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The Estrous Cycle

* The CL bulges above the surface of ovary and can be felt by palpation - the embryo will be ipsilateral to the functioning CL.

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During rectal palpation:

  • CL feels like a nipple

  • follicle is smooth 

    • max size is species specific​

    • in cows, around 2.5 cm

  • cyst is larger than normal follicle size

After ovulation, the CL makes vessel promoting factors (angiopromoters) so that a dense network of capillaries forms. A CL will regress if the animal does not become pregnant. After a few days, it becomes scar tissue known as the corpus albicans (CA). The CA has no function. It is simply an indicator of past ovulation.

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Luteolysis of the CL may be induced by PGF2a and progesterone

Cows come into heat for about 18 hours, a time during which there are observable behavioral signs. Monitoring behavior is very important, as ovulation occurs 24 - 28 hours AFTER the cow is in heat. If the heat is missed, an opportunity for conception may also be lost. It is especially crucial for AI (artificial insemination) to know when the animal came into heat. Once ovulation has occurred, the egg cannot survive long in the reproductive tract. If the animal is not bred, it will have about 17 estrous cycles in one year.

Follicular fluid starts forming

Puberty at first ovulation

blood-filled follicle

makes P4 throughout pregnancy

Luteotropic Hormone : tells CL to keep working

When the CL dies, it is called luteolysis

Graph E1

 

1. Pre-ovulatory follicle produces estrogen above threshold concentration needed to elicit a positive response from the anterior pituitary.

2. Anterior pituitary releases luteinizing hormone (LH). 

3. Estrus coincides with this LH surge. Estrus behaviors include seeking out the male, standing when pressure is applied to the back, and mounting of other females. More details are outlined below.

4. The dominant follicle ovulates within 28 hours of the LH surge, and estrogen concentrations drop.

5. The CL produces progesterone which suppresses uterine contractions.

Without inhibition by estrogen and inhibin, FSH levels are high and the first wave of new follicles begin to grow but these become atretic. 

*progesterone is also an immunosuppressant 

6. Without conception, the CL dies under the influence of hormone PGF2a from the uterine lining.

7. A second wave of follicular development occurs, and a dominant follicle develops. Inhibin selectively prevents the release of FSH, so that smaller follicles (about 99%) become atretic. 

8. Estrogen meets threshold concentration needed to induce LH surge, and the cycle repeats.

The estrous cycle is a repetitive sequence of physiological changes that occur in the female including estrus, ovulation, and changes in the uterine lining. In cows and horses the cycle takes about 21 days. Seasonal breeding animals do not come into estrus during certain times of the year. Rather, the onset of estrus and ovulation are stimulated by changes in day length (thought to be a response to levels of melatonin produced). When not cycling, animals are said to be anestrous.

Graph E1

1.

5.

7.

8.

4.

3.

6.

2.

Elevated P4 past estrous OR lack of PGF2a:

1) Pregnant

2) Pyometra

EARLY

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Detection patch unchanged

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Mingling with others in heat

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Rides other cattle

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Does not stand for other cattle

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Mucus discharge from vulva

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~ TOO EARLY TO BREED ~

STANDING HEAT

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Continue to ride others

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Stands still when mounted

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Detection patch solid color

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Tail head hair rough/worn

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Mingles with others in heat

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~ LASTS AVG. 18 HOURS ~

~ BREED 12-24 HOURS AFTER OBSERVED ~

POST STANDING HEAT

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May or may not mingle with other cattle

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Will not stand when mounted

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May or may not ride other cattle

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Clear mucus discharge from vulva

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Detection patch solid color or missing

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~ OCCURS 0-10 HRS AFTER STANDING HEAT ~

~ BREED NOW!~

Estrus Detection in Cattle

TOO LATE (for breeding)

- Bloody discharge observed

- no interest in other cattle

CIDR use in Cattle

CIDR  stands for controlled internal drug release. The device may be used as a management tool to synchronize estrous cycles or "kick-start" ovarian function in non-cycling cows. Progesterone is the active hormone, serving to 1) stimulate ovulation and 2) stimulate ovarian function in non-cyling cows.

CIDRs greatly increase profits to farmers

  • CIDR + off sync program --> conception rate almost as good as normal cycling cows

  • Adding CIDR to program doubled conception rate to the first service

  • In young heifers, conception at first service increased from 28-46%

  • Addition of CIDR decreased the cost per pregnancy by $30 because of the increased number of calves conceived

  • Initial costs are more but increase in pregnancy compensate - return on investment outweighs initial cost

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There are 2 types of normal vulvar discharge from the heifer:

- clear mucus during standing estrus

- bloody discharge during metestrus (2-3 days after ovulation)

HOW TO PLACE A CIDR IN A COW

Materials: Lube, CIDR, applicator, topical disinfectant (novisan or dermisol), paper towel

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  1. Have two buckets, one with disinfectant and one with clean, warm water. Soak the CIDR applicator in disinfectant and then rinse with water

  2.  With gloves on, handle the CIDR. It is coated in progesterone and will get on your hands. Bend the nose up and insert CIDR into the applicator with the groove up and tail down

  3. Cover the applicator with lube

  4. Wipe manure from vulva, insert device in vulva to anterior vagina and remove plunger

  5. To remove the CIDR at any time, pull the string

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CIDRs are best used to help control the onset of estrus. Progesterone prevents the cow from coming into heat. When the CIDR is removed, the hormone disappears, making estrus possible. It may also be used 7 days prior to natural service, although vaginitis is a risk.

The CL has a blood supply, but follicles do not. 

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If there is not PGF2a, the CL will continue to grow. This could either be indicative of pregnancy or pyometra.

If PGF2a is made in abnormally low quantities, the CL will last longer than in an animal with higher levels of prostaglandin. Estrous will consequently be longer than 21 days.

The key is the proximity of the ovarian artery to the uterine vein. They intertwine in the reproductive tract, so PGF2a is able to diffuse directly from the vein to the artery.

Prostaglandin, PGF2a, is largely responsible for inducing luteolysis of the CL. It is produced by the uterine epithelium and transported via the uterine vein. If the hormone were to travel all the way through the vascular system, it would not reach the ovary intact. Enzymatic action in the lung tissue digests PGF2a. So how does it get from the uterine vein to the ovarian artery?

PGF2a Production and entry into bloodstream

​TESTOSTERONE

  • responsible for development and maintenance of the male reproductive tract

  • development of secondary sex characteristics associated with masculinity, such as the crest and heavily muscled shoulders of a bull

  • factor in the normal sex drive and behavior of the male

  • increases muscular and skeletal growth

  • normal sperm formation

OVARY

FEMALE vs. MALE

 

Just as in the female, the male releases GnRH from the hypothalamus. Androgens and inhibin from the male gonads have a negative feedback response on the hypothalamus. 

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GnRH stimulates the anterior pituitary gland to release FSH and LH, and inhibin inhibits the release of FSH.

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         Leydig Cells (instead of theca cells) –                            manufacture/release testosterone & other                      androgens

  • LH promotes production of testosterone

  • Testosterone goes to the Sertoli cells and other target tissues through the blood stream

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         Sertoli Cells are responsible for sperm                          development. They respond positively to                        testosterone.

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Luteotropic Hormone : tells CL to keep working

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Estrogen

--> increases growth of mammary gland

--> uterus contractions

--> negative and positive feedback on the brain

---> development of sex characteristics

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Progesterone

--> growth of mammary gland

--> brain

--> uterus (inhibits contractions)

--> anti-estrus

--> produced by CL and placenta

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Human Chorionic Gonadotropin = found in pregnant women's urine, may be used as exogenous gonadotropin in cow

HORMONES

Hormone Functions in Female:

GnRH - +ve on anterior pituitary, stimulates release of FSH and LH

GnIH - -ve on GnRH neurons, inhibits GnRH secretion

Inhibin - -ve feedback on anterior pituitary

FSH - +ve on granulosa/luteal cells, stimulates release of oestradiol and progesterone

  • Follicle growth

LH - +ve on theca cells, stimulates androstenedione release

  • Luteinization of follicle

Androstenedione – negative feedback on hypothalamus

  • +ve feedback on tissues and skin

Oestradiol – negative feedback on hypothalamus

  • +ve feedback on anterior pituitary

  • Positive feedback on target tissues

Progesterone – negative feedback on hypothalamus

Positive feedback on target tissues

Hormone Functions in Male:

GnRH – stimulates anterior pituitary to produce FSH and LH

GnIH – Inhibits GnRH neurons from secreting GnRH

Inhibin – negative feedback on hypothalamus and pituitary gland

LH – Stimulates Leydig cells, initiating testosterone & androgen production

FSH – Stimulates Sertoli cells for sperm development

Androgens – Negative feedback on hypothalamus, goes to other target tissues

Testosterone – +ve feedback on Sertoli cells, -ve on hypothalamus

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