation, oocyte collection,
fertilization and cleavage in vitro, these precious embryos could be exposed to
an intense light which might impair their ongoing development. We therefore
dimmed this light during transfer to avoid any damage to the embryos in the last
stage of their ex-utero existence. Several investigators have disparaged my
attitude, and they may even be right when they claim that human embryos can
tolerate this degree of intense light exposure. But I have never seen any evidence
on this point from these investigators, and it is surely better to be safe than sorry.
So I still use many of these precautions.
I am therefore very sympathetic to Jacques Cohen's stress on quality in the
laboratory environment. His concentration on the atmosphere is welcome. It not
difficult to establish the correct conditions when a laboratory is designed, and
then maintain them during routine maintenance. It is essential in our field that
these precautions are taken, provided of course that all attempts are taken to
show they are relevant.
Cytoplasmic transfers on the 1-cell stage
Jacques' second topic was the transfer of cytoplasm to improve oocyte quality.
This work is a continuation of the move to micromanipulation in our field of
study. The preimplantation diagnosis of genetic disease in embryos was the first
example of the need for delicate micromanipulative studies. Another example,
more recent as intracytoplasmic sperm injection (ICSI) has spread worldwide.
Some of the pioneers in this field of micromanipulation included Tarkowski and
his Polish group of workers, who for example transferred nuclei from many
types of cell into oocytes and pronucleate eggs to study the early regulation of
mammalian development. Lin, Gardner and I used it to investigate blastocysts,
establish what are called transgenic animals today, and for the preimplantation
diagnosis of genetic disease in animals. Reik et al. (1993) are modern exponents
of the art, with their method of establishing artificial diploid androgenones and
gynogenones in mice by exchanging male and female pronuclei between
fertilized eggs.
Transferring ooplasm must be undertaken with care. We know well in mice
and other laboratory animals that repressive factors are produced in ooplasm,
and even differ in concentration and distribution in different regions of the
oocyte. The oocyte is not simply a bag of cytoplasm with two nuclei, it is a
most organized and dynamic cell, highly polarized with some maternal mRNAs
and proteins restricted to particular areas, e.g. a limited zone in cortex, and which
actually shift their position from one specific site to another as the oocyte matures
or the pronucleate egg develops (Antczak and Van Blerkom, 1997; Edwards and
Beard, 1997). One consequence of this tight control is that the male pronucleus
is four times more active transcriptionally that the female pronucleus, at least in
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mice. What we need is more information about the oocyte, its activity and
regulation, so that we interfere seriously with development in a knowing and
planned fashion. For example, the difference between pronuclei could be crucial
to the early regulation of the egg, and any upset in this difference between male
and female pronuclei could impair its normal growth. Transfers of cytoplasm
must also be made with synchronous donors and recipients. The cytoplasm must
also be placed in its correct position in the recipient egg unless the egg is capable
of a considerable regulation. I am not sure that Jacques and his team take these
precautions. Many important issues are at stake, such as the regulation of minor
and major transcription beginning in the 2-cell stage.
Embryo quality and cryopreservation
We have been indebted for many years to Jacqueline Mandelbaum and her
colleagues in Paris for their very careful analyses on the early human embryo.
They specialized in understanding the nature of growth and the consequences of
fragmentation in relation to the chances of implantation. I am still very surprised
that IVF embryologists have not extended their methods in simple and practical
ways. To find out about cleavage times and fragmentation, it would be very
simple to score embryos three times per day. At present, scoring is done once
daily in virtually all IVF clinics, so the embryologist does not know of an embryo
cleaved 1 h before it was examined, or 23 h before. How can cleavage or
fragmentation be fully judged under this once-daily regimen? This is an important
matter because cleavage history is fundamental in identifying the embryos most
likely to implant. Our own earlier work, which involved scoring embryos six or
more times daily, even during the night if necessary, evaluated the exact cleavage
time from 1-cell to 2-cell, and from 2-cell to 4-cell in human embryos in culture.
Those that divided first had an immense advantage at implantation (Edwards
et al, 1984). There is no difficulty in scoring embryos three times daily, and the
method could be incorporated into IVF clinics with minimal difficulty.
Jacqueline concentrated on the results of cryopreservation over the years since
the early 1980s. She has collaborated with other centres to produce a first-rate
report on the role of cryopreservation in human assisted conception. Her topics
ranged from the immense amounts of data now available on the standard methods
of cryostorage of pronucleate and cleaving embryos to the newer approaches of
cryopreserving blastocysts and mature oocytes, and to the very recent work on
follicles, immature oocytes and ovarian slices. All these approaches will be of
great significance in the coming years, helping patients to overcome specific
problems of their own. Oocyte storage is needed urgently for many purposes,
and together with ovarian slices, should help women facing chemoradiotherapy
for cancer. We will also soon see the day when oocytes are produced from
follicles grown or stored in vitro. I was also very glad to hear Jacqueline give
credit to Debora Gook for her work on oocyte cryopreservation, since I believe
that she and her team were among the forerunners in renovating this field of study.
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Her main theme also included the practical and social results of cryopreserving
pronucleate and cleaving embryos. The methods are increasingly standardized,
becoming more widespread, and offering extra hope to many couples. An
increment of 8% on births after the fresh embryos have been used is more than
welcome. A livebirth rate of 6% per transferred thawed embryo is excellent
news; in the best clinics, a rate of 10% with fresh embryos must be near the
average, and these births utilize the best embryos. The advantage of cryopreserved
embryos is also reflected in their 16% clinical pregnancy rate, again placing
matters in perspective. We must soon make use of this information. Critics of
cryopreservation programmes insist that the effort is too great, and that similar
results could be obtained less expensively and more simply by carrying out extra
cycles with fresh embryos. What, then, happens to those fresh embryos in excess
of the number replaced? They belong to their parents, and should be cryopreserved.
As far as I know, there are no risks to a child's health from cryopreservation,
and other statistics produced by Jacqueline deny these critics.
In this respect I was impressed by her data showing how 86% of the
cryopreserved embryos are thawed for the parents over 10 years. This is an
excellent sign of laboratory responsibility, of great care in the storage process
and of the affection of parents for their embryos. Most embryos are replaced
within 5 years. These facts certainly answer critics who say that most embryos
will be abandoned in storage by their parents, an accusation now shown to be
untrue. What is needed is an improvement in the implantation rate, just as with
fresh embryos. This would enable cryopreservation of embryos to help those
patients even more who fail to become pregnant with their fresh embryo, or who
have a miscarriage. They could have a second or more welcome attempts at
pregnancy, and many do so successfully.
I was also very impressed by Jacqueline's analysis of cryopreservation
programmes. She recommended that 5 years would see the majority of embryos
transferred or donated. It is interesting that the British Human Fertilisation and
Embryology Authority recommended 5 years initially as the period for cryostorage. However, at the end of this period, many embryos were still in store, and
there were no instructions from the parents about their fate. Consent was essential
for donation, destruction or use in research, so the Authority ruled that those
without such a consent after 5 years must be destroyed. This resulted in the
destruction of 3000 cryopreserved embryos, at a time when some of them might
have been saved for donation or research. Many of us are still highly critical at
this wanton destruction of embryos, which forces us to conclude that central
regulatory authorities really have little understanding of the necessities in our
work. Even 10% of those embryos, which could well have been saved if a short
delay in gaining consent had been granted, they would have been sufficient for
various controlled trials, e.g. David Gardner's media, of feeder layers or zona
drilling, or to analyse gene expression at successive stages of development.
The difference between the values of analysis on cryopreservation stands in
contrast to studies on co-cultures. Controlled clinical trials to formally reach a
decision about their value are still lacking. Vero cells are Green monkey kidney
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cells, and it would be wiser to abandon them in view of the number of viruses
affecting human embryos. Supporting cells must be a human oviductal or uterine
cell. With Vero cells or fibroblasts, there is always a suspicion that any beneficial
effects are due to the chelation of metals or other factors from culture medium.
Even human feeder layers are risky, in the present days of human immunodeficiancy virus (HIV) and Creutzfeld-Jakob disease (CJD).
Genetic control of embryo quality
Carol Warner has analysed, dissected may be a better word, two major aspects
of early human growth in vitro, fragmentation and cleavage rates. Her contribution
raised the possibility of identifying the risks to the embryo inherent in the genes
inherited from its two parents. These genes, Q7 and Q9 differ by only a single
nucleotide in the coding region. Fast embryos express either one, slow embryos
express neither. This minor genetic change converts embryos from fast- to slowdevelopers. Now the genes are identified, their time and mode of action is rapidly
being clarified, the possible association between Ped and HLA-G, and even to
an embryonic form of HLA-G is fascinating. Nevertheless, I still find it difficult
to see how these genes act throughout pregnancy if fast embryos are one cleavage
ahead by the blastocyst stage, and the fetus is still one day ahead at birth. The
possibility of converting slow-growing human embryos into those dividing more
rapidly could improve implantation rates. Her analyses are still largely confined
to mice, but in these days the jump to humans can be very fast indeed.
All this work raises the need for more knowledge on an exact programme of
human embryo growth. We wish to know about the switch-on of genes at specific
developmental stages, and to find out if their transcripts or proteins can be
detected in the developing embryo. We have had some markers for blastocysts
for some time, namely the presence of various receptors on the trophectoderm of
expanding and hatching blastocysts. Helen Beard and I are currently constructing
models of early growth (Edwards and Beard, 1997). If these are confirmed, then
they open new ideas of how cells might interact and differentiate. We deeply
hope that the model is largely correct because it is time to use the wealth of
information accruing from the genome project to understand the regulation of
our early beginnings, both for scientific and clinical purposes.
There was little discussion on apoptosis and its genetic control, yet the roles
of bcl-2 and bax are of fundamental significance in this process. I was delighted
to know that bax has been identified in the human embryo. These enormous
species homologies are now familiar to us all, and it is a brave person who
would challenge the concept that every human gene has a mouse homologue,
literally awaiting a molecular biologist to draw the parallels. It was again
interesting to see Carol's classification of fragmentation, with apoptosis being
linked especially to type IV embryos which have fragments equal in size to
blastomeres. Fragmentation is a serious problem in IVF, which challenges our
methods of fertilization and embryo growth in vitro. The mRMA of bax persists
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throughout preimplantation growth, so apoptosis can be promoted throughout
these stages, bcl-2 is absent from oocytes, and perhaps appears after fertilization
to deflect apoptosis. These concepts are highly novel, and they open new avenues
of understanding of early development, The fact that the oocyte has not switched
on its destruct button may also be an evolutionary adaptation to cope with the
variable time of arrival of spermatozoa in the oviduct. Or it may be a mechanism
to stop the embryonic clock while awaiting the stimulus of fertilization to activate
the genetic processes in the fertilized egg. I do not know if it will ever be
feasible to control the expression of the bax and bcl-2 genes.
Culture media and embryo quality
Embryo quality is also depends highly on the properties of culture media. New
approaches to the design on media have recently emerged from several groups
of workers, and an excellent example of their effectiveness were given by David
Gardner. Almost all animal embryologists stress the need to transfer blastocysts
into the uterus, based on experience with so many non-human species. In his
words, the blastocyst has now 'come of age in human IVF'. When designing his
media, he showed how it was necessary to focus on preparing several media, for
specific embryonic stages. The relatively inert zygote, utilising little glucose, is
transformed into a blastocyst utilising high levels of oxygen as it increases its
aerobic glycolysis. These changes are reflected in the oviductal and uterine
environments. The judicious introduction of amino-acids, glucose, control of
ammonium and reactive oxygen species, and the need for changing media at
different embryological stages has now led to new approaches to embryo culture.
The non-essential amino acids are needed in early stages and essential amino
acids later. Culture to 4-cell requires the former, blastocysts the latter. Development
of the inner cell mass (ICM) responds to essential amino acids, i.e. Gardner's
G2 medium. He now obtains many human blastocysts using this system, with
pregnancy rates improving to 50%, after transferring two blastocysts. This
promises to give us a selection system to choose the best blastocysts. Once
again, we need a controlled clinical trial, this time to find out if growing embryos
to blastocysts adds an extra dimension to pregnancy rates, or whether those
embryos capable of implanting as blastocysts are the same as in 4-cell stages.
Low rates of implantation have plagued us since the conception of Louise
Brown. All the changes in hormonal stimulation, embryo culture, ICSI, etc,
introduced in recent times have not helped much to improve implantation rates.
Some clinics claim to have achieved this step, and if they have then we must all
adopt their methods. It is difficult to introduce simpler methods of ovarian
stimulation while implantation rates per embryo remain so low. Perhaps the rates
we are currently obtaining are the same as those established in vivo, since much
data indicates that only 20% of cycles result in pregnancy in couples desiring
pregnancy. If this is so, then we will have to improve on Nature to gain IVF
pregnancy rates approaching 50%. One consequence of the low implantation rate
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is the need to replace two or more embryos, which leads to multiple pregnancies,
one of the major problems in IVF today.
Multiple pregnancies
Jean Cohen discussed this most important aspect of assisted conception with the
greatest of thoroughness and detail. He provided an urgently-needed review
assessing the widest implications of the problem, and was very careful to present
much data on facts and figures, the risks and costs of multiple pregnancies, the
various risk factors that can predicted from laboratory data, and the use of these
factors in IVF clinics. Despite 40 years of experience with ovarian stimulation,
we still suffer from multiple births, with all their inconvenience, clinical risks
and social issues.
He outlined various approaches to reduce the frequency of multiple pregnancies,
and he discussed the benefits of a voluntary limit on the number of replaced
embryos, reduction in the degree of ovarian stimulation, and the benefits of
embryo cryopreservation. I still remain unconvinced about the conclusions drawn
from replacements of three or an elective two embryos. I agree with Walters
who stressed that the laws of statistics cannot be so lightly jettisoned, and that
three embryos are certain to provide more pregnancies (and multipregnancies)
unless there is another factor in the equation. This factor can only be embryo
selection, when two can be chosen from a wide group of embryos.
Fetal reduction was described by Jean as ethically questionable. I agree, and
fully support his stance on the enormous impact of triplets on the couple and
their family life, to say nothing of the other social costs. It is a despairing
approach, often resulting from the need in clinics to compete against neighbouring
clinics, which results in the replacement of five, six or more embryos. This step
places the unfortunate patient and the surgeon in the position of having to destroy
several of the fetuses they have themselves just established. What a sad comment
on the value of intrauterine human life! It is also immensely wasteful, for the
extra transferred embryos which produced reduced fetuses could have been
cryopreserved for a later transfer. They could have been cryopreserved as cleaving
embryos or blastocysts. I suggest that this attitude to replacing many embryos is
a matter for law, in the hope of banning the practice.
Despite our agreed attitude in this meeting to the risks and danger of multiple
pregnancies, attempts to restrict the numbers of embryos transferred seem to be
doomed to failure in many countries. Too many embryos will continue to be
replaced, in view of the demands of economics, high pregnancy rates, and the
best possible results with fresh embryos for league tables as argued by many
participants in the Discussion. One way of paying for it is to charge the clinics
for the cost of their multipregnancies, including the costs of fetal and maternal
morbidity and perhaps a compensation for the stress of a couple having to raise
multiple children. Hitting the profits seems to be a very good idea! There is
indeed only one excusable item in the whole process of multiple embryos and
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this is identical twinning, which has so far proved to be beyond man's wit
to control.
Gene insertion into uterine cells
We may be doubly fortunate in that the improvements in embryo quality promised
in the papers reviewed above come at a time when new approaches to the uterine
aspects of implantation are under intense investigation. Detailed analyses of the
roles of various cytokines including gene knock-out has revealed the importance
of leukaemia inhibitory factor (LIF). Steve Smith reminded us how trophectoderm
of mouse but not human blastocysts expresses LIF, which is also expressed for
a short time during the endometrium during implantation phase. Interleukin-6 is
also elevated during the implantation phase. Inner cell mass does not express
this cytokine in either species, but it does expresses LIF-Rb and gpl30 at the
blastocyst stage. Such evidence hints at the existence of redundancy mechanisms
at this stage of pregnancy. Knock-out of LIF-Rb does not prevent mouse females
from carrying their pregnancies to full term, despite the occurrence of placental
anomalies of gpl30 in mice results in embryo death in late gestation.
Steve Smith has widened the case of genetic technologies to study implantation
by using liposomes to transfect uterine tissues with the cytomegalovirus (CMV)
promoter. There was high incidence of transfection. This approach opens prospects
of short- or long-term control of uterine function, by transporting various kinds
of genetic information into recipient cells of the uterus. This team of investigators
is fully aware of the advantages and of the potential difficulties.
While the technique of gene insertion to regulate tissues is highly welcome as
a novel approach to the improvement of implantation, it could raise concern
about effects of accidental gene contamination on the oocyte or embryo. Genetic
engineering is highly criticized in most countries, and the risk of accidental
contamination by the use of intrauterine liposomes could well risk this approbation.
There is no control of where the liposomes could get to, since once injected into
the uterine cavity they could penetrate to the oviduct and ovary. They may even
get into circulation, for example as menstruation is shed and vessels re-grow
immediately. Would some incorporated DNA persist over many years, and exert
effects within the uterus or systemic system? I am sure that Steve Smith and his
team have considered these points, and as an experimental model, their work
holds an excellent promise of making new advances into studies of implantation.
It might even be possible one to transfect the trophectoderm cell line in early
embryos, so that this tissue expresses factors that activate responses in uterine
epithelium. Let us remember that the very low rates of pregnancy in women are
due to a failure at the first stage of implantation, i.e. attachment of the embryos
to the uterus (Edwards, 1997). Why the control of this stage should have become
so loosely controlled during human evolution is far from clear. Other mammals
do not seem to suffer from such low rates of implantation. Any evidence on
embryo-uterine interaction in humans is therefore most welcome, and genetic
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techniques are certain to offer excellent opportunities of gaining the required
knowledge.
It would, perhaps, be preferable to transfect uterine cells with mRNA rather
than DNA. There could be various reasons for this proposal. A short-term effect
is all that is needed to influence uterine epithelium for purposes of implantation.
The stimulus should be given at the appropriate time, perhaps even at embryo
transfer, and cover the period of embryo attachment. If a longer term transfection
of epithelium is needed, it may have to be given earlier in the cycle. A longterm action is not needed, so these is no requirement for a permanent transfection
with DNA. The RNA would have to be incorporated, and persist over a few
days in a functional state. These conditions could no doubt be easily established
using liposomes or modifying the RNA molecule. Epithelium is easily accessible
for such treatments. Moreover, it is the primary uterine tissue engaged in the
first stage of the implantation process, and the optimal tissue to transfect.
Implantation in vitro
New and welcome advances in understanding the first stage of human implantation
were presented by Carlos Simon. He has worked for some years on the role of
the interleukins in signalling between embryo and uterus. Over this time, he has
improved his methods of co-culture of embryos and uterine epithelium to a level
where fundamental studies on the comparative roles of blastocyst and uterine
epithelium can be assessed. He now believes that these compounds have two
roles: an initial mutual signalling and then the initiation of adhesion.
It is clear today that steroids initiate the actions of cytokines and many other
factors involved in the implantation process. It was most interesting to hear that
heparin-binding epidermal growth factor (EGF) may be among the initial markers
in uterine epithelium of impending implantation. Later, LIF is expressed and is
fundamental for implantation; its actions are impeded by agents such as interleukin
(IL)-R antagonist (IL-ra). This data implies that embryonic IL-1R initiates
responses in the epithelium. Using this basis for his studies, combined with his
novel methods of culture, he was able to expose uterine epithelium to growing
blastocysts, or to blastocyst-conditioned medium, which was found to contain
IL-la and (3. Many embryos released these interleukins into conditioned medium
when co-cultured with uterine epithelium which contained these interleukins and
IL-ra.
Here, then, is a possible assay of blastocysts, to divide them into producers
and non-producers. Producers stimulate uterine epithelium to produce integrin
(33; non-producers do not. Co-cultures with stroma were ineffective. He produced
further information by the judicious use of inhibitors combined with the
appreciation of inhibitors or blocking agents. As the uterine epithelium responded
uterine morphology became modified, with the production of more microvilli
and structures similar to pinopods.
This is a double plus, because it is essential that we clarify the origin,
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maintenance and function of pinopods. Their functions were discussed two years
ago, in the previous Bourn Hall meeting. Studies on pinopods go back many
years, to the work of Enders, Jones, Leroy and of course Alex Psychoyos (Nikas
et ah, 1995). The general consensus is that they evacuate uterine fluid, and so
reduce the volume of the uterine cavity so that the blastocyst becomes tightly
trapped and adherent to the epithelium. This enables chemical binding to occur
between them, so pinopods actually function among the first part of the first
stage of implantation. Successive stages involve a permanent binding and the
initial movement of trophectoderm between the epithelial cells.
Integrin expression on uterine epithelium
We were highly fortunate to listen to Bruce Lessey immediately after Carlos
Simon. He also analyses fundamental relationships between embryo and uterus
during implantation. Between them, these two investigators enable fundamental
studies to be made on uterine epithelium including pinopods, the implanting
embryo, and cross-talk between them. This is exactly the sort of model we need.
He has concentrated for some years on the roles of integrins in implantation,
and has now moved a step further by applying his work to disease states in the
uterus and their effects on the implantation window. There may be sub-groups
of integrins, each with closely related but distinct functions. New forms of
integrins are being discovered. Steroid hormones do not seem to be involved in
integrin expression in certain cell types, whereas other integrins are regulated in
different ways, including some essential for implantation, and progesterone is
known to stimulate their expression of (Xj integrin.
The relationships between the different integrins and their pregestational
regulation and expression in epithelium has been clarified by Bruce Lessey's
studies. Epithelial cells lose progesterone receptors at this time, perhaps due to
rising levels of progesterone. If withdrawal of steroids is effective in permitting
the expression of integrins, there are two distinct phases of action in the secretory
phase. After down-regulation of epithelial steroid receptors, steroids can only
function through their actions on stromal receptors. Progesterone is thus ultimately
responsible for this shift and a polarized epithelial cell seems to be essential.
These steps may lead to the second phase of implantation. The integrins may
activate the metalloproteases, which would provide another link in the chain of
embryo/uterine interactions at this critical time in the establishment of pregnancy.
Unexplained infertility, endometriosis and hydrosalpinges could impair this
system of intercommunication, for example by releasing toxins or improper
regulatory factors such as inflammatory cytokines into the uterine cavity.
This is the kind of knowledge urgently needed on the uterus at implantation.
Details of the cytokine cascade must be clarified as the first stage of implantation
blends into successive stages of trophoblast penetration and invasion (Bishof and
Campana, 1996). There is clearly much emphasis on this field today, and the
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subject is becoming a discipline in its own right (Jauniaux et ah, 1997). All this
is most exciting and we will look forward to further developments in this field.
Recent analyses of glycodelins
Markku Seppala is one of our stalwarts, responsible over many years for
fundamental studies on the uterine proteins. He was responsible for much of the
early work of PP14. Now he and his colleagues have emerged with glycodelin,
with the accent on carbohydrates. They seem to have a role in cell-cell
communication.
He showed how endometrial and seminal plasma glycodelins may differ in
their sialic acid content. Both can inhibit sperm-egg binding, and the actions
between immune cells and their targets. Seminal levels of glycodelin correlate
with the percentage of motile spermatozoa, so it may be a marker of male
fertility. The synthesis of endometrial glycodelin A is progesterone-dependent,
glandular in origin and first detectable 5 days after ovulation. A potential role in
ovulation or some other physiological systems is clearly indicated by these
findings. An earlier appearance of relaxin in the uterus implies that this compound
may be a regulator of glycodelin production.
Another novelty in this field lies in the use of glycodelins as contraceptives.
Aberrant seminal forms may impair sperm-zona binding. Levels seem to be low
in most men, but they could be higher in infertile men. Uterine glycodelins could
exert a similar function. Their contraceptive action could be due to the inhibition
of fertilization in vivo during normal cycles. Markku Seppala and his colleagues
suggested this anti-spermatozoa property could be related to the restriction of
fertilization to intercourses occurring in the 6 days preceding ovulation, when
glycodelins are not being released from the uterine glands. Its later production
ends the fertile period, or the fertilization window. In other words, the uterus is
pro-conceptive until it become anti-conceptive soon after ovulation. This is a
novel idea, since most attention has been diverted to cervical meiosis as the
factor restricting the access of spermatozoa to the oviduct. We must clearly think
a little more widely about the nature of the fertile period and its manipulation
for contraceptive purposes.
I wish to close by once again thanking Peter Brinsden and the Bourn Hall
staff for their constant kindness and courtesy to us during our stay here. We are
again deeply indebted to Colin Howies and Serono for his enthusiasm in
supporting this meeting. Speakers and discussants have freely shared their results
and ideas. This fourth Bourn Hall meeting has lived fully up to its initial
traditions, with open discussion of many new concepts inside and outside the
debating chamber. And Howard, Jean and Lars have achieved their aim again,
to organize a small highly distinguished study group dedicated to the analysis of
some of the most recent advances in our field.
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References
Antczak, M. and van Blerkom, J. (1997) Oocyte influences on early development: the regulatory
proteins leptin and STAT3 are polarized in mouse and human oocytes and differentially
distributed within the cells of the preimplantation stage embryo. Mol. Hum. Reprod.,3,1067-1086.
Bischof, P. and Campana, A. (1996) A model for implantation of the human blastocyst and early
placentation. Hum. Reprod. Update, 2, 262-21Q.
Edwards, R.G. (1997) The pre-implantation and implanting human embryo. In Jauniaux, E.,
Barnea, E. and Edwards, R.G. (eds), Embryonic Medicine and Therapy. Oxford University Press,
Oxford, UK, pp.3-31.
Edwards, R.G. and Beard, H.K. (1997) Oocyte polarity and cell determination in early mammalian
development. Mol. Hum. Reprod., 3, 863-906.
Edwards, R.G., Mettler L.E. and Walters, D.E., (1986) Identical twins and in-vitro fertilization. J.
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