Maternal and embryonic factors associated with prenatal loss in mammals I. Wilmut, D. I. Sales and C. J. Ashworth A.F.R.C. Animal Breeding Research Organisation, Dryden Laboratory, Roslin, Midlothian EH25 9PS, U.K.

Introduction

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 under- stand 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, par- ticularly 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 the subject of extensive research and of several reviews. Hanly (1961) concluded that 'while numerous factors have been shown to influence control has eliminated it embryonic death ... there is no factor or combination of factors whose in a group or population of animals. This residual embryonic loss... would seem to be accounted for by a more universally active factor than any so far investigated.' In a speculative essay Bishop (1964) judged there to be several causes of prenatal loss, and suggested that much of the 'basal loss' characterized by Hanly (1961) was a result of the selective loss of genetically abnormal embryos. He pointed out that any such loss should be regarded as unavoidable, and indeed desirable, as it prevents the more frequent birth of abnormal offspring and, because the loss occurs early in gestation, it hastens the opportunity for a further mating in animals with only one embryo. During the past two decades, information has been collected in relation to the mechanisms that may cause prenatal loss. The purpose of this paper is to present a broad perspective in which the role of many different factors can be considered. To achieve this, information will be drawn from a number of species. It is not intended to imply that each factor is equally important in every species, for this is almost certainly not the case. Rather, the objective is to demonstrate that very many stages of reproduction may be subject to variation which prejudices prenatal survival and so to promote further studies on different aspects of the subject.

The incidence of prenatal mortality

The literature on prenatal mortality is confused by variation in terminology. This review is concerned with death at any stage between fertilization and birth. Detailed comparisons of loss in different species are difficult and may be misleading because the estimates have been obtained by a variety of methods and the species may not all have been in an optimum environment. Neverthe¬ less, several effects are clear: death occurs in all species studied, the extent of loss can be markedly

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access influenced by environmental factors, and overall the greatest mortality probably occurs in human. The pattern of loss in three species, sheep, mouse and human, is considered.

Prenatal loss in human

Successful pregnancy occurs in only 18-28% of menstrual cycles in women having intercourse frequently without using contraception (Short, 1984). A complete analysis of the extent of failure at different stages of pregnancy is not available, as there is no estimate of the proportion of eggs fertilized. New insight was obtained recently by monitoring hCG concentrations during 207 cycles in 82 women who wished to conceive (Edmonds, Lindsay, Miller, Williamson & Wood, 1982). A level of hCG judged to indicate the presence of an embryo was detected on Day 6-18 after ovulation in 60% of cycles, and the incidence of loss before this is not known. Only 38% of these pregnancies came to term to give an overall estimate of successful pregnancies occurring in 22-8% of cycles. Thus, 62% of pregnancies recognized by changes in hCG values did not come to term; in 57% of cases a pregnancy was never recognized clinically, and in the other 5% there was a spontaneous abortion. Similar results were obtained in a study of early pregnancy factor (Smart, Fraser, Roberts, Clancy & Cripps, 1982): this factor was detected in 67% of 21 cycles and 49% of conceptions came to term.

Prenatal loss in ewes

In mated ewes some 20-40% of ovulations are not represented by live births (Edey, 1979). Fertilization failure accounts for the loss of 5-10% of eggs and by far the greatest proportion of the remaining loss occurs before implantation during the first 3 weeks of gestation. This pattern of loss is similar to that observed in cattle (Ayalon, 1978), but a rather greater loss occurs in pigs (Hanly, 1961).

Prenatal loss in mice

Prenatal mortality in laboratory mice is influenced by a number of factors including genotype (Falconer, 1960; Bradford, 1969; Larsen & Generoso, 1984), age (Talbert & Krohn, 1966; Gosden, 1973) and density of population (see Sadleir, 1969). Under typical conditions it amounts to 20-25%, divided approximately equally between preimplantation and post-implantation mortality (see Bradford, 1969). The incidence of fertilization failure is 3-5%.

Factors associated with prenatal mortality

There are three routes leading to prenatal death, although some factors may act through more than one route: the embryo may be abnormal, the maternal environment may be unable to support normal development or there may be an inappropriate relationship between the embryo and the mother. In the last case, both the mother and conceptus are considered intrinsically normal.

Abnormal embryo Embryos may be abnormal because of inherited defects, errors at meiosis or fertilization or as a result of an environmental factor having a direct effect on the embryo. Evidence concerning the incidence of abnormality is in three forms. Inherited factors have been defined in laboratory and farm animals. Embryo transfer has been used to demonstrate that death is primarily associated with the embryo rather than the uterine environment of the donor female. Finally, cytogenetic analysis has been used to define the incidence of gross chromosomal anomalies.

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access Inherited factors include translocations and specific mutations that affect development. A reduction in prenatal survival has been associated with certain translocations in pigs (Akesson & Henricson, 1972) and cattle (e.g. Dyrendahl & Gustavsson, 1979). There may be species differences in the effects of translocations on fertility. Studies in New Zealand have failed to detect any reduction in fertility associated with several different translocations in rams (Bruère, 1975). A number of mutations affecting prenatal development have been described for mice. Embryos homozygous for the yellow alíele of the agouti locus die in the late blastocyst stage, although they do provoke a decidual reaction (Pedersen, 1974). By contrast, the recessive mutant gene 'siren' causes death late in pregnancy or early in the post-natal period (Schreiner & Hoornbeck, 1973). Similar mutations are probably present, but undefined, in other species. In sheep, greater prenatal loss is associated with the embryo in young females (Quirke & Hanrahan, 1977), in lactating ewes (Cognie, Hernandez-Barreto & Saumande, 1975) and in heat stressed animals (Alliston & Ulberg, 1961). Embryo survival was not restored to typical levels by transfer of embryos to a control female in any of these cases. The precise anomaly in the embryos is not known. Cytogenetic analysis has been carried out in many situations in which prenatal loss occurs, and in some cases an increased incidence of gross chromosomal anomaly has been associated with the loss. As the methods used detect gross changes, but not minor changes, in chromosome structure, they underestimate the incidence of anomaly. The incidence is strikingly high in spontaneous human abortuses (Hassold et al, 1980a) and this has led some to extrapolate from this observation and to judge that much of the earlier loss in human is a result ofsimilar errors (Short, 1979; Jacobs, 1984), and that most loss in farm animals also reflects the influence of genetic anomalies (Short, 1979). It is important to appreciate that these are extrapolations and to treat them with caution. Gross chromosomal abnormalities were present in 50—61-5% of spontaneous human abortuses of the 1st and 2nd trimester (Boue, Boue & Lazar, 1975; Hassold et al, 1980a). In some studies the incidence of abnormalities was higher in abortuses from earlier in pregnancy (Boué et al, 1975), but this was not always the case (Hassold et al, 1980a). Trisomy was present in almost half the cases and was by far the most common anomaly (Hassold et al, 1980a), although the frequency of the different errors was not consistent over a period of time (Morton, Hassold, Funkhouser, McKenna & Lew, 1982). It has been argued that the complementary monosomy occurs with equal frequency, but that such embryos die during very early pregnancy, contributing to premenstrual loss (e.g. Jacobs, 1984). An experimental model involving crosses between the laboratory mouse and the tobacco mouse (Mus poschiavinus) has provided direct evidence that monosomic conceptuses are lost earlier in pregnancy than are trisomie fetuses (review by Bomsel-Helmreich, 1974). A number of studies have been carried out in domestic species. In one particular study of various species, an analysis was made of 2 spontaneous abortions, 12 stillborn animals, 5 neonatal deaths and 14 congenitally abnormal offspring (Berepubo & Long, 1983): only 4 of these 33 cases had detectable chromosome anomalies (12%). Analysis in early pregnancy (Day 2-16) in sheep (Long & Williams, 1980), cattle (McFeely & Rajakoski, 1968; Gayerie de Abreu, Lamming & Shaw, 1984) and pigs (McFeely, 1967) revealed a frequency of anomalies of 14-6%, 7-5% and 10% respectively. By contrast, in two studies later in the preimplantation period no anomalies were found in sheep blastocysts (Long, 1977) and only 1-9% of blastocysts from superovulated cattle had gross chromosomal abnormalities (Hare et al, 1980). Death of chromosomally abnormal embryos earlier in the preimplantation period may account for this difference. Analysis of the embryos of laboratory animals also reveals a relatively low incidence of abnormality. In mice and rabbits about 5% of preimplantation blastocysts were abnormal (Gosden, 1973; Fechheimer & Beatty, 1974) while a greater incidence (13%) was observed at the first cleavage division in mice (Kaufman, 1973). On balance it seems probable that the incidence of gross chromosomal abnormality is greatest in human, perhaps corresponding to the greater loss in this species and that it is inappropriate to

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access extrapolate from human to other species. Even in humans, as observations have only been made later in pregnancy, the proportion of loss associated with chromosomal abnormality has not been determined. The cause of the difference between species is not clear. Studies of experimental animals such as rabbit have shown that the incidence of abnormalities, particularly of polyploids, is increased by ageing of female gametes (Shaver & Carr, 1967). In the absence of a defined period of oestrus, it has been suggested that ageing of gametes is more likely to occur in human than other species. However, the characteristic most significantly associated with frequency of chromosomal abnormality in human is maternal age and the most commonly occurring error is trisomy rather than polyploidy (Hassold, Jacobs, Kline, Stein & Warburton, 1980b). Older women are more likely to produce children with Down's syndrome (trisomy of chromosome 21), are more likely to suffer a spontaneous abortion (Jacobs, 1984) and have more embryos with chromosomal abnormalities, particularly trisomy (Fujimato, Sato, Yamagami & Arai, 1978). The cause and extent of chromosomal abnormalities particularly in early human embryos, requires further study.

Abnormal maternal environment

The maternal environment may be inadequate for the support a normal pregnancy either because the reproductive tract is inherently abnormal or as a result of an inappropriate hormone pattern. A striking reduction in prenatal survival has been observed in aged females and embryo transfer in mice and rabbits has shown this to be due largely to reduced uterine capability (Talbert & Krohn, 1966; Adams, 1970; Gosden, 1979). There is a reduced decidual response in aged mice (Finn, 1966) and it has been suggested that this reflects differences in vascular supply and collagen content. As the reduction in litter size occurs gradually, it seems probable that at least some of the change in the uterus occurs over a considerable portion of the life-span. One can speculate that even during the time of maximum litter size, an occasional conceptus is lost because of a local inadequacy of the uterine environment. The most direct evidence of maternal environment affecting prenatal survival has been obtained in mice selected for differences in reproductive performance (Falconer, 1960; Bradford, 1969). Prenatal loss was reduced from 19% to 10% without a reduction in number of ovulations by direct selection over 11 generations (Bradford, 1969). By contrast, it was increased to 43% by 26 gener¬ ations of selection for reduced litter size (Bradford, 1979). Embryo transfer between these popu¬ lations revealed that the greater change had been in the maternal environment, although the small differences in embryo development already present at the time of transfer should not be ignored (Moler, Donahue, Anderson & Bradford, 1981). The number of offspring in the small litter size line was restored to control levels by daily injection of progesterone during the early post-implantation period although blood progesterone concentrations in control line and small-litter size line did not differ at this stage of gestation (Michael, Geschwind, Bradford & Stabenfeldt, 1975). Analysis by cross-breeding revealed that the maternal genetic effects on prenatal loss were due to recessive alíeles (Bradford, 1979). These experiments show that in some circumstances the maternal environment is inappropriate for maximum embryo survival. Treatment with maternal hormone was able to restore post- implantation survival to normal levels. Conclusive evidence of a similar effect in women and farm animals is not available, although several observations are compatible with that interpretation. Inbreeding of pigs is associated with a reduction in litter size due more to changes in the maternal environment than to reduced viability of the embryos (Hill & Webb, 1982). Embryo transfer experiments in sheep have shown that normal embryo development depends upon a sequence of changes in uterine secretions (Wilmut & Sales, 1981) that are induced by a par¬ ticular pattern of ovarian steroids (Miller & Moore, 1976; Wilmut, Sales & Ashworth, 1985a, b). After transfer of embryos to recipients that were not in oestrus at the same time as the donors, the rate of embryo development was altered (Wilmut & Sales, 1981; Lawson, Parr & Cahill. 1983).

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access Fig. 1. Schematic representation of the effect on sheep embryo development of transfer to a recipient ewe not in oestrus at the same time as the donor (asynchronous transfer). (Reproduced from Wilmut et al, 1985a.)

Development was more rapid in recipients in oestrus before the donor, but slower in those in oestrus after the donor (Fig. 1). Over a period of time this has the effect of bringing the embryo to a stage more like a normal embryo for that particular uterine environment. However, as a result of an extreme degree of asynchrony, the embryos did not implant, and it seems that they had become abnormal (Wilmut & Sales, 1981). During the second week of pregnancy, changes in uterine secretions are apparently critical for embryo development. A hormone profile sufficient for the establishment of pregnancy and for embryo survival at levels similar to that in intact control ewes was defined by administration of steroids to ovari¬ ectomized ewes, before transfer of an embryo to assess uterine competence (Miller & Moore, 1976; Moore, 1985; Wilmut et al, 1985a, b). A sequence of 4 phases was found to be essential; a period of progesterone before mating, oestradiol equivalent to that which induces oestrus, a low peri¬ ovulatory value of progesterone and a higher value of progesterone typical of the luteal period in the oestrous cycle (Fig. 2). It also seemed that the time of changes in uterine function was governed by the time of the rise to luteal levels (Lawson & Cahill, 1983). When luteal levels were generated on the day of oestrus, pregnancy could follow transfer of embryos provided that the time of transfer was adjusted. Pregnancy was established after transfer of Day 10 embryos to Day-6 treated ewes, but not to Day-10 treated ewes. A significant association between progesterone profile after mating and embryo survival has been shown in ewes of a prolific breed maintained under normal husbandry conditions (Ashworth, Sales & Wilmut, 1984; Ashworth, 1985). Blood samples were collected three times per day to deter¬ mine the time of the preovulatory peak of LH. Detection of oestrus was carried out twice daily and at each time any oestrous females were mated by at least 2 rams of proven fertility. Plasma was col¬ lected twice daily for 16 days after mating to permit an analysis of the progesterone profile. The numbers of ovulations and fetuses were determined during laparotomy on Day 30, before abortion was induced by administration of prostaglandin. This series of observations was repeated after allowing at least one oestrous cycle without treatment. In this way 2 or 3 observations were made on the same ewes during the same breeding season. The hormone concentrations were determined Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access Luteal phase cone, of Priming luteal progesterone phase cone. of progesterone I

? Periovulatory progesterone

— -r -1-1-1- -1-1-1-1 -4 -3 -2 12 3 4 5 6 7 8 Days

Fig. 2. Diagrammatic representation of a hormone profile sufficient for the establishment of pregnancy with embryos transferred to the uterus of an ovariectomized ewe on Day 4 or 6 after oestrus (Day 0). in all ewes that suffered loss at any time in the season and in the ewes in which all embryos survived on three occasions: a total of 51 cycles in 18 ewes. Because ewes differed in the extent of embryo loss (P < 001) variation was analysed within-ewes. Progesterone concentration was significantly lower during the periovulatory periods (Day 0 (= onset of oestrus) and 1) which preceded prenatal loss (P < 005). No other aspect of the progesterone profile was significantly associated with survival. Loss was greater at the end of the season (March) than after mating in September, November or January (P < 0-05). In statistical terms, the seasonal effect could be accounted for by differences in progesterone concentration. As the number of ovulations increased the proportion of embryos lost also increased (P < 0-01) and this could in part be accounted for by differences in progesterone. The loss in relation to number of ovulations that was not attributed to differences in progesterone may reflect greater variation in embryo stage. The sources of the progesterone in the periovulatory period are not clearly defined. An increased production of progesterone by the ovary has been detected immediately before ovulation (Wheeler, Baird, Land & Scaramuzzi, 1975). It seems probable that similar amounts are produced by the larger follicles and the adrenal gland (Wheeler et al, 1975; Harrison & Heap, 1978). This is the first observation of an association between prenatal survival and any aspect of the progesterone profile in ewes maintained under normal conditions. There are a number of possible causes of the association. In view of the requirement for exogenous progesterone in the peri¬ ovulatory period in ovariectomized ewes (Wilmut et al, 1985a) it may be that uterine function was abnormal after a low periovulatory level in the present experiment. The association may reflect pro¬ duction of less progesterone by poorer follicles that also release abnormal oocytes. The seasonal changes parallel those observed in male fertility (Colas, Guérin, Claret & Solari, 1985). They may be a result of changes in the male, or there may be seasonal changes in hormone concentrations in the females that disrupt their fertility. Experiments are in hand to test these hypotheses by administration of progesterone to mated ewes. A similar association between periovulatory progesterone and conception ratio was observed in dairy cattle (Lee & Ax, 1984). The concen¬ tration of progesterone in milk was higher on Days 1-3 in those animals that become pregnant. The Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access results of these two experiments suggest that interesting relationships in early pregnancy remain to be investigated. In sheep, a number of environmental factors, including nutrition and stress, influence prenatal survival and affect progesterone concentration. Pregnancy depends upon a sequence of different concentrations of progesterone and loss may be caused by an excess of progesterone or an inadequate amount. During the periovulatory period an excess of progesterone would initiate changes in uterine function prematurely, while an inadequate amount during the luteal phase preju¬ dices survival (Wilmut et al, 1985a). The effects of nutrition and body condition on reproductive performance in sheep are complex and not well understood (reviewed by Gunn, 1983; Robinson, 1983). There is evidence of an intermediate optimum level of nutrition after mating and of an effect of level of food intake on peripheral progesterone concentration. Reduced embryo survival has been associated with feeding of sub-maintenance rations (Edey, 1966; Cumming, Blockey, Winfield, Parr & Williams, 1975) or of overfeeding (Cumming et al, 1975; Brien, Cumming & Baxter, 1977). However, these effects were not observed in some other studies (Edey, 1970a, b; Gunn, Doney & Rüssel, 1972; Parr, Cumming & Clarke, 1982; Parr & Williams, 1982). In the two studies in which three levels of feeding were contrasted, embryo survival was maximal in the ewes receiving maintenance rations rather than 25% or 200% of maintenance (Cumming et al, 1975; Williams & Cumming, 1982). There is an inverse relationship between progesterone level and food intake (Lamond, Gaddy & Kennedy, 1972; Brien, Cumming, Clarke & Cocks, 1981; Williams & Cumming, 1982). As the effect was shown in ovariectomized ewes receiving progesterone by injec¬ tion, it has been suggested that it reflects changes in catabolism due to differences in hepatic blood flow (Parr et al, 1982). Climatic stress intended to mimic that of rainfall caused a reduction in prenatal survival when applied after mating (Griffiths, Gunn & Doney, 1970). A subsequent study showed that the effect could be reproduced by daily injection of ACTH (Doney, Smith & Gunn, 1976). Increased pro¬ gesterone release by the adrenal gland (De Silva, Kaltenbach & Dunn, 1983) may be the cause of the reduction in survival because pregnancy is disrupted by an excess of progesterone early in preg¬ nancy (Lawson & Cahill, 1983). In the one study carried out to test this hypothesis, the stress applied did not affect embryo survival to Day 14-16 or progesterone concentration (Rhind, Doney, Gunn & Leslie, 1984). In view of the complex relationships between the effects of ewe, season, stress, nutrition, number of ovulations, embryo survival and progesterone production and clearance it is not sur¬ prising that associations between peripheral plasma concentrations of progesterone and embryo survival are not always apparent. Although there are fewer studies on the endocrine control of early pregnancy in cattle, there are indications that the mechanisms are similar to those in sheep. There are a number of observations on the relationships between prenatal survival, progesterone, season and nutrition that are compar¬ able to those described for sheep (reviewed by Wilmut et al, 1985b). An association of lower progesterone concentration with a failure to conceive has been observed in some studies (e.g. Erb, Garverick, Randel, Brown & Callahan, 1976; Thompson, Clekis, Kiser, Chen & Smith, 1980; Hansel, 1981; Maurer & Echternkamp, 1982; Bosu & Leslie, 1984; Lee & Ax, 1984) but not in others (Echternkamp & Maurer, 1983; Roche, Ireland, Boland & McGeady, 1985). Differences in progesterone concentration during the luteal phase have been related to a luteotrophic effect of the embryo stimulating production of the steroid (Thompson et al, 1980; Hansel, 1981) although the number of animals in each group was very small in the first study. Progesterone given to cattle during the luteal phase has caused a relatively consistent, but non-significant, improvement in con¬ ception ratio (see Sreenan & Diskin, 1983). As large numbers of animals are needed to test the effect of such treatment, none of the experiments carried out so far provides an adequate test. There is great variation in hormone profile in women and a number of different facets of this variation have been associated with some infertility in women (see Cooke et al, 1977). The con¬ centrations of hCG increased at an earlier stage of the cycle in some women who lost the embryo

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access than in those who delivered a child (Edmonds et al, 1982). The authors suggested that the embryos may have passed through a diapause. However, since the time of ovulation was not known in these patients, it may be that ovulation occurred at an unusual stage of the cycle, that the patients had unusual steroid hormone levels and as a result were less likely to conceive despite having a poten¬ tially viable embryo. In such circumstances hCG would be present at unusual times relative to menstruation. The details of these interrelationships require further study.

An inappropriate relationship Survival may be prejudiced by a failure of one of several aspects of the relationship between embryo and mother, despite the fact that both are normal. Embryos may die because they are not at the correct stage of development for the particular uterine environment. Loss may result from an inappropriate distribution of embryos within the uterus. Finally, the immunological response provoked by the embryo may be inadequate for full development. Embryo transfer between animals that were not in oestrus at the same time established that during early pregnancy embryo development depends upon a sequence of changes in the uterine secretions. Deviation from this relationship causes abnormal development and ultimately the death of the embryo (Dickmann & Noyes, 1960; Adams, 1971; Wilmut & Sales, 1981). Variation in the stage of embryo development within a polytocous female can be considerable. In a study with pigs to determine whether such variation within a female is sufficient to prejudice survival (D. I. Sales, I. Wilmut & C. J. Ashworth, unpublished), embryos were flushed from 19 donor sows on Days 4-8, after double ligatures had been placed on each uterine horn some 25 cm from the utero-tubal junction. The embryos were divided into groups by one person, according to a subjective assess¬ ment of their stage of development. The segment of each horn that had not been flushed was divided into two by placing two ligatures at the bifurcation of the horns and two further ligatures midway along the segment. Each group of embryos was then transferred into a region of the uterus of the donor female that had not been flushed. In the first experiment the sows were slaughtered before luteolysis could occur. The proportion of embryos surviving was 67%, 49%, 55% and 31% (s.e. of means 8%) in the four groups judged to be most advanced to least well developed respectively. The relatively more advanced embryos were more likely to survive (P < 0-05). These observations were extended in experiments in which hormones were given in attempts to ensure that luteal phase levels of progesterone were present until slaughter on Day 25-27. Neither of the treatments used was effective, suggesting that the surgical procedures had made the female abnormal. Variation in survival associated with differences in relative stage was observed in the first experiment. There are several possible explanations for the effect of stage of development. The least well developed embryos may be so because they died before the surgery and so the differences in development could be an effect of death rather than a cause. Alternatively, the least developed embryos may have died after the surgery because they were at a inappropriate stage of develop¬ ment. The possible sources of variation in embryo stage are discussed elsewhere (Wilmut et al, 1985a). These are the first studies of the effect of variation within a female and further experiments are needed to confirm the effect and define the underlying mechanisms. The existence of variation in embryo development in mice has been defined and it has been suggested that it may lead to prenatal loss. Analysis of inbred strains of mice has revealed the presence of a gene or a group of genes (Ped), linked to the MHC locus, that affects the time of first cleavage and the rate of subsequent divisions (Warner, Gollnick & Goldbard, 1984). A thorough investigation of the influence of the gene on prenatal survival has not been carried out, but breeding records suggest an association of the slow alíele with poor reproductive performance (L. Flaherty, cited by Warner et al, 1984). Two other studies led to other workers independently concluding that a slow cleavage rate was associated with greater post-implantation loss in mice. Gates (1965) contrasted the survival of slow

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access cleaving and fast-cleaving embryos after their transfer to the opposite horns of recipient mice. Cleavage rate did not influence the proportion of embryos able to implant, but significantly more of the slow embryos died after implantation. The effect was observed both if the embryos were divided into groups according to their stage immediately after recovery on Day 3-5, and when the embryos were all cultured from the 2-cell to the blastocyst stage before being divided into groups according to the interval required for them to complete this development. In the latter case the embryos were at a similar stage when transferred, implying that the death of slow embryos may reflect an influence of cleavage rate after implantation rather than stage of development at the time of transfer. Maternal genotype apparently influences the incidence of post-implantation death in mice being used to study dominant-lethal effects (Larsen & Generoso, 1984). This higher incidence was associated with slow development during culture and with embryos being less well developed 48 h after insemination. Together these observations provide a strong indication that variation in embryo stage causes prenatal loss in mice, and there is no reason to imagine that the effect is species specific. However, the definitive experiments to demonstrate the effect have not been carried out in any species and there are alternative explanations for all of the observations described. There is a limit to the uterine capacity of every female and it would be generally accepted that 'crowding' causes prenatal loss. However, there may be other more subtle aspects to the relation¬ ship between embryo and uterus. Several studies of sheep suggest that survival is maximal if ovu¬ lation has occurred from both ovaries (Doney, Gunn & Smith, 1973; Meyer, 1985). There appears to be a small price to pay for being in a uterine horn contralateral to the corpora lutea, but a greater penalty if embryos are crowded into one horn (Doney et al, 1973). The mechanism underlying this effect is not known. The immunological means by which the fetus, an allograft, survives in the mother is not known. However, the current models describe a need for an immune response, but not such as to destroy the fetus (Redman, Sargent & Sutton, 1984). There may be circumstances in which this mechanism fails, either because the response is inadequate or because it destroys the fetus. In some studies, but not all, recurrent abortion in women has been associated with histocompatibility (Redman et al, 1984). Similarly, in another study in a population who did not practise contraception, the interval between births was greater in women who shared HLA antigens with their husbands. In Hutterite women sharing 0, 1 or more than one HLA antigen with their partners, the interval between marriage and the birth of their fifth child, for example, was 6-98, 717 and 7-42 years respectively (Ober et al, 1983). The difference became more marked at each parity recorded, up to 10 children. Further evidence of an effect of histocompatibility was obtained when the frequency of successful pregnancy in women with a history of repeated miscarriage was increased after immunization with their partner's lymphocytes (Mowbray et al, 1985). There are other explanations for this effect, which requires confirmation and further study, but if an inappropriate immunological response causes loss in one species it may well do so in others.

General conclusions

The physiological research carried out during the past two decades has established associations between several factors and prenatal mortality. However, it is not yet possible to provide a complete analysis for any species of the proportion of loss that is associated with the different factors. Prenatal death is associated with genetic anomalies as predicted by Bishop (1964). However, the most commonly cited evidence for this association concerns spontaneous human abortuses that constitute only a portion of prenatal deaths in that species. The only published observations on conceptuses early in pregnancy were made on embryos obtained after in-vitro fertilization (Angeli, Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access Immunological anomaly Site in uterus

Crowding Inappropriate stage

Lethals Errors in meiosis Errors in fertilization Teratogens Disease High temperature

(Disease and age)

Ewe ^ Number of ovulations Nutrition Temperature stress

Fig. 3. Factors associated with prenatal loss, particularly in sheep, shown according to the route by which they act. Those at the top of the diagram disrupt the relationship between embryo and mother, those on the right cause abnormal embryos and those at the bottom of the diagram act through the maternal environment (see text for references).

Aitken, Van Look, Lumsden & Templeton, 1983). Studies on early embryos of other species suggest a far smaller proportion of anomalies than are detected in human abortuses. Completion of a successful pregnancy depends upon a large number of mechanisms operating successfully. Evidence has been presented to suggest that many of them, perhaps all, fail during some pregnancies. This failure may reflect an influence of environmental factors or physiological variation. In future studies of the underlying physiology, it is important that the extent of variation in the systems be defined to provide estimates of the frequency of failure. There are a great number of environmental factors that influence reproduction (Bronson, 1985) and many of them influence prenatal survival (Fig. 3). The occurrence ofprenatal mortality despite selection for efficient reproduction may reflect the influence of two advantages. First, there are circumstances in which it is beneficial to lose a conceptus. Second, it may be the biological cost of the ability to respond to changes in the environment. Despite the superficial paradox, it may be that for many species maximum fitness is obtained, despite some loss of embryos, when the animal is able to modify its performance in response to changes in the environment. The change may be in reproduction, as, for example, an increase in number of ovulations or in metabolism with an indirect effect on reproduction. The change in progesterone catabolism as a result of nutritional differences is an example of the latter type of response. This analysis leads to the prediction that Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access prenatal loss would be lower in species that have a small number of chances to mate during a breeding season and in those species with a prolonged delay to implantation during an annual breeding cycle. The disadvantage of prenatal loss in such circumstances would be much greater than for other species. There are two opportunities to increase prenatal survival. Understanding of the physiological mechanisms underlying prenatal death may allow pharmaceutical intervention where appropriate, particularly in women. For farm animals, the costs and the practical difficulties may be prohibitive, except for dairy cattle. However, in these species there is the hope of improving survival by genetic means. Direct selection for greater prenatal survival is being carried out in pigs, and there is evidence in sheep of variation in prenatal survival between breeds (Hanrahan, 1985) and between females within a breed (C. J. Ashworth, D. I. Sales & I. Wilmut, unpublished). In the longer term, there is also the possibility of transfer of genes with major effects on development of farm animals.

C.J.A. is in receipt of a postgraduate scholarship from the A.F.R.C.

References

Adams, CE. (1970) Ageing and reproduction in the prospective epidemiological studies of 1500 karyo- female mammal with particular reference to the rab¬ typed spontaneous human abortions. Teratology 12, bit. J. Reprod. Fert., Suppl. 12, 1-16. 11-26. Adams, CE. (1971) The fate of fertilized eggs transferred Bradford, G.E. (1969) Genetic control of ovulation rate to the uterus or oviduct during advancing pseudo¬ and embryo survival in mice. 1. Response to selec¬ pregnancy in rabbits. J. Reprod. Fert. 26, 99-111. tion. Genetics, Princeton 61, 905-921. Akesson, A. & Henricson, B. (1972) Embryonic death Bradford, G.E. (1979) Genetic variation in prenatal in pigs caused by unbalanced karyotype. Ada vet. survival and litter size. J. Anim. Sci. 49, (Suppl. 2), scand. 13, 151-160. 66-74. Alliston, C.W. & Ulberg, L.C (1961) Early pregnancy Brien, F.D., Cumming, I.A. & Baxter, R.W. (1977) Effect loss in sheep at ambient temperatures of 70° and 90" F of feeding a lupin grain supplement on reproductive as determined by embryo transfer. J. Anim. Sci. 20, performance of maiden and mature ewes. J. agrie. 608-613. Sci., Camb. 9,9,431-443. Angell, R.R., Aitken, R.J., Van Look, P.F.A., Lumsden, Brien, F.D., Cumming, I.A., Clarke, I.J. & Cocks, OS. M.A. & Templeton, A.A. (1983) Chromosome abnor¬ (1981) Role of plasma progesterone concentrations in malities in human embryos after in vitro fertilisation. early pregnancy of the ewe. Aust. J. exp. Agrie. Anim. Nature, Lond. 303, 336-338. Husb. 21, 562-565. Ashworth, C.J. (1985) Maternal factors affecting early Bronson, F.H. (1985) Mammalian reproduction: an pregnancy in sheep. Ph.D. thesis, University of ecological perspective. Biol. Reprod. 32, 1-26. Edinburgh. Bruère, A.N. (1975) Further evidence of normal fertility Ashworth, C.J., Sales, D.I. & Wilmut, I. (1984) Patterns and the formation of balanced gametes in sheep with of progesterone secretion and embryonic survival one or more different Robertsonian translocations. J. during repeated pregnancies in Damline ewes. Proc. Reprod. Fert. 45, 323-331. 10th Int. Congr. Anim. Reprod. & A.L, Urbana 2, 74. Cognie, V., Hernandez-Barreto, M. & Saumande, J. Ayalon, N. (1978) A review of embryonic mortality in (1975) Low fertility in nursing ewes during the non- cattle. J. Reprod. Fert. 54,483^93. breeding season. Annls Biol. anim. Biochim. Biophys. Berepubo, N.A. & Long, S.E. (1983) A study of the re¬ 15,329-343. lationship between chromosome anomalies and repro¬ Colas, G., Guérin, Y., Claret, V. & Solari, . (1985) ductive wastage in domestic animals. Theriogenology Influence de la durée d'éclairement sur la production 20, 177-190. et la fècondance des spermatozoïdes chez le bélier Bishop, M. (1964) Paternal contribution to embryonic adulte Ile-de-France. Reprod. Nutr. Develop. 25, death. J. Reprod. Fert. 1, 383-396. 101-111. Bomsel-Helmreich, O. (1974) Experimental induction of Cooke, I.D., Lenton, E.A., Adams, M., Pearce, M.A., chromosome abnormalities. Proc. 1st World Cong. Fahmy, D. & Evans, C.R. (1977) Some clinical aspects Genetics applied to Livestock Production, Madrid 1, of pituitary-ovarian relationships in women with 201-213. ovulatory infertility. J. Reprod. Fert. 51, 203-213. Bosu, W.T.K. & Leslie, K.E. (1984) The effects of GNRH Cumming, I.A., Blockey, M.A. de ., \\ infield, CG., administration four days after insemination on first- Parr, R.A. & Williams, A.H. (1975) A study of service conception rates and corpus luteum function relationships of breed, time of mating, level of in dairy cows. Proc. 10th Int. Congr. Anim. Reprod. & nutrition, live weight, body condition and face cover A.I.,Urbana2,309. to embryo survival in ewes. J. agrie. Sci., Camb. 84, Boue, J. Boue, A. & Lazar, P. (1975) Retrospective and 559-565. Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access De Silva, M., Kaltenbach, C.C. & Dunn, T.G. (1983) implantation mouse embryos in relation to maternal Serum cortisol and progesterone after administration age. J. Reprod. Fert. 35, 351-354. of adrenocorticotropin and (or) prolactin to sheep. J. Gosden, R.G. (1979) Effects of age and parity on the Anim. Sci. SI, 1525-1529. breeding potential of mice with one or two ovaries. J. Dickmann, . & Noyes, R.W. (1960) The fate of ova Reprod. Fert. 57, 477-487. transferred into the uterus of the rat. J. Reprod. Fert. Griffiths, J.G., Gunn, R.G. & Doney, J.M. (1970) 1, 197-212. Fertility in Scottish Blackface ewes as influenced by Doney, J.M., Gunn, R.G. & Smith, W.F. (1973) Trans- climatic stress. J. agrie. Sci., Camb. 75,485-488. uterine migration and embryo survival in sheep. J. Gunn, R.G. (1983) The influence of nutrition on the Reprod. Fert. 34, 363-367. reproductive performance of ewes. In Sheep Pro¬ Doney, J.M., Smith, W.F. & Gunn, R.G. (1976) Effects of duction, 2nd edn, pp. 99-110. Ed. W. Haresign. post mating environmental stress or administration Butterworths, London. of ACTH on early embryo loss in sheep. J. agrie. Sci., Gunn, R.G., Doney, J.M. & Rüssel, A.J.F. (1972) Camb. 87,133-136. Embryo mortality in Scottish Blackface ewes as influ¬ Dyrendahl, I. & Gustavsson, I. (1979) Sexual functions, enced by body condition at mating and by post semen characteristics and fertility of bulls carrying mating nutrition. J. agrie. Sci., Camb. 79, 19-25. the 1/29 chromosome translocation. Hereditas 90, Hanly, S. (1961) Prenatal mortality in farm animals. J. 281-289. Reprod. Fert. 2, 182-194. Echternkamp, S.E. & Maurer, R.R. (1983) Conception, Hanrahan, J.P. (1985) Reproductive efficiency in sheep. embryonic development and corpus luteum function In Exploiting New Technologies in Animal Breeding: in beef cattle open for two consecutive breeding Genetic Developments. Eds C. Smith, J. W. B. King & seasons. Theriogenology 20,627-637. J. McKay. C.E.C. Brussels (in press). Edey, T.N. (1966) Nutritional stress and preimplantation Hansel, W. (1981) Plasma hormone concentrations embryonic mortality in Merino sheep. J. agrie. Sci., associated with early embryo mortality in heifers. /. Camb. 67, 287-302. Reprod. Fert., Suppl. 30, 231-239. Edey, T.N. (1970a) Nutritional stress and preimplan¬ Hare, W.C.D., Singh, E.L., Betteridge, K.J., Eaglesome, tation embryonic mortality in Merino sheep. 1966. J. M.D., Randall. G.C.B., Mitchell, D., Bilton, R.J. & agrie. Sci., Camb. 74, 187-192. Trounson, A.O. (1980) Chromosomal analysis of 159 Edey, T.N. (1970b) Nutritional stress and preimplan¬ bovine embryos collected 12-18 days after estrus. tation embryonic mortality in Merino sheep. 1967. J. Can. J. Genet. Cytol. 22, 615-626. agrie. Sci., Camb. 74, 193-198. Harrison, F.A. & Heap, R.B. (1978) Progesterone se¬ Edey, T.N. (1979) Embryo Mortality. In Sheep Breeding, cretion during pregnancy in sheep with an autotrans- pp. 315-325. Eds G. J. Tomes, D. E. Robertson, R. J. planted adrenal and an autotransplanted ovary. J. Lightfoot & W. Haresign. Butterworths, London. Reprod. Fert. 54, 153-157. Edmonds, D.K., Lindsay, K.S., Miller, J.F., Williamson, Hassold, T., Chen, N.J., Funkhouser, J., Jooss. T., E. & Wood, P.J. (1982) Early embryonic mortality in Manuel, B., Matsura, J., Matsuyarma, ., Wilson, C, women. Fert. Steril. 38, 447-453. Yamane, J.A. & Jacobs, P.A. (1980a) A cytogenetic Erb, R.E., Garverick, H.A., Randel, R.D., Brown, B.L. study of 1000 spontaneous abortions. Annls Hum. & (allanan. C.J. (1976) Profiles of reproductive Genet. 44, 151-178. hormones associated with fertile and non-fertile Hassold, T., Jacobs, P., Kline, J., Stein, Z. & Warburton, inseminations of dairy cows. Theriogenology 5, D. (1980b) Effect of maternal age on autosomal 227-242. trisomies. Annls hum. Genet. 44, 29-36. Falconer, D.S. (1960) The genetics of litter size in mice. J. Hill, W.G. & Webb, A.J. (1982) Genetics of reproduction cell. comp. Physiol. 56, (Suppl. 1), 153-167. in the pig. In The Control of Pig Reproduction, Fechheimer, N.S. & Beatty, R.A. (1974) Chromosomal pp. 561-564. Eds D. J. A. Cole & G. R. Foxcroft. abnormalities and sex ratio in rabbit blastocysts. J. Butterworths, London. Reprod. Fert. 37, 331-341. Jacobs, P.A. (1984) Pregnancy losses and birth defects. Finn, CA. (1966) The initiation of the decidual cell reac¬ In Reproduction in Mammals: Vol. 2. Embryonic and tion in the uterus of the aged mouse. J. Reprod. Fert. Fetal Development, pp. 142-158. Eds C. R. Austin & 11,423-428. R. V. Short. Cambridge University Press. Fujimato, S., Sato, C, Yamagami, Y. & Arai, E. (1978) Kaufman, M.H. (1973) Analysis of the first cleavage Chromosomal anomalies of preimplantation rabbit division to determine the sex-ratio and incidence of blastocysts and human artificial abortuses in relation chromosome anomalies at conception in the mouse. to maternal age. Int. J. Fert. 23,207-212. J. Reprod. Fert. 35, 67-72. Gates, A.H. ( 1965) Rate of ovular development as a fac¬ Lamond, D.R., Gaddy, R.G. & Kennedy, S.W. (1972) tor in embryonic survival. In Preimplantation Stages Influence of season and nutrition on luteal plasma of Pregnancy (Ciba Fdn Symp.), pp. 270-293. Eds. progesterone in Rambouillet ewes. J. Anim. Sci. 34, G. E. W. Wolstenholme & M. O'Connor. Churchill 626-629. Livingstone, London. Larsen, M.M. & Generoso, W.M. (1984) Analysis of spon¬ Gayerie de Abreu, F., Lamming, G.E. & Shaw, R.C. taneous early embryonic lethality in mice. Mutation (1984) A cytogenetic investigation of early stage Res. 128,65-72. bovine embryos; relation with embryos mortality. Lawson, R.A.S. & Cahill, L.P. (1983) Modification of the Proc. 10th Int. Congr. Anim. Reprod. A.L, Urbana embryo maternal relationship in ewes by proges¬ 2,82. terone treatment early in the oestrous cycle. J. Gosden, R.G. (1973) Chromosomal anomalies of pre- Reprod. Fert. 67, 473-475. Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access Lawson, R.A.S., Parr, R.A. & CahiU, L.P. (1983) of maternal nutrition and plasma progesterone con¬ Evidence for maternal control of blastocyst growth centrations on survival and growth of the sheep after asynchronous transfer of embryos to the uterus embryo in early gestation. J. agrie. Sci., Camb. 98, of the ewe. J. Reprod. Fert. 67, 477-483. 39-46. Lee, C.N. & Ax, R.L. (1984) Milk progesterone of dairy Pedersen, R.A. (1974) Development of lethal yellow cows injected with gonadotrophin releasing hormone (Ay/Ay) mouse embryos in vitro. J. exp. Zool. 188, at the first post partum breeding. Proc. 10th Int. 307-320. Cong. Anim. Reprod. &A.I., Urbana 2, 401. Quinlivan, T.D., Martin, CA., Tayler, W.B. & Cairney, Long, S.E. (1977) Cytogenetic examination of pre¬ I.M. (1966) I. Pre- and perinatal mortality in those implantation blastocysts of ewes mated to rams ewes that conceived to one service. J. Reprod. Fert. heterozygous for the Massey 1 (ti) translocation. 11,379-390. Cytogenet. Cell Genet. 18, 82-89. Quirke, J.F. & Hanrahan, J.P. (1977) Comparison of the Long, S.E. & Williams, CV. (1980) Frequency of survival in the uteri of adult ewes of cleaved ova from chromosomal abnormalities in early embryos of adult ewes and ewe lambs. J. Reprod. Fert. 51, domestic sheep (Ovis aries). J. Reprod. Fert. 58, 487-489. 197-201. Redman, C.W.G., Sargent, I.L. & Sutton, L. (1984) Maurer, R.R. & Echternkamp, S.E. (1982) Hormonal Immunological aspects of human pregnancy and its asynchrony and embryonic development. Theriogen¬ disorders. In Immunological Aspects of Reproduction ology 17', 11-22. in Mammals pp. 219-250. Ed. D. B. Crighton. McFeely, R.A. (1967) Chromosome abnormalities in Butterworths, London. early embryos of the pig. J. Reprod. Fert. 13, Rhind, S.M., Doney, J.M., Gunn, R.G. & Leslie. I.D. 579-581. (1984) Effects of body condition and environmental McFeely, R.A. & Rajakoski, E. (1968) Chromosome stress on ovulation rate, embryo survival, and associ¬ studies on early embryos of the cow. Proc. 6th Int. ated plasma follicle stimulating hormone, luteinizing Cong. Reprod. Anim. & A.L, Paris 2, 905-907. hormone, prolactin and progesterone profiles in Meyer, H.H. (1985) Breed differences in ovulation rate Scottish Blackface ewes. Anim. Prod. 38, 201-209. and and uterine efficiency their contribution to Robinson, J.J. (1983) Nutrition of the pregnant ewe. In fecundity. In Genetics of Reproduction in Sheep, Sheep Production, pp. 111-131. Ed. W. Haresign. pp. 185-191. Eds R. B. Land & D. W. Robinson. Butterworths, London. Butterworths, London. Roche, J.F., Ireland, J.J., Boland, M.P. & McGeady, Michael, S.D., Geschwind, I.I., Bradford, G.E. & T.M. (1985) Concentrations of luteinising hormone Stabenfeldt, G.H. (1975) Pregnancy in mice selected and progesterone in pregnant and non-pregnant for small litter size: Reproductive hormone levels and heifers. Vet. Ree. 116,153-155. effect of exogenous hormones. Biol. Reprod. 12, Sadleir, R.M.F.S. (1969) The Ecology of Reproduction in 400-407. Wild and Domestic Mammals. Methuen, London. Miller, B.G. & Moore, N.W. (1976) Effects of proges¬ Schreiner, CA. & Hoombeck, F.K. (1973) Developmen¬ terone and oestradiol on RNA and protein metabo¬ tal aspects of sirenomelia in the mouse. J. Morph. lism in the genital tract and on survival of embryos in 141,345-358. the ovariectomised ewe. Aust. J. biol. Sci. 29, Shaver, E.L. & Carr, D.H. (1967) Chromosome abnor¬ 565-573. malities in rabbit blastocysts following delayed Moler, T.L., Donahue, S.E., Anderson, G.B. & Bradford, implantation. J. Reprod. Fert. 14,415—120. G.E. (1981) Effects of maternal and embryonic Short, R.V. (1979) When a conception fails to become a genotype in prenatal survival in two selected mouse pregnancy. In Maternal Recognition of Pregnancy lines. J. Anim. Sci. 51, 300-303. (Ciba Fdn Colloq. No. 64), pp. 377-387. Ed. J. Moore, N.W. (1985) The use of embryo transfer and Whelan. Elsevier/Excerpta Medica/North Holland, steroid hormone replacement therapy in the study of Amsterdam. prenatal mortality. Theriogenology 23, 121-128. Short, R.V. (1984) Species differences in reproductive Morton, N.E., Hassold T.J., Funkhouser, J., McKenna. mechanisms. In Reproduction in Mammals: 4. Repro¬ P.W. & Lew, R. (1982) Cytogenetic surveillance of ductive Fitness, pp. 24-61. Eds C. R. Austin & R. V. spontaneous abortions. Cytogenet. Cell Genet. 33, Short. Cambridge University Press. 232-239. Smart, Y.C, Fraser, I.S., Roberts, T.K., Clancy, R.L. & Mowbray, J.F., Liddell, H., Underwood, J.L., Gibbings, Cripps, A.W. (1982) Fertilization and early preg¬ C, Reginald, P.W. & Beard, R.W. (1985) Controlled nancy loss in healthy women attempting conception. trial of treatment of recurrent spontaneous abortion Clin. Reprod. Fert. 1, 177-184. by immunisation with paternal cells. Lancet 1, Sreenan, J.M. & Diskin, M.G. (1983) Early embryonic 941-943. mortality in the cow: its relationship with pro¬ Ober, CL., Martin, A.O., Simpson, J.L., Hauck, W.W., gesterone concentration. Vet. Ree. 112, 517-521. Amos, D.B., Kostyu, D.D., Potino, M. & Allen, F.H., Talbert, G.B. & Krohn, P.L. (1966) Effect of maternal HLA Jr (1983) Shared antigens and reproductive age on viability of ova and uterine support of preg¬ performance among Hutterites. Am. J. hum. Genet. nancy in mice. J. Reprod. Fert. 11, 399-406. 35,994-1004. Thompson, F.N., Clekis, T., Kiser, T.E., Chen, H.J. & R.A. & A.H. Nutrition of the Parr, Williams, (1982) ewe Smith, C.K. (1980) Serum progesterone concen¬ and embryo growth during early pregnancy. Aust. J. trations in pregnant and non-pregnant heifers and biol. Sci. 35,271-276. after gonadotropin releasing hormone in luteal phase Parr, R.A., Cumming, I.A. & Clarke, I.J. (1982) Effects heifers. Theriogenology 13,407^117. Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access Warner, CM., GoUnick, S.O. & Goldbard, S.B. (1984) Wilmut, I., Sales, D.I. & Ashworth, C.J. (1985a) Physio¬ Genetic control of early embryonic development by logical criteria for embryo mortality—is asynchrony gene(s) in the MHC (H-2 complex). Proc. 10th Int. between embryo and ewe a significant factor. In The Cong. Anim. Reprod. & A.L, Urbana 4, XIII, 10-15. Genetics of Reproduction in Sheep, pp. 275-289. Eds Wheeler, A.G., Baird, D.T., Land, R.B. & Scaramuzzi, R. B. Land & D. Robinson. Butterworths, London. R.J. (1975) Increased secretion of progesterone from Wilmut, I., Sales, D.I. & Ashworth, C.J. (1985b) The the ovary of the ewe during the pre-ovulatory period. influence of variation in embryo stage and maternal J. Reprod. Fert. 45, 419-522. hormone profiles on embryo survival in farm Williams, A.H. & Cumming, I.A. (1982) Inverse relation¬ animals. Theriogenology 23, 107-119. ship between concentration of progesterone and nutrition in ewes. J. agrie. Sci., Camb. 98, 517-522. Wilmut, I. & Sales, D.I. (1981) Effect of an asynchronous environment on embryonic development in sheep. J. Reprod. Fert. 61, 179-184.

Downloaded from Bioscientifica.com at 09/29/2021 08:56:28AM via free access