Reproduction in Domestic Animals

138 KM Mortonet al. 1999). Increases in both the number and in vitrodevelopmental competence of oocytes derived from veryyoung donors have been reported recently (Mortonet al. 2005c). This improvement has been attributed tothe hormone stimulation regime and an optimized IVPsystem, and has resulted in the birth of offspring fromIVP embryos derived from prepubertal lambs as youngas 3–4 weeks of age (Morton et al. 2004a, 2005d) andthe first pre-sexed JIVET offspring (Morton et al.2004a). The current status of JIVET and a number offactors affecting its efficiency including selection of thedonor, hormone stimulation prior to oocyte collectionas well as the developmental capabilities of prepubertaloocytes will be described.Donor SelectionThe ideal donor lamb will produce large numbers offollicles and developmentally competent oocytes inresponse to hormone stimulation. Prepubertal lambs,like their adult counterparts, display a wide variation inthe number of ovarian follicles (both pre- and posthormonestimulation) and in the number of oocytescollected (O’Brien et al. 1997a; Morton et al. 2005b; c,d). Approximately, 20% of lambs failed to respond tohormone stimulation (Ptak et al. 1999, 2003), which issimilar to adult animals (Cognie 1999). Oocyte developmentto the blastocyst stage is highly dependant onindividual lambs (Ptak et al. 2003; Morton et al. 2005c),with the number of embryos produced per lamb rangingfrom 0 to 55 (Morton et al. 2005b, c). Thus, selection oflambs which produce large numbers of follicles andoocytes with a high developmental potential becomesparamount.Various methods have been studied for selectinglambs although most have been inconclusive. Bodyweight prior to hormone stimulation and growth rateare not related to ovarian weight or the number ofovarian follicles after hormone stimulation (Mortonet al. 2005c). Donors have been selected on the basis ofovarian follicular status assessed by laparoscopy priorto hormone stimulation (Earl et al. 1995b); the numberof follicles observed was highly related (R = 0.98) tothe number post-hormone stimulation. Yet, this requirestwo laporscopic procedures within a fortnight and thereis no correlation between the number of ovarian folliclesand the in vitro developmental capabilities of theseoocytes. Ptak et al. (2003) observed similarities betweenfive sibling lambs derived from a lamb with oocytes of ahigh developmental capability, highlighting the role ofgenotype in oocyte developmental competence.Age of oocyte donorsArmstrong et al. (1997) suggest that the ideal age tocollect oocytes from prepubertal lambs is between 4 and6 weeks of age as this is the time of most follicularresponsiveness. Yet, in cattle oocytes obtained fromolder donors display an increased response to hormonestimulation (Armstrong et al. 1992, 1994), oocyte developmentin vitro (Presicce et al. 1995; Tervit et al. 1997)and pregnancy rate (Yang et al. 1997) when comparedwith oocytes from younger animals. Earl et al. (1995a)reported similar rates of in vitro development for oocytesfrom 8- to 9-week-hormone stimulated and 4-monthunstimulatedlambs and O’Brien et al. (1997a) reportedthat development of oocytes in vitro was higher from3- to 6-week-old hormone stimulated lambs than 16–24-week-old unstimulated lambs. Yet, Morton et al.(2005d) observed similar rates of oocyte developmentto the blastocyst stage for oocytes derived from 16- to24-week-unstimulated prepubertal lambs (29.0%) andadult ewes (39.3%) but was reduced for oocytes derivedfrom 3- to 6-week-hormone stimulated lambs (27.8%).These studies compared the development of oocytesfrom younger hormone stimulated lambs with olderunstimulated lambs, thereby precluding the identificationof the relative contributions of lamb age andhormone stimulation.Comparing hormone stimulated lambs of differingages (3–4 and 6–7 weeks), Morton et al. (2005b)observed that lamb age affected uterine weight, whileovarian weight, the number of ovarian follicles, thenumber of oocytes recovered or the proportion ofoocytes suitable for IVP were not affected. Yet, oocytedevelopment to the blastocyst stage was higher for 6–7 week (27.9%) than 3–4 week (20.4%) lambs demonstratingthe increase in oocyte development with donorage. Despite these results and those of Morton et al.(2005d), the greater follicular responsiveness and numberof oocytes obtained from younger lambs (4–6 weeksof age) outweights the increase in development whenoocytes are obtained from older lambs agreeing withArmstrong et al. (1997).In summary, a window of opportunity which existswhen lambs are aged between 3 and 6 weeks, to collectlarge numbers of oocytes which are capable of fulldevelopment in vitro and in vivo. Yet, the lack of efficientmethods for selecting oocyte donors lambs prevents fullexploitation of this opportunity and further research isrequired to develop an efficient method of selecting 3–6-week-old lambs for JIVET.Hormone Stimulation of DonorsHormone stimulation prior to oocyte collection is usedto increase the number of ovarian follicles and oocytesharvested (O’Brien et al. 1997a; Ledda et al. 1999;Morton et al. 2005b), collect oocytes at a synchronousstage of development (Armstrong et al. 1997) andincrease oocyte development in vitro (O’Brien et al.1997a; Morton et al. 2005b).While the precise mechanisms responsible for thisincrease in developmental competence are not known,the effects are not mediated solely through an increase infollicle size, but also by the activation of biosyntheticprocesses within the oocyte that are required forsuccessful embryo development (Armstrong 2001). Biochemicalchanges in the follicle cells and intrafollicularenvironment that would not have otherwise occurreduntil after puberty are also induced by hormonetreatment (O’Brien et al. 1997a), for example, increasingthe production of gonadotrophin receptors in granulosacells resulting in an enhanced ability of the follicle torespond to gonadotrophin stimulation, thereby improvingoocyte quality and function.Ó 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag