Browsing by Subject "production animal reproduction"

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  • Haen, Silke (Helsingin yliopisto, 2020)
    The hypothalamic-pituitary-gonadal axis is the endocrinological pathway of reproduction, where fertility is adjusted and fine-tuned with respect to intrinsic and extrinsic information. The pituitary gland receives the information that determines secretion of luteinizing hormone (LH) from the hypothalamus in the form of gonadotropin releasing hormone (GnRH), it is the frequency and amplitude of the GnRH pulses that regulate LH release. LH transports the information from the brain to the gonads, which feeds back to the hypothalamus and the pituitary gland in the form of steroids. This thesis work focused on the orchestration of GnRH, LH and progesterone during the early pregnancy of the pig. Therefore, gonads refer to the ovary with their functional bodies, the corpora lutea (CLs) and their principal steroid, progesterone. CLs form from the follicles after ovulation and they are either maintained, and pregnancy continues, or they regress, enabling follicle growth and new ovulations and a new opportunity for reproduction. The corpus luteum is independent of pituitary LH support for the first eleven days after ovulation. Subsequently the maternal organism is informed of the presence of embryos and the need to maintain the CLs. Thus, dependency of the CLs on LH begins at the same time as the sow receives the positive signals that indicate pregnancy. Progesterone is the main hormone of pregnancy and its concentration is highest around the time of maternal recognition of pregnancy. LH supports the CLs, but it remains unclear for exactly how long during pregnancy the LH stimulus directly triggers secretion of progesterone. The objective of this thesis work was to study the relationship between the two hormones, LH and progesterone, during early pregnancy in pigs. Firstly, we investigated the physiological release of LH and ovarian progesterone in inseminated gilts on Day 11 after oestrus, before the CLs are thought to be dependent on LH. Secondly, we looked at the pattern of progesterone release after the CLs had become dependent on LH. We studied the pattern of progesterone release from the ovary and were interested to ascertain if that pattern is related to the LH pattern. To explore a possible causal relationship between LH and progesterone further, a GnRH agonist model was used to nullify LH pulsatility around implantation of embryos. Thirdly, we were interested to know if parity, sow age and maturity affect the relationship between the two hormones. In our studies, a catheter was inserted through the vena saphena lateralis, enabling blood sampling in the vena cava caudalis next to the venous drainage of the ovary and uterus. LH and progesterone concentration were assessed after frequent blood sampling for 8–12 hours on Day 11 after oestrus in inseminated gilts, and on Days 16 and 21 in pregnant gilts. A slow-release GnRH agonist was inserted into pregnant gilts on Day 11 and LH and progesterone were studied on Day 16 and Day 21. Additionally, a vena jugularis catheter was inserted into pregnant primiparous sows and progesterone concentrations in local (vena cava caudalis) and peripheral (vena jugularis) blood samples were compared on Day 14. In gilts that were inseminated and classified post mortem whether they were pregnant or not, the progesterone release pattern measured in the vena cava caudalis had similar basal and mean progesterone concentrations, and frequency and amplitude of pulses on Day 11. On Day 11 and Day 21 of pregnancy, there was no relation between LH pulsatility and progesterone pulsatility. In gilts that were treated with a slow-release GnRH agonist on Day 11, LH pulsatility ceased but progesterone release remained pulsatile on Day 16 and Day 21 of pregnancy. On Day 21 we found that LH pulsatile release was active when progesterone release was basal. In primiparous sows, a progesterone pulse followed 60.8% of LH pulses within one hour. Mean progesterone concentration was approximately twice as high in the vena cava caudalis than in the vena jugularis in primiparous sows and differed significantly before and after feeding in the vena jugularis. GnRH-agonist-treated gilts had an elevated progesterone concentration on Day 21 compared with the control gilts. Progesterone concentration declined from Day 11 via Day 16 to Day 21 in gilts. LH pulse amplitude declined during those days, but no decline in mean or basal LH concentration or LH pulse frequency was recorded. LH pulsatility ceased on Day 16 and Day 21 in gilts treated with the GnRH agonist. LH release was synchronized in such a way that gilts under the same management conditions exhibited LH pulses around the same time during twelve hours, indicating a synchronized rhythm of secretion. We observed that progesterone release of CLs is similar in non-pregnant gilts and pregnant gilts on Day 11, before maternal recognition of pregnancy. During early pregnancy in gilts the pulsatile progesterone release of CLs on Days 11 and 16 is not responsive to – and on Days 16 and 21 it is not dependent on – LH pulsatile secretion. On Day 21 there is an alternate pattern of activity in pulsatile release of LH and progesterone in individual gilts. In primiparous sows, we found a temporal relationship between LH pulses and progesterone pulses already on Day 14 of pregnancy. We conclude that the progesterone release of the CLs functions without LH pulsatile stimulus from Day 11 to Day 21, but associations of these hormones at an individual level are evident.