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I. Wilmut and D. I. Sales

Summary. Sheep embryos which were transferred to recipients in oestrus 3 days before or 3 days after the donors were unable to implant even when a synchronously transferred embryo was developing successfully in the same uterus. Embryos which had spent 3 days in an advanced recipient were transferred to a recipient synchronous with the original donor. Embryos first transferred on Day 3 were slightly accelerated in their development, but retained the ability to implant normally in the 2nd recipient. By contrast, embryos first transferred on Day 6 were markedly stimulated and less able to implant in the second recipient.

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I. Wilmut, D. I. Sales and C. J. Ashworth


Substantial prenatal mortality has been observed in all mammals studied, although there are significant differences between species in the extent and timing of the death. There are two reasons why it is important to define the causes of such loss. First, it is of fundamental interest to understand why prenatal loss continues to occur despite natural selection for efficient reproduction. Second, there may be practical applications arising from such knowledge if it can be used to increase the survival of embryos. Great distress is caused to those people who suffer infertility, particularly if it is a result of repeated miscarriage. Furthermore, a substantial economic loss follows prenatal death in farm animals. This death leads to a reduction in litter size in pigs and prolific sheep and, in cattle and sheep with only one ovulation at each oestrus, an increased interval between births.

The causes of prenatal mortality have been

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C. J. Ashworth, D. I. Sales and I. Wilmut

Summary. Plasma progesterone concentration and embryo survival were determined during successive pregnancies in ewes throughout one breeding season. The probability of an embryo surviving was associated with the progesterone concentration on the days around ovulation, with the timing of the increase from periovulatory to luteal values, and with the rate at which progesterone concentrations increased. Individual embryo survival decreased both as the number of corpora lutea increased, and towards the end of the breeding season; the latter effect could be explained entirely by differences in progesterone concentration. Considerable variation in progesterone secretion and in embryo survival was observed within the same ewes during successive pregnancies. Such variability in progesterone concentrations during early pregnancy may be a cause of some embryo mortality.

Keywords: embryo survival; fertility; progesterone; sheep; season

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I. Wilmut, C. J. Ashworth, A. J. Springbett and D. I. Sales

Summary. Embryos at different stages of development were transferred to recipient ewes on Day 6 to investigate the effect of variation in stage of development on embryo survival and growth. Three groups of ewes received 2 embryos that were at the same stage of development, Day 4, Day 6 or Day 8. A fourth group received 1 Day-4 and 1 Day-8 embryo. At autopsy on recipient Day 34 there were no significant differences in embryo survival (Day 4, 34%; Day 6, 50%; Day 8, 46%; and Day 4 and 8, 48%). Fetuses developing from Day-8 embryos were heavier than others (Day 4, 1·10 ± 0·06 g; Day 6, 1·15 ± 0·06 g; Day 8, 1·41 ± 0·08 g; P < 0·05). In Group 4 neither survival nor growth of embryos was significantly affected by the presence of an embryo at a different stage of development.

The ability of the uterus to stimulate development of a relatively retarded embryo is confirmed. Apparently the uterus has less effect in slowing the development of advanced embryos.

Keywords: embryo transfer; embryo survival; embryo growth; sheep