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Summary. When 25 mg progesterone/day were injected into ewes on Days 0–3 of the oestrous cycle, (i) the subsequent cycle was shortened by 4 days and (ii) on Day 6 such ewes provided an acceptable uterine environment for the survival of 10-day-old embryos. We suggest that exposure of the non-pregnant uterus to approximately 8 days of normal luteal concentrations of progesterone may be necessary to initiate luteolysis.
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Embryonic development will proceed normally in ewes ovariectomized 4 days after ovulation if the animals are treated daily with progesterone (Bindon, 1971). Oestrogens are required for implantation to proceed in rats and mice (Nalbandov, 1971) but this requirement has not been established for sheep. While oestrogens of ovarian origin have been excluded as being necessary for the establishment of ovine pregnancy, it is possible that oestrogen production by the adrenals could supply any requirements at this time (McLaren, 1973).
Adrenalectomized ewes can be mated and become pregnant (Harrison, Heap & Paterson, 1972) provided they are supplied with glucocorticoids and mineralocorticoids. Tilton, Hoffmann, Light & Buchanan (1971) report that adrenalectomy did not not reduce mating activity, fertilization rate or conception rate.
Reliable estimates of oestrogen production by the adrenal have been difficult to achieve because of the very low secretion rates. Baird (1968) found approximately 1 ng oestrone in each of
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Summary. The fate of embryos transferred asynchronously in the ewe was investigated when the functional life of the corpus luteum was prolonged by both hemi-hysterectomy and by the presence of a second synchronously transferred embryo. The development of asynchronously transferred embryos was assessed at progressively later stages after transfer. Prolongation of luteal function did not enable asynchronously transferred embryos to persist. Embryos from Day 4 donors were found to be retarded in their rate of development when placed in 'younger' Day 1 or 2 uteri and appeared unable to develop beyond the early blastocyst stage. Conversely, embryos from Day 4 donors placed in 'older' Day 6 or 7 uteri showed accelerated growth and development which was maintained until the uterus reached Day 12. Thereafter further growth of the asynchronously transferred embryos was retarded, although synchronously transferred embryos then entered the phase of rapid blastodermic vesicle elongation. Asynchronously transferred embryos disappeared from the uterus when the ewe entered pro-oestrus.
The experiments demonstrate the existence of an active relationship between the embryo and the maternal environment during mid-cycle and an apparent lack of association between embryo size, growth rate and physiological maturation.
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The normal length of pseudopregnancy in hamsters (9·2 days) is doubled (18 days) following hysterectomy. Transplanting either one hamster uterine horn, or pieces of endometrium to the cheek pouch of hysterectomized animals significantly reduces the length of induced pseudopregnancy from 18 to 13·5 days (Caldwell, Mazer & Wright, 1967). These authors also reported that transplanting rat uteri to the cheek pouch also reduced pseudopregnancy in hysterectomized hamsters (14·8 days), whereas the effects of transplanting rabbit uteri (15·5 days) were not as conclusive. Human endometrium had no apparent shortening influence on the pseudopregnancy cycle length (17·5 days).
These results suggested that the regulatory effect of the uterus on the life span of corpora lutea may not be species specific and the present study was initiated to investigate the influence of sheep endometrium and endometrial preparations on the
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Summary.
In order to investigate the influences of maternal and embryonic genotype on litter size, fertilized eggs were transferred, at a rate of five per recipient, from Clun Forest ewes to Romney Marsh, Suffolk and Finnish Landrace ewes, and also from Romney Marsh and Finnish Landrace ewes to Romney Marsh ewes. The numbers of natural ovulations (corpora lutea) were counted in recipient ewes at the time of transfer. All ewes which became pregnant were allowed to lamb.
Differences in conception rate and litter size at birth between the three breeds which received Clun Forest eggs were not significant, but, taken together, these differences resulted in the survival of significantly more of the transferred eggs when the recipient ewes were Finnish Landrace (63%) than when they were Romney Marsh (47%) or Suffolk (43%). In Romney Marsh ewes, the survival rate of Romney Marsh eggs tended to be poorer (33%) than that of alien Clun Forest (47%) or Finnish Landrace (44%) eggs but this effect was not significant.
Irrespective of the genotype of the eggs transferred, the mean size of litters from Romney Marsh ewes following transfer of five eggs was significantly greater than the potential litter size of this breed determined by natural ovulation rate.
It was concluded that ovulation rate was the major factor limiting fertility in the Romney Marsh breed. Differences do seem to exist between breeds of sheep in the numbers of offspring which the uterus is able to support, but, in the breeds studied, this control did not appear to limit litter size below levels which are desirable in practice.
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Summary.
Eggs were transferred surgically from fifty-three donor heifers to ninety-nine recipients which were allocated to different groups according to the degree of synchronization of oestrus between donor and recipient.
The pregnancy rate obtained was 0%, 30%, 52·2%, 91·1%, 56·5%, 40% and 20% where the degree of synchronization of the recipient in relation to the donor was −3, −2, −1, 0, +1, +2, +3 days, respectively. Storage of eggs for periods of up to 6 hr did not influence the pregnancy rate.
The high pregnancy rate obtained where synchronization was exact may have been due to the rejection of retarded or abnormal eggs before transfer.
It is concluded that the synchronization requirements for successful egg transfer in the cow are slightly more acute than in the sheep and should not vary by more than ±1 day.
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Summary.
Antisera to 14-day-old sheep embryos were raised in rabbits and used to detect antigens specific to pregnancy by immunofluorescent staining and haemagglutination. Non-specific antibodies were removed by repeated absorptions of the antisera with homogenates of liver and kidney from non-pregnant ewes. The pregnancy-specific antigens were detected using immunofluorescence in the embryo, myometrium, maternal blood and CL as early as Day 8. No fluorescence was detected in pituitary, hypothalamus, liver, kidney, skeletal muscle or endometrium of pregnant ewes, or in any tissues of non-pregnant ewes. Haemagglutination occurred when a 1:8 dilution of rabbit anti-sheep embryo sera was added to blood obtained from ewes between Days 6 and 50 of pregnancy, but not when added to blood from nonpregnant ewes, rams and wethers or from pregnant mares, sows and cows. The immunological activity was removed from the anti-sheep embryo sera by absorption with homogenates of 14-day-old sheep embryo or pregnant uterus, or erythrocytes from Day 14 pregnant ewes, confirming that the antigens were specific to pregnancy.
The presence of these antigens provides a basis for a haemagglutination test for pregnancy from Day 6 after mating and may be involved in the maternal recognition of pregnancy in the ewe.
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Summary.
The effects on embryo survival of procedures used in transferring eggs non-surgically were investigated in three experiments in ewes and heifers. In Exp. 1, two techniques for introducing eggs into the uterus through the cervix in heifers were compared; namely (i) deposition of the eggs high into the uterine horn or (ii) into the body of the uterus. Both methods were followed by inflation of the uterus with carbon dioxide. Out of a total of 34 heifers, only one became pregnant by the use of Method (i).
Non-surgical egg transfers early (Days 3 to 5) or later (Days 6 to 9) in the oestrous cycles of heifers were carried out in Exp. 2. Three transfer procedures were compared: (i) pipette transfer of an egg into the body of the uterus through the cervix (control), (ii) the control procedure performed under Fluothane anaesthesia, or (iii) followed by inflation of the uterus with carbon dioxide. With transfers carried out early in the cycle, pregnancies resulted in 1/10, 0/10 and 1/10 of the heifers in the control, carbon dioxide and Fluothane groups, respectively. With late transfers, 7/20, 1/10 and 8/20 heifers became pregnant in the respective treatment groups. This trend for pregnancy rate to be improved when late transfers were done in the control and Fluothane groups was significant only at the 10% level of probability when both groups were pooled. It was tentatively concluded, however, that non-surgical transfers of fertilized eggs to heifers may be best done during mid-cycle, after Day 6. Fluothane anaesthesia did not improve conception rate. Inflation of the uterus with carbon dioxide appeared to be deleterious when used at the mid-cycle stage in heifers.
In Exp. 3, it was found that inflation of the ewe's uterus with carbon dioxide or nitrogen following the surgical transfer of an egg did not affect the incidence of pregnancy. The introduction of 50 μl liquid Fluothane into the lumen of the uterus was embryotoxic.
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Summary.
A total of fifty-two heifers was used in an egg transfer experiment to study methods of inducing twinning in cattle.
Two groups of eleven recipients were made pregnant to determine the effect of either transferring two eggs to a single uterine horn or one egg to each. Seven animals in each group were slaughtered and the remaining four allowed to go to term.
Of the heifers into which two eggs were transferred to a single horn, 73% became pregnant and 45% had twins at slaughter or calving. In a single case, one of the eggs had undergone transuterine migration and developed in the opposite horn to that into which it was transferred. Of the eleven heifers receiving one egg in each uterine horn, 73% had twins and a pregnancy rate of 72% was obtained in this group. A much greater degree of embryonic loss was thus found in the slaughtered group which had received two eggs to a single uterine horn.
The number of cotyledons which developed was much greater in heifers with a bilateral twin pregnancy.
The experiments indicate that the development of twin embryos within a single uterine horn results in competition for nutrients and sometimes the loss of one or both embryos but, with bilateral transfers, a high percentage of normal twins is produced.
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Cytological examination of the oocytes of a number of mammalian species has indicated that they will resume meiosis upon liberation from the Graafian follicle into a simple culture medium (Chang, 1955; Edwards, 1962, 1965a, b). Evaluation of the physiological status of such oocytes after periods of maturation would best be tested by noting their ability to undergo fertilization, cleavage and, ultimately, to produce viable young. Recent experiments have attempted in-vitro fertilization of mammalian eggs matured in culture and, whilst this has apparently yielded some success with human material (Edwards, Bavister & Steptoe, 1969) and in the rabbit (Thibault & Gérard, 1970), the contrary has proved the case in the large domestic species. In many experiments involving cow and pig eggs, sperm penetration of the zona pellucida of artificially matured oocytes was never
