Estrous Cycles

  • Reproduction: series of physiological and psychological events that must be properly timed

    • need a functional male and a willing female

    • Females are the rate-limiting factor

  • breeding patterns in mammals

    • once per cycle desire to mate

      • domestic animal species

      • Ab-Libitum — at will; breed whenever they want

    • Estrus: standing heat, one event

    • Estrous Cycles: progressive accomplishment of alteration of reproductive tract characteristics of estrus, metestrus, diestrus and proestrus produced by changes in ovarian hormones, entire cycle

  • Breeding Exhaustion: point where male has mated too many times and is producing too few sperm to allow for reasonable chance for pregnancy

  • assisted reproductive techniques must take into account, where the animal is in her cycle

  • Estrous Cycle

    • Proestrus, Estrus, Metestrus, Diestrus:

      • Progressive accomplishments and alterations in genital tract demonstrating characteristics of estrus, metestrus, diestrus, and proestrus produced by changes in ovarian hormones

      • Occurs in non primate females

    • Estrous cycles: the growth of follicles, the ovlation of follicles around he time of standing heat, and the formation of the corpus luteum

      • If the animal is not pregnant, the process will start over again

        • Estrous phases: major events occurring during that time period

    • Animal will ovulate sometime during or just after estrus, the ovary will switch gears and go from growing follicles to creating a corpus luteum

    • Structure on ovaries dictate phase of the cycle

      • Corpus hemorrhagcium: the major anatomical feature on the ovary during metestrus

      • corpus luteum: the longest phase of the cylcel, diestrus

      • Follicular growth: termed proestrus (pro meaning before)

    • Anestrus: period between diestrus and proestrus when nothing is happening. It is not part of the cycle. Period when the ovary and hormones are quiescent (quiet) like they were before puberty

    • Proestrus (Follicular phase)

      • Days 17 to 21 of estrous cycle of cow

      • Ovarian follicles are growing rapidly

      • Increase in E2B secretion by growing follicles

      • Decrease in P4 from CL

      • Mucosal layer of vagina and uterus multiply

      • Must occur first to have an ovulation

      • begins as the corups luteum from the previous cycle becomes atretic and stops function, no more progesterone

      • Uterine changes

        • increase in the number of cells in both the uterus and vagina, and the uterus is also said to be in the follicular phase

    • Estrus

      • period of sexual receptivity in female

      • commonly called “heat”

      • can be designated DAY 0 or DAY 1 of cycle

      • End of the follicular phase

      • Reproductive Receptivity: time when female in estrus stands to be mated

      • LH Surge: triggers ovulation and begins the transformation of the theca and granulosa of the follicle into the progesterone producing luteal cells of the corpus luteum

  • Types of Estrous Cycles

    • Continuous:

      • occurs in rabbits and other induced ovulators

      • LH release is induced by mating

      • ovulation occurs 8 to 12 hours after mating

      • Act of mating sends nervous impulse that triggers the LH surge

    • Continuous Estrus: estrous animals with constant desire to mate and whose ovulation is triggered by the act of mating

      • Follicles are constantly developing, then regressing, no luteal phase

      • Lack of receptors for estrogen on the LH producing cells

      • Estrogen still induces all other effects

    • Induced Ovulation: ovulation triggered by the act of mating

      • nervous impulse, during mating, triggers the LH surge

      • extremely high conception rates, sperm in tract same time as ovulation

    • Monoestrus:

      • single cycle per year

        • EX: bears, wolves, foxes, and many breeds of domesticated dogs

    • Polyestrus: animals that have two or more estrus cycles per year

      • Two catgeories:

        • True polyestrus: two or more estrous cycles per year UNLESS interrupted by pregnancy

        • Seasonally Polyestrus: same as above except last diestrus of breeding season would proceed to anestrus

      • Proposed mechanism of Seasonal Polyestrus:

        • retina of eye is sensor for light

        • impulses travel by way of optic nerce to pineal gland

        • pineal gland releases melatonin

        • melatonin serves as a mediator between photoreceptors, hypothalamus, and/or anterior pituitary

    • Type of Estrus: “standing heat”

      • Characteristics: During this phase, the female is receptive to mating and displays behavioral changes such as increased vocalization, restlessness, and a willingness to mate.

    • Postpartum estrus: estrus occurring within a few days after delivery

      • rarely seen in cattle or sheep that have an extended postpartum anestrus period to allow repair of the reproductive tract; however, in species with shorter postpartum recovery times, such as pigs, this phenomenon can be more common, allowing for quicker return to breeding.

  • Specialized types of Estrus

    • Sow heat: exhibit a postpartum estrus approximately 3-10 days after farrowing, they rarely ovulate; meaning the cycle is sub or infertile

    • Foal heat: between 5 and 15 days after foaling, can ovulate and become pregnant on these cycles, they usually have lower fertility die to the damage to the reproductive tract during parturition, but it provides a valuable opportunity for early breeding.

      • energy demands: own physiology, lactating for foal, pregnancy to maintain

    • Silent or Quiet estrus or quiet ovulation

      • during estrus, ovulation occurs, but there are none of the psychological signs of standing heat

      • lack of receptors for estrogen in the brain, which form in response to having “seen: progesterone from a pervious cycle

      • Seen in farm animals on cycle before first standing heat, or after long anestrus period

      • will not stand to be mated

        • cause: heat stress, stress, iodine limiting, predisposed

    • anovulatory estrus:

      • estrus without ovulation

        • occurs in all farm animals

        • best example is postpartum in sows

    • Nymphomania: characterized as a continuous psychological desire to mate by female animal that is not in heat

      • ovulation rarely occurs

      • occurs most commonly in cows, less commonly in mares and rarely in ewes or sows

      • cystic ovaries usually accompany nymphomania condition

      • not all individuals with cystic ovaries display nymphomania

  • Different types of ovulation and the influence of mating

    • Spontaneous ovulation:

      • most common on we have discussed, no male needed

      • repeated ovulation at regular intervals except during pregnancy

      • LH release is cyclic and independent of mating stimulus

    • Induced ovulation: ovulation occurs after stimulation of vagina and/or cervix, requires mating

    • non pregnant cycles for rodents: growing follicles, forming a corpus hemmorrhagicum, not pregnant, skipping formation of the corpus luteum and returning to the follicular phase of the cycle

  • The Hormone-Producing Cells of the Follicle/CL

    • Hormones during the estrus cycle

      • ovarian hormones originate from the follicle/corpus luteum

        • 3 Types of Cells in Mature Follicles:

          • oocyte or egg cell

          • the Theca

          • The Granulosa

        • Theca and Granulosa are both needed to make: Two Cell Theory

    • Theca, luteal cells conversion: turn off the enzyme that converts progesterone to androgens , happens quickly, metestrus

    • Ganulosa, luteal conversion: few days to reorganize and become functional. At diestrus both cells transform into the luteal cells and accelerate progesterone production. Diestrus

    • Both cells types must be to luteal cells to be in diestrus