Reproduction in Domestic Animals

Reprod Dom Anim 43 (Suppl. 2), 137–143 (2008); doi: 10.1111/j.1439-0531.2008.01153.xISSN 0936-6768Developmental Capabilities of Embryos Produced In Vitro from Prepubertal LambOocytesKM MortonThe Camel Reproduction Centre, Dubai, United Arab EmiratesContentsBreeding from prepubertal females, known as juvenile in vitroembryo transfer (JIVET), reduces the generation interval andincreases the rate of genetic gain in animal breeding programs.While the birth of the first lambs from prepubertal ewesoccurred nearly 30 years ago; and there is considerable interestin the commercialization of this technology, its efficiencyremains too low. The advent of in vitro production (IVP) ofembryo resulted in the more widespread use of JIVET.Morphologic and metabolic differences coupled with reducedin vitro and in vivo development of oocytes derived fromprepubertal animals have been reported. Research has beenundertaken to optimize donor selection and hormone stimulationmethods in an attempt to reduce the variability andincrease the proportion of donors responding to hormonestimulation and increase oocyte developmental competence.Yet, this variation persists and the development of oocytes andembryos from prepubertal animals remains reduced whencompared with adults. Recent improvements to JIVET,resulting from a modified hormone stimulation regime, haveeliminated the failure of donors to respond to hormonestimulation, and increased both the number and developmentalcompetence of oocytes harvested from very young prepubertallambs. This increased efficiency has facilitated theincorporation of other reproductive technologies such assperm sexing with JIVET, resulting in the birth of lambs of apre-determined sex from prepubertal lambs. Increased rates ofgenetic gain in sheep breeding programs can be achieved bycombining sexed sperm with oocytes obtained from lambs asyoung as 3–4 weeks of age. Continued increases in theefficiency of JIVET resulting from further improvements tohormone stimulation regimes and an increased understandingof the differences between oocytes from adult and prepubertalanimals will result in the commercialization of this technology.IntroductionThe use of prepubertal ewes for breeding programsprovides an exciting opportunity for sheep producers toreduce the generation interval and thereby, increase therate of genetic gain in their flocks (Nicholas 1996;Armstrong et al. 1997). This can be performed using aform of assisted reproductive technology, known asjuvenile in vitro embryo transfer (JIVET), to produceoffspring after the transfer of in vitro produced (IVP)embryos derived from oocytes obtained from prepubertal(juvenile) animals.Traditional sheep multiple ovulation and embryotransfer (MOET) schemes, which use adult ewes, resultin a generation interval of at least 12 months. Using 8–12-week-old lambs in a JIVET scheme, reduces thegeneration interval to 7 months (van der Werf 2005).Using oocytes obtained from 3- to 4-week-old lambsfurther reduces this generation interval to 6 months, andoffers producers a substantial increase in the rate ofgenetic gain when compared with other reproductivetechnologies such as MOET. Further increases in therate of genetic gain (of approximately 5%) can beachieved by combining sexed sperm with JIVET, whichis most advantageous at the commercial level (van derWerf 2005).During the early post-natal period in sheep, there isremarkable growth of the reproductive tract. Ovarianweight increased to 6.8 and 10.8 times the weight at birthby 4 and 8 weeks of age, respectively, and the number ofgrowing ovarian follicles increased significantly by4 weeks of age (Kennedy et al. 1974). Both the numberof growing follicles and ovarian weight declining by 8–12 weeks of age remained constant until 33 weeks of age(Kennedy et al. 1974). Ovarian follicles from 4-week-oldlambs began to show signs of atresia whilst the numberof follicles displaying signs of advanced atresia werehigher from 8- to 10-week-old lambs (Tassell et al. 1978).These results suggest this post-natal flourish of folliculargrowth provides a ‘window of opportunity’ to collectlarge numbers of oocytes from non-atretic follicles whenlambs are aged between 3 and 8 weeks of age.Initial attempts to produce embryos from prepubertallambs were achieved using MOET and, while prepubertallambs were capable of responding to exogenousgonadotrophins (Tassell et al. 1978; Worthington andKennedy 1979), the results were highly variable and thesuperovulatory response was generally low. In addition,embryos which were recovered from prepubertal lambsdisplayed a reduced development during in vitro culture(Wright et al. 1976) and after transfer to recipients(Quirke and Hanrahan 1977; McMillan and McDonald1985) when compared with those obtained from adultdonors. Despite these difficulties, a lamb was successfullyborn after the transfer of embryos derived from 10-to 16-week-old Welsh Mountain ewe lambs (Trounsonet al. 1977). The development of IVP, combined withjuvenile oocytes resulted in the birth of a calf (Armstronget al. 1992) followed closely by the birth of lambs(Armstrong et al. 1994; Earl et al. 1994).The use of JIVET was considerably expanded by thedevelopment of IVP and increased knowledge about thedifferences between oocytes from prepubertal and adultanimals. Despite considerable research, the efficiency ofJIVET has remained below commercially acceptablelevels, and increases in efficiency have been hindered bythe high proportion of lambs which do not respond tohormone stimulation, the highly variable response tohormone stimulation, the low proportion of oocytes thatreach the blastocyst stage and the high rates of foetalloss and malformations observed after the transfer ofIVP embryos derived from prepubertal animals (PtakÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag