Studies in superovulated ewes of factors influencing the yield of fertilised ova and their capacity for development
Border Leicester x Scottish Blackface ewes were used in a series of experiments to investigate factors affecting the number of transferable ova produced in response to superovulation with follicle stimulating hormone or pregnant mare serum gonadotrophin. The effect of the method of oestrous synchronisation on ova production was studied and it was suggested that during the progesterone priming period the application of progestagens via intravaginal pessaries resulted in a higher ovulation rate than pure progesterone. There was also evidence that ewes primed with 40 as opposed to 30mg fluorogestone acetate produced ova of higher viability after transfer to recipient ewes. However, a sustained increase in plasma concentrations of progesterone, within physiological limits, did not improve ovulation rate or the development of 5-day old ova in an in vitro culture system. Failure to maintain adequate progesterone concentrations throughout the entire priming period leading to mistiming of superovulatory treatment in relation to follicular development was identified as a possible cause of reduced ovulation rates. Laparoscopic technique for intrauterine insemination and ovum recovery were used successfully in experimental and commercial pedigree ewes. Insemination close to the expected time of ovulation resulted in high fertilisation rates of ova. Delaying the insemination until after ovulation was expected to be complete improved the ovum recovery rate but resulted in an increased number of retarded and unfertilised ova. It was hypothesised that this was the result of oocyte ageing prior to fertilisation. Attempts to investigate the development of early stage (two-day old) ova in in vitro co-culture with oviductal cells demonstrated the unreliability of morphology as a guide to ovine ovum viability and the need for additional tests such as nuclear staining. Through the thesis the implications of the findings for application in commercial ovum transfer schemes are discussed.