Pre-implantation maternal uterine effects on embryo growth and development : an investigation using models of maternal constraint in sheep : a thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Turitea, Palmerston North, New Zealand

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Prenatal development and growth are critical to survival of the fetus and neonate. Recent evidence suggests that a critical period for determining growth is the pre-implantation period of pregnancy during which differentiation, organogenesis and development of the embryo occur and the embryo is considerably vulnerable to uterine environmental factors. The objectives of the present study were to examine the effects of restrictive uterine environments on embryo development using two sheep models of maternal constraint: litter size and dam size, and to identify embryonic and maternally-driven mechanisms that regulate development of the peri-implantation sheep embryo. Morphometric analysis (embryo length, width and heart bulge width) of the embryos in peri-implantation single and twin embryos was inconclusive; as was the transcriptomics analysis of whole embryos using RNA-seq to examine differential gene expression that may be responsible for differential regulation of growth. In a dam size model, large-breed Suffolk embryos gestated in small-breed Cheviot ewes (constrained environment) were smaller than Suffolk embryos gestated in Suffolk ewes (control) at day 19 of pregnancy, confirming previous findings that maternal constraint is evident in early pregnancy when limitations of space are not of consequence. Progesterone administered in the post-ovulatory period, day 0 to 6, alleviates this apparent constraint such that Suffolk embryos gestated in Cheviot ewes that received progesterone are larger than those gestated in Cheviot ewes that did not. Further, differential gene expression analysis of maternal uterine tissues showed that at day 6 and day 19 endometrial genes that encode for histotroph secretion and uterine receptivity are altered by post-ovulatory progesterone administration. Timing of administration of progesterone is critical not only to embryo growth but also to embryo survival. There were lower pregnancy rates in the ewes that received progesterone from day 0 than those that received progesterone from day 2. The results of this thesis indicate that progesterone exerts its effects by regulation of genes that encode for uterine structural and secretory activity to advance the uterus. This likely forces the asynchronous embryo to accelerate its growth in order to adapt to its environment. These findings contribute to the knowledge of the regulatory mechanisms controlling early embryo growth and present a platform within the livestock industry and human reproductive technology practice to manipulate embryo growth to improve survival of offspring.
Listed in 2017 Dean's List of Exceptional Theses
Sheep, Reproduction, Embryos, Research Subject Categories::FORESTRY, AGRICULTURAL SCIENCES and LANDSCAPE PLANNING::Animal production::Animal breeding, Dean's List of Exceptional Theses