This paper reviews the status of embryo transfer and the major technologies
applied to preimplantation of embryos in sheep. Embryo production from sup
erovulated ewes is hindered by an unpredictable response to hormonal treatm
ent. Progress in this area should be expected by an appropriated control of
follicular development with gonadotropin-releasing hormone (GnRH) agonist
or antagonist prior to gonadotrophin administration. Simple protocols for t
he cryopreservation of sheep embryos by vitrification are already available
and the development of frozen-thawed blastocysts to term is close to the f
resh ones. Further research is required to identify factors able to promote
the maturation in vitro of oocytes, namely those obtained from prepubertal
animals. Semen and embryo sexing procedures are available in cattle althou
gh much less attention was paid to their application to sheep. Among all th
e reproductive technologies, cloning with embryonic and foetal cells has pr
ogressed dramatically in sheep and nuclear transfer has been used to produc
e transgenic animals as an alternative to pronuclear injection. The product
ion of the first lamb cloned from a somatic cell opened new opportunities i
n animal breeding as well as exciting lines of basic research. The overall
conclusions are that, apart from superovulation, the application of in vitr
o technologies is likely to evolve rapidly and once applied, a great impact
on traditional and new animal productions should be expected. However, a b
etter understanding of the changes in gene expression, induced in embryos b
y different in vitro manipulation procedures, is necessary to prevent abnor
mal foetal development. (C) Inra Elsevier, Paris.