Show simple item record

dc.contributor.authorUtomo, Desianto Budien
dc.date.accessioned2018-05-14T10:16:51Z
dc.date.available2018-05-14T10:16:51Z
dc.date.issued1997en
dc.identifier.urihttp://hdl.handle.net/1842/30032
dc.description.abstracten
dc.description.abstractMost modern highly productive poultry strains have been developed in temperate countries, with little opportunity for heat tolerance to be a selection factor. When these birds are moved to tropical or subtropical countries their egg production, egg quality, food intake and growth rate decrease at high ambient temperature. Varying reports on the effects of heat stress on egg and yolk production may be attributable to the differences of the range and duration of thermal loads employed, different bird strains, ages, diets and food intake responses. There are, however, only a few studies which attempt to explain the actual physiological mechanisms involved in the adaptation of laying hens to thermal loads, especially those relating to the changes in egg yolk precursors including vitellogenin and very low density lipoprotein (VLDL) synthesis, secretion and deposition in the oocytes.en
dc.description.abstractThe major yolk precursors are vitellogenin, a metalobinding protein, and VLDL, a specialised type of triglyceride-rich lipoprotein found in abundance in the plasma of laying hens. Both vitellogenin and VLDL are synthesised in the liver in response to oestrogen stimulation, transported to the ovary and transferred and deposited into the growing oocytes by a selective mechanism. Chronic heat stress in laying hens reduces yolk size and total yolk production considerably. Any changes in yolk precursor synthesis mediating such a response may involve altered oestradiol secretion or reduced sensitivity of the hepatocytes to oestrogen stimulation. Other possible mechanisms of reduced yolk accumulation include disturbances in the transport of vitellogenin in the blood stream and changes in the level of ovarian uptake of egg yolk precursors by the growing oocytes. The present project examined the changes in egg yolk precursor levels in the plasma of laying hens chronically exposed to different degrees of thermal stress (32 and 35°C with various relative humidities) and precursor uptake by the oocytes. The study also addressed the role of oestrogen in yolk precursor responses to chronic heat stress. In addition, the role of nutritional vitamin E supplementation as a strategy for alleviating the effects of heat stress has been examined. The studies have confirmed that heat stress reduces yolk and egg production concomitant with reduced circulating vitellogenin and VLDL in the plasma. The decreased plasma concentrations of vitellogenin and VLDL resulting from exposure to high ambient temperatures are not simply a consequence of reduced food intake. Studies applying some degree ofrestriction on feeding the birds (85, 70 and 55%) compared to the control group {ad libitum; 100%) and a paired-feeding study, where a control group was fed with the same amount as the heat stressed group consumed were also undertaken.en
dc.description.abstractThe present project examined the changes in egg yolk precursor levels in the plasma of laying hens chronically exposed to different degrees of thermal stress (32 and 35°C with various relative humidities) and precursor uptake by the oocytes. The study also addressed the role of oestrogen in yolk precursor responses to chronic heat stress. In addition, the role of nutritional vitamin E supplementation as a strategy for alleviating the effects of heat stress has been examined. The studies have confirmed that heat stress reduces yolk and egg production concomitant with reduced circulating vitellogenin and VLDL in the plasma. The decreased plasma concentrations of vitellogenin and VLDL resulting from exposure to high ambient temperatures are not simply a consequence of reduced food intake. Studies applying some degree ofrestriction on feeding the birds (85, 70 and 55%) compared to the control group {ad libitum; 100%) and a paired-feeding study, where a control group was fed with the same amount as the heat stressed group consumed were also undertaken.en
dc.description.abstractIt was also demonstrated by oestrogen administration in vivo under different heat loads (30°C and 80% relative humidity [RH]; 35°C and 55% RH), that oestrogen increases the availability of both vitellogenin and VLDL in the circulation of heat stressed and control (thermoneutral) hens although their responses were not identical quantitatively. There was a doubling of the concentration of VLDL in response to oestrogen treatment during the second week of heat stress. The results of this exogenous oestrogen treatment suggest that the liver is capable of synthesising vitellogenin and VLDL and this also means that hepatocyte sensitivity is not altered during heat stress.en
dc.description.abstractOther experiments in this study were also carried out to determine whether the mechanism mediating reduced egg production in heat-stressed hens involved a change in the rate of uptake of yolk precursors by the oocytes. This led to the establishment and validation of a new technique which allows characterisation of oocyte yolk precursor uptake in vivo. The labelling of natural vitellogenin present in high concentrations in fresh laying hen plasma by incubation with 65Zn proved extremely effective. The findings indicate that the novel technique is appropriate for the measurement of oocyte vitellogenin uptake in vivo and therefore for the elucidation of the mechanisms controlling oocyte development and yolk accumulation in response to environmental challenges.en
dc.description.abstractFurther studies have been carried out in an attempt to alleviate the effects of heat stress in laying hens using supplementation of the diet with a-tocopherol (vitamin E), a biological antioxidant. Supplementation of the diet with 500mg/kg vitamin E increased circulating egg yolk precursor concentrations during exposure to high heat loads and this may support the observed improvement in yolk and egg production. It is suggested that vitamin E supplementation represents a possible strategy for reducing the effects of heat stress. The study provides important evidence of a possible physiological basis for the changes in yolk and egg production induced by chronic heat stress in laying hens.en
dc.publisherThe University of Edinburghen
dc.relation.isreferencedbyAlready catalogueden
dc.subjectAnnexe Thesis Digitisation Project 2018 Block 18en
dc.titlePhysiological responses and mechanisms of yolk precursors and egg production in laying hens exposed to high ambient temperatureen
dc.typeThesis or Dissertationen
dc.type.qualificationlevelDoctoralen
dc.type.qualificationnamePhD Doctor of Philosophyen


Files in this item

This item appears in the following Collection(s)

Show simple item record