Estrus, preovulatory LH surge and oocyte maturation in Goettingen miniature pigs (Sus scrofa domesticus)

Many studies dealing with ovarian activity and oocyte maturation in pigs were conducted on gilts and sows in which ovarian activity had been induced with synchronizing hormones. The present study was conducted to characterize the naturally occurring processes. Duration of the estrus cycle and estrous behavior, monitored over 229 estrus cycles of 50 gilts (4.6 cycles/gilt) were 21.3 (SEM 0.08) d and 2.90 (SEM 0.06) d, respectively. Periestrous serum concentrations of progesterone and LH were determined in 50 gilts provided with permanently indwelling jugular catheters. The LH surge commenced 1.6 (SEM 1.6) h before the onset of the immobility reflex and climaxed 8.6 (SEM 1.5) h later. The duration of the LH surge was 27.9 (SEM 1.3) h; its amplitude was 2.8 (SEM 0.3) ng/mL. There was considerable individual variation in timing and pattern of LH release. Before and 6, 12, 18, 24, 30 and 36 h after the onset of the immobility reflex 7 to 10 gilts were sacrificed. Ovarian structures were recorded and cumulus oocyte complexes (COCs) from small follicles (<4 mm), large follicles (≥4 mm), ovarian bursa and oviduct were subjected to cytogenetic (meiotic progression) and molecular (mRNA concentration) studies. Oocytes from follicles <4 mm in diameter did not respond to the preovulatory LH surge, whereas oocytes in follicles ≥4 mm in diameter resumed meiotic progress to metaphase II. The relative abundance of CDC2, Cyclin B, ERK2 and c-mos gene transcripts was assessed by semi-quantitative RT-PCR in oocytes from follicles (<4 mm and ≥4 mm in diameter) as well as in post ovulatory oocytes. The amount of mRNA for each of the four genes investigated was higher in oocytes from follicles <4 mm than in those from follicles ≥4 mm in diameter. Differences were significant after 20 h for CDC2 and ERK2 transcripts. mRNA content for each of the four genes was significantly lower in ovulated than in non-matured (follicular) oocytes. It is concluded that in follicles ≥4 mm in diameter the LH surge induces oocyte maturation, a process where stored mRNA is used to synthesize the proteins relevant for continued oocyte maturation, viz. maturation promoting factor (MPF), mitogen activated protein (MAP) and c-mos.