REPRODUCTIVE HORMONES IN THE MARE PART II


 

Follicle Stimulating Hormone (FSH)

FSH is a glycoprotein hormone, produced in the anterior pituitary and released into the blood in response to GnRH (GnRH is released from the hypothalamus).

·      Glycoprotein hormones are protein hormones with carbohydrate chains. This carbohydrate group protects the FSH molecule from rapid breakdown and metabolism. This shielding group adds to the hormone's half-life, which in most species is several hours.

·      The FSH is transported to the ovary, enters the developing follicle, and stimulates the granulosa cells to produce oestrogen. Granulosa cells are a type of cells lining the follicle that will be described in more detail later.

·      FSH is also responsible for growth of the follicle, involving both an increase in the number of granulosa cells and secretion of follicular fluid.

·      Peak blood concentrations of FSH occur in mid-dioestrus approximately 10 days before ovulation.  This stimulates initial follicular growth (not usually palpable within the ovaries).  Low concentrations of FSH are found during oestrus.

·      As the follicle grows, the granulosa cells secrete a hormone called inhibin into the follicular fluid. This hormone is carried, through the vascular system, back to the anterior pituitary where it suppresses the secretion of FSH.

 

Luteinizing Hormone (LH)

            Similar to FSH, LH is produced in the anterior pituitary and is a

 

glycoprotein hormone, although the carbohydrate group is different from FSH.

·      LH is released due to the influence of GnRH from the hypothalamus. The LH is transported to the ovary and stimulates the theca cells surrounding the follicle to produce testosterone.

·      Circulating concentrations of LH are low in the late luteal phase.  Initial rise occurs in early oestrus and reaches peak values after ovulation.  In the mare, this hormone stimulates major follicular growth, maturation and ovulation.  Values decrease as progesterone concentrations (from the developing CL) rise.

·      Both FSH and LH have two modes of delivery. One mode is "tonic", which maintains low blood hormone levels.

·      The second mode is a "surge", which results in a sharp increase or peak in the level of the gonadotropin. Both of these delivery modes are required at various times throughout the estrous cycle.

 

Testosterone

In females, testosterone is synthesized in a layer of cells in the follicle called the theca interna. These cells surround the follicle wall. Cholesterol is used as a substrate for testosterone synthesis. Most of the cholesterol used in the synthesis of the steroid hormones is from the blood.

One of the steroid-manufacturing enzymes converts cholesterol into pregnenolone and progesterone. These steroids are converted into the androgens, androstenedione, and testosterone.  Therefore, female cells produce androgens, although the levels in the body are low. The androgens pass through the follicle wall and most are converted to estradiol by the granulosa cells using an enzyme called aromatase.

 

The estrogen produced by the granulosa cells may pass into the follicle fluid or pass back out the follicle through the follicle wall and theca cells. It can be picked up by the vascular system and transported throughout the body.

 

Oestrogens

The main physiological role of oestrogen is the development and maintenance of the female sex organs.

Concentrations of oestrogen are low during most of cycle but rise in early oestrus to reach peak values 48 hours before ovulation

 

·      It stimulates protein synthesis and mitosis (cell division) of the many reproductive organs that are oestrogen-dependent.

·      The secondary sex characteristics of the female, including changes in body conformation and growth, hair or plumage distribution, and mammary gland development are under oestrogen control.

·      After progesterone priming, estrogen is responsible for the induction of sexual receptivity to mating in the female.

·      The decline of progesterone and the increasing concentration of estrogens induce behavioral oestrus.

·      Additional functions of estrogens are:

 

1  induce pronounced vascularization, increased blood flow and hyperemia (engorgement of blood), water and salt retention, and edema of the uterus, vagina, cervix and vulva.

 

2.  cause uterine endometrial cells to increase in height and undergo mitosis (hypertrophy-cell growth, and hyperplasia-increase in cell number)

 3.increase both the amplitude and frequency of muscle contractions

 

4.  cause the uterine endometrial glands to secrete a mucus which flushes the tract

 

5  stimulate duct growth and cause development of the mammary gland. This effect is seen only at the end of pregnancy in the mare.
 

6.  reduces FSH secretion in mid-cycle but has a positive effect on LH secretion (induces the LH surge)

 

During the oestrous cycle, the increasing estrogen levels released by the growing follicle feeds back to the hypothalamus and pituitary. This positive feedback causes increased secretion of FSH and a larger surge of LH. This surge of LH has several functions:

 

1. LH initiates enzymatic changes in the follicle wall which subsequently cause breakdown of the follicle wall and ovulation of the oocyte

 

2. LH activates the oocyte within the Graafian follicle to continue meiosis

 

3. LH causes granulosa cells within the follicle to undergo luteinization, forming luteal cells that subsequently secrete progesterone (corpus luteum formation).

 

Soon after the LH surge, oestrogen levels reach their peak and the mare usually shows strongest signs of behavioral oestrus (she becomes very sexually receptive).

Following mating, the spermatozoa are carried into the cervix and uterus due to rhythmic contractions of the female reproductive tract. In the horse, as in many other species, mating usually precedes ovulation so that the spermatozoa are at the site of fertilization and ready to penetrate the coat of the ovulated egg.


The contractions involved in sperm transport are the result of the influence of oestrogen and oxytocin on the uterine and oviductal myometrium.

 

Progesterone

Progesterone is secreted by the luteal cells within the corpus luteum.

In contrast to the follicles, the corpus luteum is filled with blood vessels. Under the influence of progesterone, the superficial endometrial cells of the uterus multiply and secrete "uterine milk" for subsequent nourishment of the embryo when it arrives in the uterus.

Progesterone is the primary steroid produced by the corpus luteum of the cycling animal and by the corpus luteum of pregnancy.

Secreted initially by the corpus haemorrhagicum and later by the corpus luteum.

Blood concentrations start to rise after ovulation and reach a peak 5 to 9 days later. 

During the first 4-5 days the developing CL is refractory to prostaglandin. High concentrations of progesterone maintained until day 14, when the CL is usually lysed by endogenous PG released from the uterus. 

 

Functions of Progesterone

·      prepares the uterus for implantation and pregnancy

·      develops alveoli of mammary gland

·      inhibits the rise of LH that causes ovulation

In later pregnancy in the mare, the placenta also produces large amounts of progesterone. Progesterone is necessary for maintaining pregnancy in all mammals and abortion will occur if progesterone level is insufficient. While it was once popular to provide progesterone to mares that were habitual aborters, this process is now uncommon since there is very little indication that low progesterone levels are a common cause of abortion.

 

Prostaglandin

If pregnancy does not occur following ovulation, the uterus will produce prostaglandin F2 alpha.

This prostaglandin will lyse (regress) the corpus luteum causing a drop in progesterone production. This allows oestrogen levels to increase and preovulatory follicles to develop. The oestrous cycle starts over once again.

 

Oxytocin

Oxytocin is a 9 amino acid peptide hormone that is synthesized in the hypothalamus. It passes down the nerve axons from the hypothalamus into the posterior pituitary where it is stored.

It is released in response to neural stimuli such as suckling and mating. Oxytocin released at mating stimulates the uterine contractions that help transport sperm through the uterus, and into the oviducts.

(Oxytocin is also involved in expelling the foetus at birth and in the contraction of the myoepithelial cells that eject milk from the alveoli of the mammary gland.)

 

Prostaglandins E2 and F

Prostaglandins are a group of hormones that are fatty acids.

·      They are found in many tissues of the body but were first isolated from human semen.

·      There are many different types of prostaglandins and these have roles in functions ranging from pain and inflammation to reproduction. Granulosa cells of the preovulatory follicle produce large quantities of both prostaglandins E2 and F2 alpha that may have a role in ovulation.

·      Prostaglandins are also involved in labor. Administration of prostaglandin F2 alpha to a pregnant animal results in induction of labor, abortion, and destruction of the corpus luteum.

·      In the cycling animal, prostaglandin F2 alpha is secreted by the uterine endometrial cells.

·      This secretion occurs in the late luteal phase if the mare is not pregnant.

·      Prostaglandin F2a released from the endometrium.  Released 13 to 15 days after. Enters general circulation and if quantities reaching the CL are sufficient, causes luteolysis

·      As it leaves the uterus via the uterine vein, the prostaglandin crosses through the wall of the vein and enters the ovarian artery. This artery is wound tightly around the uterine vein so the prostaglandin can diffuse into the ovarian circulation, by-passing inactivation in the lungs. Over 95 % of prostaglandins are inactivated by one pass through the lungs.

·      Upon reaching the ovary, prostaglandin F2 alpha causes regression of the corpus luteum.

 

 

The Role of These Hormones in the Oestrous Cycle

The estrous cycle is repeated throughout the reproductive life of an animal unless it is interrupted by pregnancy. However, if pregnancy occurs, the oestrous cycle is resumed soon after parturition.

The oestrous cycle is initiated at puberty, following the winter anoestrus period or following foaling by the secretion of GnRH from the hypothalamus.

Upon reaching the anterior pituitary, GnRH stimulates the release of the gonadotropins FSH and LH into the vascular system.

LH is transported to the theca cells of the developing follicle, stimulating the production of testosterone. FSH is also transported to the follicle and it causes the conversion of testosterone to oestrogen by the granulosa cells.

As the follicle grows, inhibin is now produced by the granulosa cells and it is carried by the bloodstream to the anterior pituitary where it suppresses FSH production.

Oestrogens are also produced by the granulosa cells of the follicle and they prepare the female tract for mating. At high levels, oestrogens also act on the central nervous system to cause the psychological manifestations of oestrus.

When estrogen levels become very high, they have a positive feedback on LH release, producing even more estrogen. Because of this positive feedback loop, LH levels continue to rise producing an LH peak. This surge of LH initiates changes in the follicle wall that result in ovulation.

Along with the effects of LH on ovulation it also transforms the granulosa cells to luteal cells (process is called luteinization). Luteal cells produce progesterone instead of oestrogens. These luteal cells form the corpus luteum.

Progesterone augments the effects of oestrogen on the female tract, thus preparing the tract for pregnancy. The blood level of progesterone remains very high, and the embryo is transported into the uterus where it will implant. Progesterone is necessary for implantation to occur.

If the embryo does not produce a signal to the uterus, prostaglandin F2 alpha is release from the uterine endometrium. It travels into the uterine vein, then is passed into the ovarian artery and carried to the corpus luteum. It results in regression of the corpus luteum and a reduction in progesterone.

The decrease in progesterone secretion allows the hypothalamus to secrete GnRH and a new wave of follicles starts to develop, starting the cycle over. As long as the progesterone level is high (during pregnancy or birth control pills or oestrous synchronization agents), oestrus is prevented and the cycle is arrested.

 

 Inhibin

1.Secreted by granulosa cells in the growing follicle.

2.Function: inhibits FSH secretion without altering LH secretion 

 

Placental Hormones

•Equine Chorionic Gonadotropin (Pregnant Mare Serum Gonadotropin, PMSG)

1.Contains mainly FSH-like activity but also some LH-like activity.

2.Has a longer half-life than FSH.

3.Found in the blood and not the urine. 4.Function: stimulates follicular development during pregnancy in the mare.

•the LH-like activity stimulates some developing follicles to ovulate and form accessory CLs

 

Gonadal Polypeptide Hormones

Relaxin

1.A protein hormone. It is somewhat similar in size and structure to insulin. 2.Secreted by CL during pregnancy. 3.In the horse it is also secreted by the placenta.

4.Generally for relaxin to have effect, it requires that tissues first be exposed to estrogens for its effects.

5.Functions

•cervical dilation

•inhibits uterine contractions

Melatonin

•Secreted from the pineal gland in the brain

•Is a modified amino acid

•Functions to integrate effects of light on reproductive processes.

Acts on the hypothalamus to influence the output of GnRH. Increasing levels of melatonin are produced in the shorter days. This inhibits the release of GnRH from the pituitary. In longer days, melatonin secretion decreases and GnRH secretion can increase.

 

Review Questions:

1.Describe the following:

•endocrine gland

•exocrine gland

•hormone

•endocrinology

2.What is the function of the endocrine system?

3.Draw a diagram of a mare and indicate all the endocrine glands and the hormone(s) each produce that have an affect on reproduction.

4.What are the three chemical structure groups that hormones fall into? 

5. What sex steroid is the most prevalent in the male?

6. What two sex steroids are most prevalent in the female?

7. Inhibin decreases the production of what hormone?

8.List two hormones in each of the following categories: proteins, modified amino acids, sex steroids.

10.List the source and a function for each of the following hormones: LH, FSH, oestrogen, progesterone, relaxin, PGF, inhibin, and GnRH

 

Control of the Mare’s Reproductive System: Don’t Forget the Brain.

 

The reproductive system in all mammals is controlled by the central nervous system.

Visual, auditory, olfactory, and tactile cues feed into the higher brain centers, which in turn send stimuli to the a brain centre called the hypothalamus. For example, day length plays a prominent role in breeding activity of the mare while auditory, olfactory, and tactile stimuli are important in pigs. Within the hypothalamus, the information is processed, amplified, and converted into a humoral substance (a hormone) that is transmitted to the anterior pituitary gland.

Within the anterior pituitary, the signal is further amplified and transmitted, by way of the gonadotrophic hormones, to the gonads. The gonads, ovaries, and testes respond to these hormones in many ways. The most understood gonadal reaction is the secretion of sex hormones, which act on many target tissues including the brain and pituitary gland.