Impact of early pregnancy on prenatal development in the pig

Abstract

In the present study aspects of the impact of early pregnancy on the average prenatal development per litter and on the within-litter weight distribution at birth have been investigated. The aims of the present study are given in the introduction (chapter 1). A brief review of the literature concerning the chronology of early pregnancy and embryonic mortality in the pig is given in chapter 2.In chapter 3 an experiment is described in which it was attempted to affect the development and functioning of the uterus structurally in order to obtain a model in which the role of the uterus and its secretion'products as determinants of embryonic mortality and/or embryonic development could be studied. The possibility to affect the functional integrity of the uterus by means of a severe growth retardation during the first 80 days of life, i.e. during the period in which the uterine glands differentiate, was investigated. A group of female piglets were weaned within 24 hours after birth and either fed unrestricted (n=35) or restricted (n=34). The restricted fed piglets were allowed to grow at a rate of approximately 50% of that of their unrestricted littermates. This contrast was maintained until day 80 after birth. Another group of piglets (control group, n=38) were normally weaned at an age of 35 days and subsequently fed unrestricted until day 80. From day 35 onwards all piglets were kept under the same conditions and from day 80 onwards they were also treated the same. A representative sample from each of the three groups (unrestricted fed (n=8), restricted fed (n=8) and control (n=10)) were slaughtered at an age of 80 days to study the histomorphology of their uterus. The remaining gilts which reached puberty and became pregnant (n=50) were slaughtered on either day 34, 35 or 36 of first pregnancy. The uterine and conceptus development were subsequently studied.At day 80 the average weight of restricted fed piglets was 45% of that of unrestricted fed piglets and 48% of that of control piglets. Although the averages for uterine length and weight, thickness of the myometrium and endometrium and the relative glandular surface area were all lower in the restricted fed than in both other groups, only the difference in thickness of the myometrium between the restricted fed group and control group was significant. All differences were merely due to differences in bodyweight. No negative effects were seen on ovulation rate, number of embryos, embryonic mortality rate, uterine length and weight or any of the parameters for conceptus development. This indicates that the functional integrity of the uterus is not affected by a severe growth retardation during the first 80 days of life.With the data collected for the gilts which were slaughtered on day 35 of pregnancy in this experiment (n=31) and data collected for another group of Dutch Landrace gilts which were also slaughtered on day 35 of pregnancy (n=40), the relationship between conceptus development and embryonic mortality was studied. The results of this study are described in chapter 4. For purposes of interpretation of the results, especially concerning the development of the placentae, relevant uterine parameters were also considered. The results indicate that the embryonic weight and length significantly decrease with an increasing embryonic mortality rate, whereas the placental weight and length significantly increase with an increasing embryonic mortality rate. Statistical analyses have shown that this decrease in embryonic development with an increasing embryonic mortality rate was not due to the concomitant increase in placental development. After correction for differences in number of embryos, these relationships (except for placental length) were no longer significant, indicating that embryonic mortality is related to conceptus development through its relationship with number of embryos. It was concluded that factors that caused a high embryonic mortality - and thus small litters - also caused a low embryonic weight and length. From the fact that the uterine length available per embryo significantly increased with an increasing embryonic mortality, even after correction for the number of embryos, it was concluded that the relationship of embryonic mortality with placental development might be due to the effect that embryos which died had on the uterine length before their death.To study whether the accelerated growth of the placentae in gilts with a high embryonic mortality rate will be beneficial for the foetal development until birth, the relationship between foetal development and the prenatal mortality rate was studied. The results of this study for which 195 pregnant Dutch Landrace gilts were slaughtered between day 35 and day 112 of pregnancy, are described in chapter 5. From these results it was clear that the weight gain of foetuses between day 35 and day 100 of pregnancy was higher within gilts with a high prenatal mortality rate than within gilts with a low prenatal mortality rate. There were also marked differences between these two groups for placental development during this period. Within the group of gilts with a high prenatal mortality rate the placental weight and length increased until day 95 of pregnancy, to decrease thereafter. Within the group of gilts with a low prenatal mortality rate it increased until day 65, remained fairly constant until day 95, to increase rapidly thereafter. Although the prenatal mortality rate during the early foetal stage is most probably still a good estimator of the embryonic mortality rate, towards the end of pregnancy this is almost certainly no longer the case. The present study therefore does not allow decisive conclusions as to the relationship between embryonic mortality rate and the average birthweight.In chapter 4 and 5 the emphasis has been on the variation in prenatal development between litters. In chapter 6 and 7 one aspect of differences in prenatal development within litters (the within-litter weight distribution) has been studied. Although it is generally assumed that the within-litter weight distribution is normal (Gaussian), evidence has been provided for the fact that in approximately one-third of litters the distribution is abnormal in a sense that two subpopulations can be identified within these litters. On the other hand there is an accumulation of indications that within-litter variation in embryonic developmental stage can occur very early in pregnancy and that this variation is reduced before or shortly after the time of maternal recognition of pregnancy (day 11) through selective mortality of the less developed embryos. The results described in chapter 6 confirms the fact that the within-litter weight distribution is abnormal in 33% of all litters studied (466 foetal litters and 915 litters at term). On the basis of the within-litter weight distribution 5 littertypes were defined: normal litters (one normal distribution), litters with one or two runts (two discrete subpopulations: one main distribution and one or two growth retarded outliers), litters with left-hand individuals (two discrete subpopulations, the left-hand distribution numerically smaller than or equal to the right-hand distribution and both larger than 2), litters with right-hand individuals (two discrete subpopulations, the right hand distribution numerically smaller than the left-hand distribution and both larger than 2) and litters with one or two giants (two discrete subpopulations: one main distribution and 1 or 2 growth accelerated outliers). The results strongly suggest that the within-litter weight distribution as found at term has been established by the end of the embryonic stage (day 35) of pregnancy, at least in the majority of litters. The average embryonic mortality rate differed between littertypes. The lowest mortality rate was found for the litters with lefthand individuals, the highest for litters with giants.The preweaning death risk and preweaning growth rate of piglets in relation to the within-litter weight distribution of the litter in which they were born and suckled were subsequently studied. The results, based on data for 819 litters, are described in chapter 7. The preweaning death risk of piglets from litters with two discrete subpopulations was not higher than that for piglets from litters with a normal within-litter weight distribution. In all 5 littertypes the death risk was similarly dependent on birthweight, litter size and within-litter variation in birthweight. The average preweaning growth rate per litter differed between littertypes but was entirely due to differences in average litter size at birth. Within each of the 4 littertypes with two discrete subpopulations the average growth rate of the piglettype with the lower average birthweight was always less than that for the piglettype with the higher average birthweight. Within litters with runts and left-hand individuals this remained the case even after correction for birthweight, indicating that the growth rate of runts and left-hand individuals is less than would be expected on the basis of their birthweights.A general discussion of the results and the conclusions are given in chapter 8. The results of the present study allows the following conclusions:1. The functional integrity of the uterus of gilts as measured by its ability to support the survival and normal development of embryos, can not be adversely affected by a regular, severe growth retardation during the first 80 days of life, i.e. during the period of differentiation of the uterine glands.2. Day 35 embryos in gilts with a low embryonic mortality rate (the larger litters) are better developed than day 35 embryos in gilts with a high embryonic mortality rate (the smaller litters), indicating that factors that are favourable for embryonic survival are also favourable for embryonic development or are associated with factors that are favourable for embryonic development.3. The placentae of day 35 embryos in gilts with a high embryonic mortality rate (the smaller litters) are better developed than the placentae of day 35 embryos in gilts with a low embryonic mortality rate (the larger litters). As a consequence the foetuses in gilts with a high embryonic mortality will grow faster than foetuses in gilts with a low embryonic mortality rate.4. The within-litter weight distribution is normal (Gaussian) in approximately 67% of all litters; within the remaining litters two discrete subpopulations of variable size can be identified.5. The within-litter weight distribution at birth is established at the end of the embryonic stage of pregnancy (day 35) and is related to the embryonic mortality rate.6. The within-litter weight distribution at birth is neither an important determinant of the within-litter variation in birthweight, nor an important determinant of the preweaning death risk of liveborn piglets.</p