Bioscientifica Proceedings

Extended exposure of progesterone and conceptus estrogen influences the vascular compartment of the uterus and expression of many factors, such as prostaglandins (PGs), growth factors, extracellular matrix and adhesion molecules, cytokines and transcription factors. One of the supportive mechanisms by which the conceptus inhibits luteolysis is by changing PG synthesis in favor of luteoprotective PGE2. Alteration in PG synthesis may result from increased PGE synthase (mPG...

The pre-ovulatory LH surge triggers luteinisation of follicle tissue, but subsequent development of corpora lutea to full size is independent of LH up to around day 12 of pregnancy. Thereafter, severe (pharmacological) inhibition of LH secretion for 3 to 5 days will result in luteal failure and loss of pregnancy. It is unlikely that nutritional circumstances will have a similar effect, although scenarios with severe undernutrition have hardly been studied during early pr...

Experiments were conducted to further our understanding of the cellular and molecular mechanisms that regulate luteal function in ewes. Inhibition of protein kinase A (PKA) reduced (P < 0.05) secretion of progesterone from both small and large steroidogenic luteal cells. In addition, the relative phosphorylation state of steriodogenic acute regulatory protein (StAR) was more than twice as high (P&hairsp;<&hairsp;0.05) in large vs small luteal cells. Large steroidogenic...

During luteal regression episodic pulses of oxytocin secretion become coupled to the release of prostaglandin F2α (PGF2α) following synthesis of endometrial oxytocin receptors, but in early pregnancy the inhibition of oxytocin receptor formation by the conceptus prevents the development of the pulsatile pattern of PGF2α release needed to achieve luteolysis. Oxytocin receptors are present on the luminal epithelium in ovariectomize...

In males, including the ram, testosterone, acting via its primary metabolites oestradiol and dihydrotestosterone (DHT), suppresses circulating LH concentrations. This effect is due primarily, although not totally, to decreased frequency of gonadotrophin-releasing hormone (GnRH) pulses. The arcuate–ventromedial region (ARC–VMR) of the mediobasal hypothalamus and possibly the medial preoptic area (mPOA) are sites at which oestradiol acts to suppress GnRH, but the site ...

As milk production has increased during the past four decades, conception rates in lactating cows have declined. Although reduced reproductive performance has been associated with high milk yields, measures of postpartum ovarian activity have been more closely related to energy balance. The relationship between daily energy balance and postpartum reproductive activity is confirmed by longer intervals to first ovulation in cows with greater body condition loss. Patterns in dail...

Our current understanding of pre-elongation embryo metabolism and its regulation by factors both intrinsic to the embryo and present in its immediate environment is limited mainly to studies in rodents and of ruminant embryos that have been cultured in vitro. Energy metabolism in such embryos is initially low and dependent on oxidative phosphorylation for the generation of ATP. The embryo exhibits substrate preference for carboxylic acids, such as pyruvate, during thi...

The formation of primordial follicles to establish a reservoir of resting follicles and the gradual depletion of that reservoir to provide a succession of growing follicles are key to female fertility, but little is known about the regulation of these early stages of follicular development. This review summarizes the efforts of our laboratory to elucidate these critical processes in cattle. Primordial follicles first appear in fetal ovaries around the end of the first trimeste...

Summary. Using the cow, three questions related to the importance of the maternal immune system for the developing embryo are addressed: role of semen-induced inflammation for pregnancy establishment, regulation of preimplantation development by molecules that function as soluble mediators of immune cells, and immunological aspects of embryonic signaling by interferon-τ. Unlike rodents, there is no indication that semen modifies the physiology of the moth...

Summary. European breeds of rams appear to be responsive to photoperiodic changes even though there are large differences between breeds in the timing and amplitude of endocrine (LH and testosterone) and gametogenetic variations before the sexual season. Light regimens such as 6-month light cycles or alternations of constant short and long days every 12–16 weeks are able to entrain the parameters of sexual activity. In these regimens in which the period ...