Assisted reproductive technologies in cattle: A review

Abstract

Over a period of approximately thirty years, commercial bovine embryo transfer has become a large international business. The technology is well established, and more than 500,000 embryos are produced annually from superovulated cows world wide. Since bovine embryos with intact zonae pellucidae can be specified pathogen-free through washing procedures, thousands of frozen embryos are routinely sold and transferred between countries. Throughout the world, approximately 15% of bovine embryos are produced by in vitro technology. Polymerase chain reaction technology is currently being used for sexing embryos on a small scale, and it is likely that this technology will be used for 'embryo diagnostics' in the future. Semen sexing is an established technology and is likely to be used on a small scale in the near future, especially in in vitro embryo production systems. The cloning of adult cattle through nuclear transfer and the production of cloned, transgenic cattle has been technically achieved. However, this is an expensive and inefficient technology, which is being used primarily by the pharmaceutical industry. Benefits in agriculture are likely to be minimal in the near future.

Full text

Rev. sci. tech. Off. int. Epiz., 2005, 24 (1), ... - ...
Assisted reproductive technologies in
cattle: a review
R.J. Mapletoft (1) & J.F. Hasler (2)
(1) Western College of Veterinary Medicine, University of
Saskatchewan, Saskatoon, SK S7N 5B4, Canada
(2) AB Technology, Bioniche Animal Health USA, Inc., Pullman,
WA 99163, United States of America
Summary
Over a period of approximately thirty years, commercial bovine
embryo transfer has become a large international business. The
technology is well established, and more than 500,000 embryos are
produced annually from superovulated cows world wide. Since bovine
embryos with intact zona pellucidae can be specified pathogen-free
through washing procedures, thousands of frozen embryos are
routinely sold and transferred between countries. Throughout the
world, approximately 15% of bovine embryos are produced by in vitro
technology. Polymerase chain reaction technology is currently being
used for sexing embryos on a small scale, and it is likely that this
technology will be used for ‘embryo diagnostics’ in the future. Semen
sexing is an established technology and is likely to be used on a small
scale in the near future, especially in in vitro embryo production
systems. The cloning of adult cattle through nuclear transfer and the
production of cloned, transgenic cattle has been technically achieved.
However, this is an expensive and inefficient technology, which is
being used primarily by the pharmaceutical industry. Benefits in
agriculture are likely to be minimal in the near future.
Keywords
Assisted reproductive technologies – Bovine – Cloning – Embryo
diagnostics – Embryo sexing – Embryo transfer – In vitro technology
– Semen sexing.
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Introduction
For an historical perspective on assisted reproduction, the reader is
referred to a recent, comprehensive review of farm animal embryo
transfer and its associated technologies (4). The period before 1970
will not be covered in any great detail in this paper, and reviews will
be referenced as often as possible to conserve space. In brief, the first
successful transfer of mammalian embryos was performed by Walter
Heape in 1890. Heape transferred two four-cell Angora rabbit
embryos into an inseminated Belgian doe, which subsequently gave
birth to four Belgian and two Angora young (3, 4, 30). There appear to
be no reports of further success in mammalian embryo transfer until
the 1920s, when several investigators again described embryo transfer
in rabbits (3). Warwick and colleagues did considerable work on
embryo transfer in sheep and goats in the 1930s and 1940s (4), but it
was Umbaugh (73) who reported on the first successful embryo
transfers in cattle in 1949. He produced four pregnancies from the
transfer of cattle embryos, but all the recipients aborted before the
pregnancies reached full term. In 1951, the first embryo transfer calf
was born following the surgical transfer of an abattoir-derived day-5
embryo (82). However, it was Rowson and colleagues at Cambridge
who developed much of the technology that later found commercial
use (4).
The bovine embryo transfer industry as it is known today arose in
North America in the early 1970s (3, 4). Continental breeds of cattle
imported into Canada were very valuable and relatively scarce
because of international health and trade restrictions. Embryo transfer
offered a means by which their numbers could be multiplied rapidly.
For several years, the most common use of embryo transfer in animal
production programmes was the proliferation of so-called desirable
phenotypes. However, in 1987, Smith (59, 60) at the University of
Guelph introduced the concept of multiple ovulation and embryo
transfer (MOET). He showed how well-designed MOET programmes
could lead to increased selection intensity and reduced generation
intervals, resulting in improved genetic gains. The establishment of
nucleus herds and subjecting heifer offspring to ‘juvenile MOET’
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could result in genetic gains that approached twice those achieved
with traditional progeny test programmes. It is noteworthy that, prior
to the work of Smith, most embryo transfers conducted in Canada
occurred in beef cattle, whereas approximately 75% of the embryo
transfer work in Canada in 2002 involved dairy cattle. Approximately
65% of embryo transfer work in the United States of America (USA)
continues to involve beef cattle (70).
Embryo transfer is now commonly used to produce artificial
insemination (AI) sires from highly proven cows and bulls (10, 68).
Although technical costs would seem to preclude the use of embryo
transfer techniques for anything but seed-stock production at this time,
the commercial cattle industry can benefit by the use of bulls
produced through well-designed MOET programmes (15, 53). The
success of MOET programmes has now led to the use of this
technology to test AI sires genetically (40). Selected cows are
superstimulated and inseminated to highly proven bulls. Male
offspring are placed in waiting while female offspring are placed into
production. Bulls are then proven by production records from siblings
rather than offspring (61). With this approach, it is possible to test a
bull genetically in three-and-a-half years, as opposed to five-and-a-
half years using traditional progeny testing schemes. Although some
accuracy may have been sacrificed, the shorter generation intervals
result in a greater overall genetic gain. Although results supported the
theory, physiology was a limiting factor. Superovulatory results made
it difficult to produce the desired number of offspring for genetic
testing.
Although the applications (42) and techniques (43) associated with
bovine embryo transfer have previously been reviewed, a brief
historical perspective may be useful. Early investigators described
non-surgical embryo recovery techniques (52), but these were not very
successful, and so all embryo recoveries and transfers performed in
the early 1970s were conducted surgically. As a consequence, the first
commercial embryo transfer programmes relied on mid-ventral
surgical exposure of the uterus and ovaries with the donor under
general anaesthesia. This required surgical facilities and limited use of
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the technology in the dairy industry because the udder of dairy cows
hindered mid-ventral access to the reproductive tract. It was not until
the mid-1970s that non-surgical embryo recovery became sufficiently
developed to be used in commercial practice (17, 19, 50). In the early
1980s, non-surgical embryo transfer techniques (51, 86) were adopted,
allowing embryo transfer to be practised on the farm and making it
especially attractive to dairy farmers.
The embryo transfer industry grew rapidly in the late 1970s, both in
terms of the number of practitioners and the number of donors
flushed. Seidel (54) reported that, in 1979, more than 17,000
pregnancies resulting from the transfer of bovine embryos were
recorded in North America. More recently, Thibier (70) reported that,
in the year 2002, 538,312 bovine embryos were transferred world
wide, of which 52% were transferred after on-farm freezing and
thawing and 15% were produced by in vitro techniques. North
America has continued to be the centre of commercial embryo transfer
activity, with more than 42,000 donor cows superstimulated and more
than 190,000 embryos transferred (35% of all reported embryo
transfers in the world). However, commercial embryo transfer in
North America is static or declining. In South America, by
comparison, commercial embryo transfer is expanding, accounting for
22% of embryo transfers throughout the world in 2002. Europe and
Asia each reported about 17% of the total number of bovine embryo
transfers in 2002 (70).
The International Embryo Transfer Society (IETS) was founded in
1974, with 82 Charter Members, representing researchers, academics
and veterinary practitioners (69). However, with the founding of
regional embryo transfer organisations, a growing number of
commercial embryo transfer practitioners have discontinued
membership in the IETS in favour of their regional organisations. It is
also clear that a growing number of the IETS membership are basic
researchers representing government, industrial or academic
institutions, including human medicine (30). However, the IETS has
played a very important role in the dissemination of basic and applied
information, assisting in the rapid growth of the embryo transfer
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industry in the 1980s and 1990s. In particular, the Import/Export
Committee of the IETS, now referred to as the Health and Safety
Advisory Committee (HASAC), has been instrumental in gathering
and disseminating scientific information on the potential for disease
control through the use of bovine embryo transfer (69). The following
contributions are of note:
– the ‘round table’ meeting on disease control issues and embryo
transfer, organised by the IETS and the World Organisation for
Animal Health (OIE) in 1985 (46)
– the formulation of disease control procedures for the international
movement of embryos, as established in the OIE Terrestrial Animal
Health Code (47)
– the organisation of the International Embryo Movement
Symposium, sponsored by the IETS at the XXXIII World Veterinary
Congress in Montreal in 1987.
These landmark events, in addition to continued close collaboration
between the IETS and the OIE, have made the international movement
of cattle embryos possible. In this regard, the Manual of the IETS: a
procedural guide and general information for the use of embryo
transfer technology emphasizing sanitary precedures (64) has become
the reference source for disease control procedures used in export
protocols.
In 1982, the American Embryo Transfer Association was formed to
unite and organise the commercial embryo transfer industry in the
USA. In 1984, the Canadian Embryo Transfer Association was
formed. The objectives of both organisations include the following:
– to establish standards for performance and conduct
– to liaise with Federal agencies for both domestic and international
embryo transfer.
These associations also co-operate directly with breed associations,
producer groups and international groups, such as the IETS. Their
purpose is to establish standards of practice which ensure confidence
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in the use of embryo transfer technology for disease control, in the
USA and Canada and throughout the world. Their certification
programmes are vital in ensuring that embryo transfer practitioners are
technically and ethically competent to handle embryos for
international trade.
There has been no appreciable increase in the number of embryos
produced per superovulated donor over the past twenty years.
However, recognising the importance of follicle wave dynamics (1, 7)
and devising methods for the synchronisation of follicular wave
emergence (7, 8) have simplified the way in which superovulation is
achieved, resulting in increased embryo production per unit of time.
Donor cows are being superstimulated more frequently than in the
past, and more embryos are being produced per year with no change
in the actual superstimulation protocol. Applying similar procedures
to the recipients has made oestrus detection, and the need to wait for
animals to ‘come into heat’, unnecessary, facilitating the management
of commercial embryo transfer programmes (8).
Disease control
Several large studies have now shown that the bovine embryo does
not transmit infectious diseases. In fact, the Research Subcommittee
of HASAC, within the IETS, has categorised disease agents based on
the risk of transmission with a bovine embryo (66). Category 1
comprises diseases or disease agents for which sufficient evidence has
accrued to show that the risk of transmission is negligible, provided
that embryos are properly handled between collection and transfer.
Proper handling includes the following:
– inspection of the zona pellucida microscopically at a magnification
of at least 50× to ensure that it is intact and free of adherent material
– ten washes of the embryo with at least 100-fold dilution of each
wash
– on occasion, two trypsin treatments to dissociate viruses that tend
to stick to the zona pellucida.
Category 1 diseases include, as follows:
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– enzootic bovine leukosis
– foot and mouth disease (cattle)
– bluetongue (cattle)
– Brucella abortus (cattle)
– infectious bovine rhinotracheitis (trypsin treatment required)
– Aujeszky’s disease (pseudorabies) in swine (trypsin treatment
required)
– bovine spongiform encephalopathy.
Category 2, 3 and 4 diseases are those for which less research
information has been generated. However, it should be noted that none
of the infectious diseases studied has been transmitted by in vivo-
produced bovine embryos, provided embryo handling procedures were
followed correctly (58). Consequently, it has been suggested that
embryo transfer be used to salvage genetic material in the event of a
disease outbreak (85), which could be a useful alternative in
establishing disease-free herds.
Embryo export and import
The intercontinental transportation of live animals costs several
thousands of dollars, whereas an entire herd can be transported, in the
form of frozen embryos, for less than the price of a single plane fare.
Additional benefits of frozen embryos in comparison to live animals
include, as follows:
– a reduced risk of disease transmission
– reduced quarantine costs
– the ability to select animals from a wider genetic base
– the ability to retain the genes of the selected animals within the
exporting country
– the ability of the animals to adapt.
Adaptation is particularly important in tropical and subtropical
environments, where the resulting calf would have the opportunity to
adapt first while in the uterus and then while suckling a recipient cow
indigenous to the area. However, the reduced risk of infectious disease
transmission is the overwhelming benefit of using embryos in
international trade.
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In 1961, the successful long-distance transportation of sheep embryos
in the oviducts of rabbits was reported (30). Although there are no
published records of cattle embryos being transported in a similar
way, the advent of reliable cryopreservation techniques has aided the
movement of cattle embryos across international borders. Over the last
ten years, embryo import regulations for many countries have been
simplified to such a degree that embryo exporters are now able to
operate in a predictable and routine manner. In 2002, approximately
30,000 embryos were frozen in North America for export. Obviously,
the growth of embryo exports is closely linked to the existence of
realistic health regulations in the importing countries. However,
changes in these regulations are often unpredictable, especially when
relatively new disease problems arise.
Although handling procedures recommended by the IETS make it
possible to safely export in vivo-derived embryos originating from
donors which are sero-positive to certain pathogens (63), the case is
very different for embryos produced with in vitro techniques (44). The
structure of the zona pellucida of in vitro-produced (IVP) bovine
embryos differs from that of in vivo-derived embryos (79). In a recent
review, it was shown that a number of pathogens are more likely to
remain associated with in vitro-derived embryos following washing
than with in vivo-derived embryos (65). This has potentially serious
ramifications for the international movement of IVP embryos.
Serological testing for the microbes in question could be performed on
donor cows that produce oocytes through transvaginal ultrasound-
guided ovum pick-up (OPU). However, there may be a serious health
risk when oocytes are recovered from ovaries derived from abattoirs
(44).
Cryopreservation: direct transfer of frozen/thawed
embryos and vitrification
The development of effective methods of freezing embryos (38, 83)
has made embryo transfer a much more efficient technology, which no
longer depends on the immediate availability of suitable recipients.
Freezing bovine embryos is now common and pregnancy rates are
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only slightly less than those achieved with fresh embryos (39).
Recently, the use of highly permeating cryoprotectants, such as
ethylene glycol, has allowed the direct transfer of bovine embryos (32,
75). In this approach, the embryo straw is thawed in a water bath,
much like semen, and its contents are deposited directly into the uterus
of the recipient, as occurs in AI. There is no need for a microscope or
complicated dilution procedures. The cryoprotectant leaves the
embryo in the uterus, without causing osmotic stress. In a recent study
of the North American embryo transfer industry, pregnancy rates from
direct-transfer embryos were comparable to those achieved with
glycerol (39). During 2002, more than half the embryos collected in
North America were frozen, and most were frozen in ethylene glycol
for direct transfer (70). Although the level of skill required to transfer
these embryos is the same as that needed for conventionally frozen
embryos, no embryologist is required at the time of thawing.
Consequently, a growing number of direct-transfer embryos are now
being transferred by technicians with experience in AI.
Freezing and thawing procedures are time-consuming and require the
use of biological freezers and a microscope. Complicated embryo
freezing procedures may soon be replaced by a relatively simple
procedure called vitrification (48). With vitrification, high
concentrations of cryoprotectants are used and the embryo in its
cryoprotectant solution is placed directly into liquid nitrogen. As a
result of the high concentration of cryoprotectants and the ultra-rapid
method of freezing, ice crystals do not form; instead the frozen
solution forms a ‘glass’. Since ice crystal formation is one of the most
damaging processes in freezing, vitrification has much to offer in the
cryopreservation of oocytes and IVP embryos. However, its greatest
advantage is its simplicity. Vitrification is now widely used
experimentally and recent results suggest that bovine embryos can be
vitrified in 0.25 ml straws for direct transfer (76).
Embryo production in vitro
Although each ovary contains hundreds of thousands of oocytes at
birth, most are lost through atresia. This process starts even before
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birth. This tremendous loss of genetic material could be reduced by
harvesting oocytes from the ovary and using IVP techniques (12, 29).
Bovine IVP is now a well-established and reasonably efficient
procedure. Moreover, OPU at frequent intervals, in combination with
in vitro fertilisation, has proved its worth in improving or increasing
the yield of embryos from designated donors. In addition, IVP can be
used to salvage irreplaceable genetic material following slaughter for
infectious disease control or culling for other reasons (30). In vitro
fertilisation has also been used to produce the thousands of embryos
needed for scientific research (26), including efforts to produce
embryonic stem cells. The constituent oocyte maturation and embryo
culture techniques also are integral parts of the procedures for cloning
by somatic cell nuclear transfer and generating transgenic cattle which
produce valuable pharmaceutical proteins in their milk (45). In vitro
fertilisation by intracytoplasmic sperm injection, so prominent in
assisted human reproduction, is feasible in cattle, even with freeze-
dried sperm (36), but not yet widely applied.
A few laboratories have reported very modest successes in producing
pregnancies from IVP of embryos from calves (18, 22, 67), which
offers the potential for increased genetic gain by decreasing generation
intervals (5). In addition, OPU has proven to be safe and very
successful in pregnant cattle and is often used when there is high
demand for offspring from a particular donor cow, or MOET
programmes require additional offspring. Oocytes with good viability
have been collected once or twice weekly, or after pre-treatment with
follicle-stimulating hormone, as late into gestation as 90 to 150 days,
with very few abortions (23, 25).
Several authors have directly addressed the question of using IVP as a
substitute for in vivo embryo production by conventional embryo
transfer procedures (11, 29, 57). It is clear that pregnancies can be
produced by IVP from donor females that are infertile both to AI and
conventional embryo transfer technology (20, 31, 41). However, it is
unclear whether IVP is a realistic alternative to conventional
superovulation and embryo transfer for producing embryos from
reproductively healthy cattle. Data for 2002 show that, on a worldwide
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basis, more than 80,000 IVP embryos (both fresh and frozen) were
transferred (70). This is nearly double the number reported for 2001,
but is accounted for, almost entirely, by the increase of activity in
Brazil.
One commercial embryo transfer unit in North America has provided
data comparing the efficacy of conventional embryo transfer to that of
IVP in cattle (11). Success rates for IVP (4.7 embryos per OPU
session, 48% blastocysts from oocytes recovered) greatly exceeded
the published results of other commercial programmes. The authors
directly compared the results of IVP and the conventional in vivo
programmes and concluded that IVP would produce about 3.4 times
more embryos and 3.2 more pregnancies in a 60-day period, assuming
only one superovulation per donor. This rate is somewhat higher than
that reported by other commercial embryo transfer practitioners (see
above). At present, under commercial conditions in North America, it
appears to be more expensive to produce pregnancies by IVP than
with conventional superovulation and embryo transfer. For most
breeders, this technology is an advantage only for extremely valuable
cows which are infertile or fail to produce embryos after
superstimulation.
Prenatal determination of sex
Determining the sex of bovine embryos before implantation, using
polymerase chain reaction (PCR), is a service offered by a moderate
number of embryo transfer businesses (71). However, removing the
biopsy from the embryo requires a high level of operator skill, and
embryo biopsy is an invasive technique that results in disruption of the
integrity of the zona pellucida and some reduction in the viability of
the embryo. Both this procedure and a successful PCR programme
also require a higher level of hygiene and care than is often practised
with “on farm” embryo transfer. Although a modest number of
livestock breeders readily accept embryo sexing, it is not a technology
that has found widespread use in the embryo transfer industry. During
2002, almost 3,800 sexed embryos were transferred in Canada, one-
third of them after freezing and thawing (70).
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In the near future, PCR assays to identify other traits of economic
importance will no doubt become available (6). The extent of the
market for this technology will depend on the value of the genes in
question to cattle breeders. Marker-assisted selection (MAS), based on
identifying genetic markers for unknown alleles of valuable traits,
probably has a similar future (24). Like genotyping of specific alleles,
MAS can potentially be applied to embryo biopsies if sufficiently
valuable markers can be identified. A PCR assay currently exists for
simultaneous detection of the bovine leucocyte adhesion deficiency
gene and the sex of embryo biopsies (30). It is probable that PCR
techniques will be developed that permit the analysis of a large
number of markers from one biopsy simultaneously leading to the
concept of “embryo diagnostics”.
The flow cytometric technology used to separate X- and Y-bearing
sperm into live fractions has been improved over the last ten years
(34, 35). Approximately 10 million live sperm of each sex can be
sorted per hour (55), with a resulting purity rate of 90%. In AI field
trials involving approximately 1,000 heifers, pregnancy rates
following insemination with 1 million sexed, frozen sperm were
reported to be 70% to 90% that of unsexed controls inseminated with
20 to 40 million sperm (56). A recent study which compared 574
calves produced from sex-sorted sperm with 385 control calves
concluded that there were no differences in gestation, neonatal deaths,
ease of calving, birth weight or survival rate to weaning (72). The
disadvantages of flow cytometry are the slow speed of sorting, the
decreased sperm viability (pregnancy rates), especially in
superovulated donor cows, the cost of the semen, and the availability
of semen from specific bulls (2). It is likely that sexed semen will
have the greatest use in IVP of bovine embryos in the near future.
Production of identical offspring
Embryos can be split before transfer to produce identical twins.
Pregnancy rates of 50% or more per demi-embryo have been reported,
resulting in a net pregnancy rate of more than 100% per original
bovine embryo (27). Cloning can also be used to produce identical
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offspring (9). Embryonic cloning was first reported by Willadsen (81).
He showed that the developmental programme for eight- and sixteen-
cell nuclei could be reset back to fertilisation by the oocyte cytoplasm.
The births of lambs cloned from cultured embryonic cells in 1996
(14), and of ‘Dolly’, cloned from a mammary cell taken from an adult
ewe (84), have resulted in a great deal of research on cloning in a
number of mammals. The results have contradicted two long-held
models in developmental biology, as follows:
a) that differentiated embryonic cells are irreversibly modified
b) that cells (in this case, somatic cells) from adult mammals could
not be re-programmed to develop into embryos.
Following the success of cloning in sheep, bovine clones have also
been produced, using the following cell types (reviewed in 30):
– foetal fibroblasts
– oviductal and cumulus cells
– granulosa cells
– skin fibroblast cells
– muscle cells.
Adult somatic cell nuclear transfer also has been used to preserve the
last surviving cow in a rare breed (80), and fibroblasts from a 21-year-
old bull were successfully used to produce a cloned calf (33). Progress
has been hindered by very poor rates of cloning efficiency, low
pregnancy rates, high abortion rates and poor calf survival (21, 37, 77,
87). Consequently, the use of this technology for multiplying elite
cattle on a large scale depends on improving the efficiency of the
procedures (28). However, it has been reported recently that more than
2,600 cloned cattle embryos have been transferred into recipients in
one programme in the USA (20). Moreover, very recently, it was
reported that it was possible to clone cattle with nothing more than
regular embryo transfer equipment (74).
Transgenics
The use of cultured somatic cells to produce clones allows workers to
genetically modify the cells through gene transfer (16). Unfortunately,
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very poor rates of cloning efficiency, low pregnancy rates, high
abortion rates and poor calf survival are common. Transfection has
largely replaced the inefficient technique of pro-nuclear micro-
injection, which was used during the early years of transgenic animal
production. Transfection has proved very successful in producing
transgenic cells with relatively short deoxyribonucleic acid (DNA)
sequences. However, longer DNA sequences, which incorporate large
and complex genes, have been successfully incorporated into human
artificial chromosomes, which were then introduced into bovine
fibroblasts and, ultimately, into bovine clones (49). This work
involved a number of steps, including the production of intermediate
foetuses, which were genetically tested and then used to produce the
desired cloned cattle. Robl et al. (49) reported that 21 calves carrying
the human artificial chromosomes were produced and that at least
some of these offspring produced human polyclonal antibodies.
Transgenic technology could also be used to produce clonal lines of
embryos that have been genetically modified (13, 45, 49, 62, 78) to:
– improve the efficiency of meat or milk production
– modify milk composition
– improve disease resistance.
However, the use of this technology to multiply elite or genetically
modified cattle on a large scale depends on major improvements in the
efficiency of the procedures. It is highly likely that cloned transgenic
embryos will be used by the pharmaceutical industry well before they
can be produced at a cost and with an efficiency that is acceptable to
the cattle industry. Thus, the availability of transgenic clones for the
cattle industry will probably be quite limited for some years to come.
Conclusion
Commercial embryo transfer in cattle has become a well-established
industry in many parts of the world, with more than 500,000 embryos
being transferred on an annual basis. Although this results in a very
small number of offspring, considering the total numbers of calves
born throughout the world each year, the impact is large because of
the quality of animals being produced. Multiple ovulation and embryo
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transfer are now being used for real genetic improvement, especially
in the dairy industry, and most semen used today comes from bulls
produced by embryo transfer. However, the real benefit of embryo
transfer is that in vivo-produced bovine embryos can be specified
pathogen-free through washing protocols, making this an ideal
procedure for disease control programmes or in the international
movement of animal genetic material. Techniques have improved over
the past thirty years so that frozen-thawed embryos can be transferred
to suitable recipients as easily and simply as in AI. In vitro embryo
production, and embryo and semen sexing are also successfully
performed, but time and cost limit their widespread use. Somatic cell
cloning and the production of transgenic, cloned embryos have also
been shown to be possible, but the high cost and inefficiency of these
procedures preclude their use in cattle improvement programmes at
this time. A combination of embryo transfer, using highly proven
cows inseminated with semen from highly proven bulls, and industry-
wide artificial insemination would appear to be the most likely use for
bovine embryo transfer in the near future.
Both French and Spanish summaries are in preparation and will
be presented here, please advise if you would like to see either, or
both, of these translations. It is not necessary.
Résumé français: titre
Résumé
Mots-clés
Resumen español: título
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Resumen
Palabras clave
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3. Betteridge K.J. (1981). – An historical look at embryo transfer.
J. Reprod. Fertil., 62 (1), 1-13.
4. Betteridge K.J. (2003). – A history of farm animal embryo
transfer and some associated techniques. Anim. Reprod. Sci., 79 (3-4),
203-244.
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