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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Date
2017
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Massey University
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Abstract
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.
Description
Listed in 2017 Dean's List of Exceptional Theses
Keywords
Sheep, Reproduction, Embryos, Research Subject Categories::FORESTRY, AGRICULTURAL SCIENCES and LANDSCAPE PLANNING::Animal production::Animal breeding, Dean's List of Exceptional Theses