Female Reproduction

Reproductive Cycles


1) The Estrus Cycle


The Estrus cycle is made up of two main phases:
  • Follicular Phase
    • there is NO corpus luteum (CL) producing P4
    • folliculargenesis
    • E2 is high
    • the Follicular Phase is about 20% of the cycle
    • ovulation occurs at the end of it
  • Luteal Phase
    • there is a functioning CL
    • P4 is high
    • the Luteal Phase is about 80% of the cycle
    • it ends with the destruction of the CL which is called Luteolysis


Within the Follicular and Luteal phases there are two more phases each :

  • Follicular Phase
    • Proestrus Phase
      • formulation of follicles on the ovaries
      • major endocrine transition from a period of P4 dominance to E2 dominance
      • not ready to me mated
      • will not stand for the bull
      • E2 is still climbing so cows in this phase will often times ride other cows but will not stand yet
    • Estrus Phase
      • sexual receptivity
      • displays standing estrus or willingness to mate
      • E2 peaks at its highest
      • ends with the release of the ovum (ovulation)
  • Luteal Phase
    • Metestrus Phase
      • the CL forms and begins to secrete P4
      • will see bloody discharge sometimes- this is NOT menstruation
    • Diestrus Phase
      • longest stage of the cycle
      • sustained secretion of P4 from the CL
      • dry and tight cervix
      • it ends when the CL is destroyed (luteolysis) OR if there is a pregnancy the CL will continue to secrete P4 to sustain the pregnancy until parturition

The Follicular Phase in depth

Four major events:
1) elevated gonadotropin (GnRH) release from the anterior lobe of the pituitary
2) follicular growth and preparation for ovulation
3) sexual receptivity
4) ovulation

Recruitment= high FSH + low LH +no inhibin + no E2
Selection= low FSH + moderate LH + low inhibin
Dominance= low FSH + high LH + high inhibin
Atresia= degeneration of follicles= 90% of ovarian follicles undergo the irreversible process of atresia which occurs continuously throughout folliculogenesis.

Cows will generally have 2-3 follicular waves in every cycle but these follicles cannot ovulate until P4 decreases and E2 increases.

During recruitment, FSH increases which prompts antral follicle growth. Entering the selection phase, inhibin and E2 are produced by the follicle and inhibit FSH secretion from the pituitary. LH and FSH roles begin to switch. FSH secretion is at its lowest point while LH secretion increases.




Super Ovulation:
"Ovulation of abnormally high numbers of ova"
You can inject a cow with FSH which will allow several eggs to become large enough to ovulate. The inhibin that is produced will not be enough to inhibit the growth that the FSH will cause. Several eggs will ovulate and these eggs can be flushed from the uterus and placed in other cows.



Superstimulated ovaries of a cow and a camel. The bottom image is a cow's ovary after
the eggs ovulated leaving nine corpora hemorraghica.

What is the process of ovulation?
  • elevated blood flow
  • breakdown of connective tissue
  • ovarian contractions

What are the two types of ovulators among mammals?
  • Spontaneous Ovulators
    • they release eggs because of endocrine control and they have no need for stimulation/copulation
    • examples are horses, cows, pigs, sheep
  • Reflex or Induced Ovulators
    • they only ovulate when their cervix and/or vagina a stimulated during copulation
    • rabbits, cats, ferrets, minks



The Luteal Phase in depth

Three major events:
1) the transformation of follicle tissue into luteal tissue after ovulation (luteinization)
  • Granulosal cells(which change to large luteal cells-LLC) and Theca Interna cells (which change to small luteal cells-SLC) mix and change to luteal cells.  This is governed by LH.
2) growth and development of the corpus luteum so that it produces high quantities of progesterone (diestrus)
  • LLC hypertrophy (growth in size) and SLC hyperplasia (growth in number of cells).
  • Both cells types produce P4 and the bigger the ovulatory follicle, the more luteal cells there are to produce more P4. Smaller follicles=lower output of P4 from the CL. Failure to maintain a pregnancy is sometimes due to low P4.
  • The target tissues of Progesterone:
    • Hypothalamus
      • P4 holds down GnRH, and as a result, LH and FSH as well. This prevents high E2, estrus, GnRH, and the LH spike and therefore ovulation
    • Uterus
      • P4 quiets the myometrium causing it to have less contractions (EXCEPT in the mare)
      • P4 causes the endometrial tissue to have enhanced secretions to support a pregnancy
    • Mammary Gland
      • P4 promotes the development of the alveoli for milk production later
  • Progesterone synthesis requires cholesterol and LH
3) destruction of the corpus luteum (luteolysis) resulting in a subsequent follicular phase
  • PGF, the hormone that destroys the CL comes from the uterus, except in primates it comes from the CL itself. In mammals (other than primates), if you removed the side of the uterus where there was a functioning CL, the animal would remain as if it was pregnant. If you removed the opposite side of the uterus as the one with the CL, the animal would continue to cycle and luteolysis would occur. So it is important to understand that the side of the uterus where the CL is, is the side that produces PGF to destroy the CL. How this happens is there is a counter-current exchange (this is present in the cow, ewe, sow, but NOT the mare) between the uterine vein and ovarian artery (kind of like the pampiniform plexus in the male scrotum). They are wrapped so tightly together that the hormones can pass through the vessel walls. First, in the CL the LLC secrete oxytocin and OT travels to the uterus and docks into the receptors. The OT has a positive feedback mechanism on PGF in the uterus and increases the release and production of it. This is were the counter-current exchange comes in and the blood carries PGF to the CL. PGF docks into the receptors on the CL and opens up a "gate" where calcium floods in from the blood stream and kills the luteal cells. The CL has to be a certain age before PGF can kill it:
    • Horses: 4-5 days
    • Cows: 5-6 days
    • Pigs: 12-14 days
  • The immune system helps clean up the dying luteal cells

Estrus Synchronization:

Horses:
  • 5 day old CL---shot of PGF---estrus 3-5 days later---ovulation about 8 days later
    • Lutalyce- 1 ml/500 lbs. (usually 2 ml. total for the mare but a cow dose is 5 ml.) In the mare this amount causes heavy sweating, muscle contractions, and cramping for at least an hour. Instead, you could give the mare .2 ml. (1/10 dose) twice, 24 hours apart and then there are no side effects and it still destroys the CL.
    • Feed regumate for 8-10 days---shot of PGF. More expensive and less precise.
    • Give 10 mg. E2 and 150 mg. P4 shot for 10 days---10th day give PGF---estrus begins about day 14---18th day give 2500 IU of HCG IV if follicle is at least 35mm in size---ovulation will occur on 19th/20th day in 90% of mares. This is supposedly the most precise and successful sync.
Pigs:
  • Gilts (prepuberial) --- PG 600 (eCG and hCG) you can't use PGF because they have no CL
  • Sows--- wean piglets off of her and it causes estrus in 3-5 days
Sheep:
  • CIDRS--insert for 5 days---give PGF at the end
  • MGA (oral P4) feed for 8 days---turn rams in. Also increased feed/energy will increase ovulation and they often will have twins and triplets
Cows:
  • PGF to kill the CL. If there is one to kill they will come into heat 3-5 days later
  • CIDR and a shot of PGF at the end
  • Feed MGA for 14 days--give PGF 19 days after---kills CL and they come into heat 2-3 days later
  • OvSynch (cows only, time breeding, no heat checking) Day 0 GnRH--Day 7 PGF--Day 9 GnRH (forced ovulation)--Day 10 A.I.
How to start Mares cycling in their off season:
This process takes about 2 months. If you want the mare to foal in January, she has to get bred in February which means you should start the artificial lights on her about Thanksgiving. She needs 14-16 hours of daylight and it is better to add light at the end of the day rather than the beginning. The amount of light needs to be enough to read a newspaper by. She will begin to shed her winter hair coat and start to show estrus. Stallions are also effected by day length and have reduced fertility in the winter months.


Types of Estrous Cycles

There are different classes of animals depending on when they cycle:

  • Polyestrus (poly means many)
    • cycle year around
    • cows, cats, pigs, rodents
  • Seasonal Polyestrus (long day breeders=spring)
    • their cycle is approximately April-July
    • they have several cycles in a season
    • horses
  • Seasonal Polyestrus (short day breeders=fall)
    • their cycle is approximately September- November
    • they have several cycles in a season
    • ewe, doe, elk, nanny
  • Monoestrus (mono means one)
    • they cycle once or maybe twice a year
    • they are in heat for awhile
    • then months go by before they will cycle again
    • dog, wolf, fox, bear

The hormone that is the key player in the seasonal polyestrus long and short day breeders is melatonin.

Anestrus

Definition: A condition in a female when she does not display estrus

Causes for Anestrus:
  • pregnancy
  • lactation
    • lactation delays estrus
    • in studies they claim that a cow sucked more than 2-3 times a day will stay in anestrus longer but will eventually start cycling again
    • the cow sucked just once or twice a day will cycle much sooner
    • if your cows are not cycling due to lactation the calves can be removed from the cows for 48 hours and the cows will generally start cycling (do this 5-6 weeks after calving)
  • presence of offspring
  • season effects (photoperiod)
  • stress/discomfort
  • poor nutrition
  • pathology/disease
    • infection such as pus in the uterus. The uterus doesn't send the PGF2 to kill the C.L.
    • 3 kinds of cysts on the ovaries:
      • Follicular Cyst
        • a big follicle on the ovary that doesn't ovulate. E2 is high and P4 is low. They keep coming in heat but don't ovulate the ovum. Can give GnRH to cause another follicle to ovulate
      • Luteal Cyst
        • corpus luteum tissue that doesn't go away. P4 is high and E2 is low. They don't show heat. Can give PGF to kill the C.L.
      • Benign Cyst
        • a fluid filled cyst that doesn't seem to effect the reproduction process. The female cycles normally and can get pregnant.
    • Cysts occur more in dairy cattle than in beef cattle. Sometimes if left alone they fix themselves.
Silent heat or ovulation- after P4 has been dominant during gestation, the brain sometimes doesn't have many receptors for E2 so it causes a silent heat or an estrus without all the typical behavioral signs. The first heat after puberty and the first heat after parturition are common times for silent heat to occur. The following estrus after the female will generally show signs of behavorial heat.

Length of Estrous Cycle:

Cow- 21 days (range 17-24d)
Mare- 21 days (range 15-26d)
Ewe- 17 days (range 13-19d)
Sow- 21 days (range 17-25d)

Duration of Estrus (heat):

Cow- 15 hours (6-24h)
Mare- 7 days (2-12d)
Ewe- 30 hours (18-48h)
Sow- 50 hours (12-96h)


2) The Menstrual Cycle

A menstrual cycle consists of the physiological events that occur between two successive menstrual periods (about 28 days in women). At the conclusion of the luteal phase in the menstrual cycle, the endometrium is sloughed to the exterior (if the female is not pregnant). No endometrial sloughing occurs in animals with estrous cycles. The timeline for the cycle begins and ends with menses, not ovulation or estrus. The follicular and luteal phase are the same length (about 14 days). "Menopause occurs in women when their ovarian supply of follicles is depleted."(Senger, 158)

  
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