Reproduction is a critical component of an animal’s fitness, but it is also energetically intensive. In order to maximize reproductive fitness, organisms modulate their reproductive investment or timing in response to environmental cues that provide information about resource availability or other indicators of reproductive success. Physiologic systems that monitor these cues then influence both reproductive readiness and more scaled measures of investment, including number of offspring or offspring provisioning effort. This kind of physiologic modulation is especially salient to mammals, for whom reproduction (gestation and lactation) is exceptionally intensive, and there is extensive documentation of variation in reproductive effort and success under specific environmental and physiological challenges. Still, the underlying mechanisms that coordinate these shifts in investment remain poorly defined and understood.
This dissertation identifies novel endocrine and paracrine mechanisms that regulate ovarian and uterine function in the context of reproductive success. Chapter 1 summarizes our current understanding of female reproductive function and modulation, ultimately identifying 4 key areas of modulation for which the molecular mechanisms are poorly understood or defined: (1) evaluating autonomic nervous system regulation of reproductive organs in ecophysiological contexts, (2) identifying new signals regulating ovary and uterine function; (3) determining the extent to which reproductive stage (e.g., pregnancy) alters regulatory networks and sensitivity; and (4) scaling function of specific cell types or organs to organism-level reproductive outcomes. Chapters 2-5 address areas 2-4 by identifying new mechanisms of mammalian reproductive system control in the ovary and uterus and by evaluating the function of established signaling networks in other reproductive states.
Chapter 2 interrogates the direct sensitivity of the ovary to physiological cues using mouse ovarian explants. By using pharmacologic inhibition of glucose metabolism, these experiments identify novel glucose sensitivity in the ovary. Chapter 3 examines the effect of neuropeptide gonadotropin inhibitory hormone (GnIH) on ovarian follicle growth, survival, and steroidogenesis in the domestic cat. As has been shown in other mammals to-date, GnIH promotes follicle degradation in vitro. These findings support a conserved role for GnIH in vertebrate ovarian folliculogenesis. Together, these chapters provide new paracrine and endocrine mechanisms of ovarian regulation that adds to our molecular understanding of how reproductive effort and investment are modulated.
Chapter 4 explores the sensitivity of feline uterine endometrial epithelial cells to sex steroids. We demonstrate that sex steroids stimulate morphological changes to endometrial epithelial cell 3D growth in vitro, and that morphological changes are associated with functional changes in gene expression. Though often manipulated as part of normal fertility treatment, we do not have a good understanding of how these hormones facilitate uterine preparedness for implantation and pregnancy in felines. Chapter 4 thus identifies specific mechanisms of reproductive modulation in the feline uterus and also beings to flesh out molecular mechanisms underlying reproductive failure at the organism level. Our results have direct application to endangered felid breeding programs, which utilize in vitro follicle maturation and in vivo hormones treatments to promote breeding success of genetically-important individuals.
Finally, chapter 5 uses a classic stress physiology paradigm to identify the mechanism(s) by which psychological stress inhibits ovarian steroid synthesis during pregnancy in mice. These experiments demonstrate that stress-dependent inhibition of ovarian function does not occur through classical, hypothalamic inhibition, emphasizing the dynamic shifts in endocrine function across mammalian pregnancy.
These studies add to our comparative understanding of reproductive function, which is critical because of its fundamental connection to organismal fitness. Taken together, these studies also highlight the utility of in vitro approaches for experimentally approaching the challenge of connecting molecular variation to organismal function. We summarize the results and their ultimate importance to female reproductive physiology research in Chapter 6.