Assistierte Reproduktion beim Pferd

In the mare, the practical use of assisted reproduction techniques
has been very limited so far. Embryo transfer is used only in a few
countries to a low extent. The main reason for this situation is
that there is no practical method available for superovulation in
the mare. The majority of equine embryos are therefore collected
from spontaneously ovulating mares. Synchronisation of the oestrous
cycle of the donor and recipient mare before transfer is of major
importance. In synchronized recipients, ovulation should occur one
day before to three days after the donor mare's ovulation.
Ovariectomized, hormone-treated mares have also been used as embryo
recipients. In young, fertile mares embryo recovery rates up to 90
% have been reported. Embryo recovery rates for older, infertile
mares are only between 20 to 40 %. Embryo recovery is generally
performed nonsurgically by transvaginal flushing of the uterus.
Nonsurgical recovery of embryos is not possible before day 6 post
ovulation. Equine embryos are transferred by surgical flank
incision or by nonsurgical, transvaginal transfer. Surgical
transfer is mainly used in commercial embryo transfer programs with
pregnancy rates up to 80%. The advantage of nonsurgical transfer is
the noninvasive technique and the low costs, but results vary
tremendously. Cooled, transported embryos offer great advantages in
the management of transfer programs. Cooling for 24 to 30 hours
does not seem to impair the viability of embryos. Conventional
methods of cryopreservation or vitrification can be used for long
term storage of equine embryos. Smaller embryos withstand
cryopreservation better than large embryos. Micromanipulation of
equine embryos can be used to produce monozygotic twins and for
embryo sexing. The efficiency of embryo production by extracorporal
fertilization in the horse is very low so far. Equine oocytes can
be recovered by one of four methods: surgically, by transcutaneous
flank puncture, by transvaginal aspiration or by aspirating,
scraping or rupturing follicles of slaughterhouse ovaries.
Different culture media can be used for in vitro oocyte maturation.
After 30 hours of incubation in TCM 199, the maturation rate of
equine oocytes to metaphase II is up to 80 % . In vivo
fertilization of oocytes can be achieved by surgical transfer into
the oviduct or follicles of inseminated recipient mares. The
procedure of gamete intrafallopian tube transfer (GIFT) involves
transfer of oocytes and spermatozoa into the oviduct of a recipient
mare. Since only low sperm numbers are required, GIFT can be used
for subfertile stallions, frozen-thawed semen and sexed
spermatozoa. Conventional methods or vitrification can be applied
for the cryopreservation of equine oocytes. Maturation rates in
vitro are lower for frozen-thawed than for fresh oocytes but
results of in vitro fertilization rates are comparable. In
contrast, embryonic development rates of frozen-thawed oocytes seem
to be affected by cryopreservation. In vitro fertilization of
matured oocytes can be performed by co-incubation of oocytes and
spermatozoa or by assisted fertilization techniques such as
mechanical manipulation of the zona pellucida, zona drilling or
intracytoplasmic sperm injection (ICSI). Fertilization rates with
conventional methods are low (0-25 %), whereas fertilization rates
up to 52% are reported with ICSI. Viable offspring has been
produced with both techniques. Fertilized oocytes can be cultured
to the stage of morula or blastocyst in vivo or in vitro.
Co-culture with somatic cells seems to be important for embryonic
development in vitro. Live foals have already been produced using
in vitro cultured embryos, which developed from ICSI of in vitro
matured oocytes.