School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemPlacental size and foetal growth in relation to maternal undernutrition during mid-pregnancy in sheep( 1989)Examination of the evidence published in the literature revealed a variety of responses in birth weight of the lamb to maternal undernutrition during mid pregnancy. Explanations of the various responses are commonly made in terms of what has been named, in this thesis, as the Size Hypothesis. This hypothesis states that placental size is limiting the transfer of nutrients across the placenta during late pregnancy, and therefore determining foetal growth. The two main assumptions of this hypothesis are that (i) placental size is an index of its maximum functional capacity, and.(ii) the placenta is functioning at its maximum capacity during late pregnancy. In this thesis the validity of both of these assumptions is questioned, and, an alternative hypothesis proposed. Chapter One reviews the factors which are involved in the movement of metabolic substrate from the dam, across the placental barrier and to the site(s) of deposition in the foetus. From the limited amount of information available in the literature, it is apparent that placental size is not necessarily an accurate indicator of the maximum functional capacity of the placenta. Therefore the Size Hypothesis cannot be functionally correct. Consideration of the evidence used to support the Size Hypothesis questioned the causal relationship between placental size, and growth of the foetus during late pregnancy. Consequently experiments were designed to: (i)investigate the effect of maternal undernutrition during mid pregnancy on placental size. (ii)determine the functional relationship between placental size and foetal growth. Maternal undernutrition during mid pregnancy was chosen as the means of manipulation of placental size because of its direct relevance to the extensive grazing system of sheep management in Southern Australia. In the first experiment (Chapter Two) maternal undernutrition imposed between 75 and 100 days post coitus tended (P=0.08) to reduce both placental size, measured at 100 days post coitus, and weight of the lamb at birth. The relationship between placental size and growth of the foetus remained unresolved. Therefore a second experiment was designed. In this experiment (Chapter Three) single bearing pregnant ewes were exposed to undernutrition during one of three periods during mid pregnancy; 30 to 96 days (Group RA), 50 to 96 days (Group RB) and 75 to 96 days (Group RC) post coitus. Placental growth was retarded in ewes exposed to maternal undernutrition between 30 and 96 days post coitus, while the shorter periods of undernutrition had no significant effect on placental growth. Despite a smaller maximum placental size being achieved for group RA compared to control (well fed) ewes, and all ewes being fed on a high plane of nutrition during late pregnancy, foetal growth during late pregnancy was not significantly different between group RA and control (well fed) This evidence supports the hypothesis that placental size per se is not the prime determinant of foetal growth. The effect of altering placental size, by maternal undernutrition between the time of conception and 70 days post coitus (Chapter Four), or the number of foetuses sharing one uterus (Chapter Four, Chapter Five), on birth weight of the lamb was investigated. One experiment in Chapter Four (Experiment One) twin lambs at birth were not significantly lighter in weight, despite being associated with a smaller placenta, when compared to the single lambs. This is further evidence to suggest that placental size per se was not limiting growth of the foetus during late pregnancy. The final two experimental chapters in this thesis quantify the relationship between placental size and some functional aspects of the placenta. Placental size was not closely related to either the volume of blood in the placenta (Chapter Six), or the rate of movement of calcium (Chapter Seven) across the placenta. All experiments discuss the possible sites of limitation, or regulation, to the movement of nutrients to the foetus. These include the regulation of the intake of feed, the digestion, metabolism and partition of the metabolic substrates in the dam as they influence growth of the foetus. The experiments in this thesis demonstrated that growth of the placenta can be improved, retarded or remain unaffected by maternal undernutrition being imposed at various times during the first 100 days post coitus. Explanation of the potential effect which nutritional stress has in altering placental growth, is often made in terms of the degree to which the mobilization of maternal body reserves may buffer the conceptus from any nutrient limitation on growth of the foetus. In contrast to this, the experiment described in Chapter Seven reported that growth of the placenta, in ewes fed at a restricted level of nutrition, was greater when compared with their control (well fed) counterparts. The factors causing this stimulation in placenta growth are yet to be identified. Regression analysis of data from all experiments revealed that both placental size near parturition (P<0.001), and the change in liveweight during the period of nutritional restriction (P=0.06), were related to weight of the foetus near parturition. Conversely the liveweight and/or condition score of the dam at the time of joining was not related to placental size or foetal weight near parturition. This observation suggested that the level of body reserves at the time of joining does not significantly modify the effect which maternal undernutrition has. on placental and foetal growth. Maternal undernutrition during the first 100 days post coitus had little or no effect on growth of the foetus, measured at the end of the restriction period, despite a significant depletion in the body reserves of the dam. In addition, the large differences in the patterns of placental growth caused by maternal feed restriction were not reflected in weight of the lamb at birth. Differences in voluntary food intake and the level of body reserves available for mobilization/deposition, between the previously restricted and control (well fed) ewes, were not reflected in the whole-body metabolism of either glucose or calcium (Chapter Seven). Therefore the Size Hypothesis was not confirmed by the observations made in this thesis. Evidence used to support the Size Hypothesis includes the observation that the rate of foetal growth during late pregnancy does not continue to increase at an increasing rate, even when the ewe is well fed during late pregnancy. It has been proposed by other workers (e.g. Mellor, 1983) that this indicates that the functional capacity of the placenta is limiting foetal growth. The experiment reported in Chapter Three demonstrated that, despite a retardation in placental size of as much as 23 percent by 96 days post coitus, growth of the foetus during the final 12 days of pregnancy was similar for both the control (well fed) and previously under fed (restricted) ewes. It was therefore concluded that placental size per se cannot be limiting foetal growth during late pregnancy. In another experiment (Chapter Seven) maternal undernutrition during mid pregnancy resulted in an increase in placental growth of 21 percent, despite the ewes being severely under fed between 30 and 96 days post coitus. The rate of transport of calcium across the placenta, and its lack of any relationship to placental size, indicated that placental size was not an appropriate indicator of its movement. In contrast to the prediction of the Size Hypothesis, foetal growth during late pregnancy was not improved despite the ewes being fed on a high plane of nutrition during the period between 96 days post coitus and the time of parturition. The circumstances in which a relationship between "size" and "function" were strong, and those in which no close relationship existed, were examined in an attempt to define the physiological conditions regulating foetal growth. An alternative hypothesis, termed the Functional Reserve Hypothesis was proposed. It aims to incorporate all available observations made in the experimental work reported in this thesis, and from the relevant literature reported elsewhere. The concept central to Functional Reserve Hypothesis is that the ovine placenta is not in a state of functional saturation during late pregnancy, and hence does not act as a limitation to growth of the foetus. In addition, the Functional Reserve Hypothesis proposes that placental size is not an accurate indicator of the capacity of the placenta to transfer nutrients. It is hypothesized that the ovine placenta has same level of functional reserve, even in the situations where it is retarded in its growth. Additionally it is proposed that factors regulating both (i) the partition and supply of nutrients from the dam, and (ii) the potential for growth, and the demand for nutrients by the foetus, all interact to regulate the rate of' foetal growth during late pregnancy and therefore ultimately the birth weight of the lamb. Before successful manipulation of foetal growth during late pregnancy is possible, the key metabolic substrates, and the factors regulating their movement to the foetus, need to be identified. Finally, how the above results contribute to the development of strategic feeding recommendations for the Australian sheep flock are discussed. Two specific areas for further research are identified.