Ovulation occurs 24 to 36 hours after the start of standing heat (estrus) for gilts and 36 to 48 hours after the beginning of standing heat for sows. After ovulation the ruptured follicular tissue forms a corpus luteum. The corpora lutea or yellow bodies produce and secrete progesterone. Progesterone has a negative effect on follicular growth by preventing the secretion of FSH and LH. Progesterone inhibits the secretion of FSH and LH by suppressing the release of GnRH from the hypothalamus and by directly influencing the secretion of FSH and LH by desensitizing the anterior pituitary to GnRH. This period of the estrous cycle when progesterone dominates is referred to as the luteal phase. The high level of progesterone causes the female to be uninterested in the boar. Around days 12 to 14 of the estrous cycle, progesterone concentrations begin to decline. If the female is not pregnant at this time, PGF is released from the uterus and causes the rapid regression or destruction of the corpora lutea (a process called luteolysis). With the destruction of the corpora lutea, progesterone levels rapidly decline, and their inhibitory control on the growth of follicles is removed.
Following luteolysis, a slight increase in FSH secretion and an increase in the number of LH pulses occur that stimulate follicular growth. This part of the estrous cycle is called the follicular phase. As the follicles develop, they produce estradiol, which increases steadily during the latter stages of the estrous cycle. Once estradiol concentrations reach a high enough level, the preovulatory surge of FSH and LH occurs, and ovulation follows. Estradiol is also responsible for the characteristic responses of a gilt or sow in estrus or standing heat (swollen and red vulva, immobile stance, and erect ears).
Estrus begins with the pituitary gland, which is located just below the brain and secretes several hormones into the blood stream, including luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH are called gonadotropins.
--------
In prepubertal (immature) gilts, gonadotropin secretion is low but dramatically increases just prior to puberty (first estrus) at 6 to 8 months of age. During the two- to three-day period just prior to estrus, increasing blood levels of LH and FSH cause the follicles on each of the two ovaries to grow rapidly. These follicles in turn secrete increased levels of the hormone estradiol into the blood, causing the behavioral and physiological changes associated with estrus. Moreover, each follicle contains an ovum, which when released and fertilized by a sperm cell, develops into an embryo.
Rising concentrations of estradiol in the blood reach a threshold which triggers a massive release of LH from the pituitary gland around the onset of estrus. This LH surge stimulates ovulation, the release of ova from the follicles into the oviducts. Though the timing of ovulation is extremely variable, on average it occurs 40 hours after the onset of estrus. The sperm cells fertilize the ova in the oviducts, the tubes between the ovaries and the horns of the uterus. The fertilized eggs then progress to the uterus, implant, and further develop into embryos and then fetuses.
The sites on the ovaries from which ova are released subsequently form structures called corpora lutea that secrete another hormone, progesterone, into the blood. During the luteal phase of the estrous cycle (approximately day 4 to day 16), progesterone inhibits LH and FSH secretion from the pituitary gland, inhibiting follicular growth.
If the ova are not fertilized during estrus, or embryos do not implant in the uterus then beginning around day16 the uterus secrets the hormone prostaglandin-F2° (PGF2° ) into the blood. PGF2° causes the regression or death of the corpora lutea and as a result, progesterone levels decline. Decreasing levels of progesterone allow LH and FSH levels to increase, follicles to grow, and estrus returns. Female swine display estrus at 18- to 22-day intervals throughout the year unless their cycling is interrupted by pregnancy and lactation, poor nutrition, disease, etc.
If fertilization occurs and pregnancy is initiated, then PGF2° is not released into the circulation. The corpora lutea are maintained and secrete high levels of progesterone into the blood stream throughout gestation. Progesterone is essential for maintenance of pregnancy. It inhibits follicular growth as well as uterine contractions.
Around day 114 of gestation, the uterus causes the corpora lutea to regress by releasing large amounts of PGF2° into the blood. Consequently, progesterone levels decrease, uterine contractions commence, and the fetuses are expelled.
During lactation, suckling by the pigs causes a suppression of LH and FSH secretion, keeping the ovaries devoid of large follicles. The removal of the suckling stimulus at weaning allows the secretion of gonadotropin to increase. Follicles grow rapidly and there is the corresponding rise in the circulating levels of estradiol. Sows return to estrus in four to seven days and estradiol elicits the surge of LH, causing ovulation.
Note:
Estrus refers to heat period while estrous refers to the cycle.
Following luteolysis, a slight increase in FSH secretion and an increase in the number of LH pulses occur that stimulate follicular growth. This part of the estrous cycle is called the follicular phase. As the follicles develop, they produce estradiol, which increases steadily during the latter stages of the estrous cycle. Once estradiol concentrations reach a high enough level, the preovulatory surge of FSH and LH occurs, and ovulation follows. Estradiol is also responsible for the characteristic responses of a gilt or sow in estrus or standing heat (swollen and red vulva, immobile stance, and erect ears).
Estrus begins with the pituitary gland, which is located just below the brain and secretes several hormones into the blood stream, including luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH are called gonadotropins.
--------
In prepubertal (immature) gilts, gonadotropin secretion is low but dramatically increases just prior to puberty (first estrus) at 6 to 8 months of age. During the two- to three-day period just prior to estrus, increasing blood levels of LH and FSH cause the follicles on each of the two ovaries to grow rapidly. These follicles in turn secrete increased levels of the hormone estradiol into the blood, causing the behavioral and physiological changes associated with estrus. Moreover, each follicle contains an ovum, which when released and fertilized by a sperm cell, develops into an embryo.
Rising concentrations of estradiol in the blood reach a threshold which triggers a massive release of LH from the pituitary gland around the onset of estrus. This LH surge stimulates ovulation, the release of ova from the follicles into the oviducts. Though the timing of ovulation is extremely variable, on average it occurs 40 hours after the onset of estrus. The sperm cells fertilize the ova in the oviducts, the tubes between the ovaries and the horns of the uterus. The fertilized eggs then progress to the uterus, implant, and further develop into embryos and then fetuses.
The sites on the ovaries from which ova are released subsequently form structures called corpora lutea that secrete another hormone, progesterone, into the blood. During the luteal phase of the estrous cycle (approximately day 4 to day 16), progesterone inhibits LH and FSH secretion from the pituitary gland, inhibiting follicular growth.
If the ova are not fertilized during estrus, or embryos do not implant in the uterus then beginning around day16 the uterus secrets the hormone prostaglandin-F2° (PGF2° ) into the blood. PGF2° causes the regression or death of the corpora lutea and as a result, progesterone levels decline. Decreasing levels of progesterone allow LH and FSH levels to increase, follicles to grow, and estrus returns. Female swine display estrus at 18- to 22-day intervals throughout the year unless their cycling is interrupted by pregnancy and lactation, poor nutrition, disease, etc.
If fertilization occurs and pregnancy is initiated, then PGF2° is not released into the circulation. The corpora lutea are maintained and secrete high levels of progesterone into the blood stream throughout gestation. Progesterone is essential for maintenance of pregnancy. It inhibits follicular growth as well as uterine contractions.
Around day 114 of gestation, the uterus causes the corpora lutea to regress by releasing large amounts of PGF2° into the blood. Consequently, progesterone levels decrease, uterine contractions commence, and the fetuses are expelled.
During lactation, suckling by the pigs causes a suppression of LH and FSH secretion, keeping the ovaries devoid of large follicles. The removal of the suckling stimulus at weaning allows the secretion of gonadotropin to increase. Follicles grow rapidly and there is the corresponding rise in the circulating levels of estradiol. Sows return to estrus in four to seven days and estradiol elicits the surge of LH, causing ovulation.
Note:
Estrus refers to heat period while estrous refers to the cycle.