REPRODUCTIVE HORMONES AND THEIR FUNCTIONS

Reproductive hormones controls the reproductive development of the body i.e. the regulation

of reproductive related process such as sperm and egg production. Reproductive hormones

often have multiple roles and operate via negative feedback systems. Hormones are key to

reproductive health in all aspects of a woman’s sexual life. They regulate menstruation,

fertility, menopause, and sex drive (libido). The main hormones affecting the menstrual cycle

and fertility are produced by glands in the brain and by the ovaries. The information below

will provide the main reproductive hormones in domestic species and their functions.

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Gonadotropin Releasing Hormone (GnRH)

GnRH is a neuropeptide (a decapeptide) that is produced in the hypothalamic

surge and tonic centres. In the male and the female, the target tissue is the anterior pituitary

gland, specifically Gonadotroph cells. In males and females, secretion of GnRH results in

the release of Follicle Stimulating Hormone (FSH) and Leutinising Hormone (LH) from

the anterior pituitary gland. 

Luteinising Hormone (LH)

LH is a type of glycoprotein that is produced in the anterior pituitary via gonadotroph cells

and serves to regulate the function of the gonads. In

males’ LH stimulates the production and secretion of testosterone from the testes via leydig

cells. In females LH stimulates the production of oestrogens and progesterone from the ovary

via theca interna cells and luteal cells. Concentrations of LH increase during ovulation and

with the formation of the corpora lutea with progesterone secretion. The secretion of LH is

regulated via the secretion of GnRH

Follicle Stimulating Hormone (FSH)

FSH is a type of glycoprotein that is produced in the anterior pituitary via gonadotroph

cells. FSH secretion is regulated by GnRH from the hypothalamus. The target tissue of FSH

in males are the sertoli cells within the testes and in the female the granulosa cells of the

ovary. FSH stimulates the maturation of germ cells within the testes and ovaries. In the

female it also stimulates follicular development and oestradiol synthesis. 

Prolactin (PRL)

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Prolactin is a protein that is produced from by the anterior pituitary via lactotroph cells.

This hormone exerts a stimulatory effect on milk synthesis within the mammary glands. It

has also been shown to have some degree of gonadal function in some domestic species and

rodents. In birds increased concentrations of prolactin have been linked with brooding

behaviours and the associated metabolic changes that birds undergo during brooding. 

Oxytocin (OT)

OT is a neuropeptide (a octapeptide) which is synthesised in the hypothalamus and stored in

the posterior pituitary. OT is primarily involved in upregulating the activity of smooth muscle

cells in the uterus and the smooth muscles surrounding the alveoli ducts of the mammary

glands. At parturition, OT causes strong contractions from the myometrium. OT is also

essential for 'milk let-down' in most domestic species. OT binds to receptors in the membrane

of target cells which activates phospholipase C. OT facilitates the generation of the driving

pressure behind pushing the milk towards the large excretory ducts and the teats.

Estradiol (E2)

Estradiol (E2) is a steroid hormone and is part of the oestrogens group of hormones and is the

principle oestrogen in females. Estrone and estriol are chemically similar to estradiol but are

found in lower concentrations and have a lower estrogenic activity. Production of oestrogens

occurs in the ovary via granulosa cells, the placenta and the Zona reticularis of the adrenal

cortex. In males in it is produced in sertoli cells found in the testes. Estradiol is synthesised

from cholestrol. 

Progesterone (P4)

Progesterone is a steroid hormone that along with oestrogens is based on a cholesterol

molecule produced by the corpus luteum and the placenta using cholesterol as the base

molecule. Progesterone is produced by the corpus luteum as well as by the feto-placental unit

and in the zona reticularis of the adrenal cortex (to a lesser extent). More detailed information

regarding corpus luteum formation andregression please use the links. Progesterone prepares

the uterus for reception of fertilized oocytes and is transported via the blood bound to plasma

proteins. Progesterone also prepares the mammary tissues for milk production as well as

inhibiting female reproductive behaviours associated with oestrous. 

Testosterone (T)

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The male sex hormone is called testosterone and this hormone is required

for spermatogenesis. Testosterone is a steroid hormone that is produced in the leydig cells

within the testes. A relatively high concentration of testosterone is maintained within the

testicular tissue and testosterone is circulated around the body by diffusion of the hormone

from the spermatic cord into the testicular veins and arteries. The primary action of

testosterone is anabolic growth, spermatogenesis promotion and promotion of secretion from

the accessory sex glands. 

Male sex hormones are regulated by negative feedback systems that operate at various levels

within the male sex hormone system. The starting point for the production of testosterone

(and therefore the production of spermatozoa)is the hypothalamus. The hypothalamus

contains neuroendocrine cells that are capable of secreting a substance called Gonadotropin-

releasing hormone or GnRH. GnRH stimulates basophilic cells in the adenohypophysis, via

the "portal system" to secrete two intermediate hormones within the male sex hormone

cycle; Luteinizing hormone (LH) and Follicle-Stimulating Hormone (FSH). 

Inhibin

Inhibin is a type of glycoprotein that is synthesised within the granulosa cells of ovarian

follicles in females and in sertoli cells located in the seminiferous tubules within the testes in

the male. In both males and females the target organ for inhibin is the adenohypophysis,

specifically the gonadotroph cells (basophilic cells). 

In the male inhibin production is stimulated via androgens. Inhibin inhibits FSH secretion,

which together with decreased concentrations of LH and testosterone results in decreased

spermatogenesis and therefore decreased sperm output and quality. 

In females some studies have suggested that inhibin may also be produced by the placenta. In

females inhibin inhibits FSH secretion. It does however not have any effect on the secretion

of LH. When inhibin is secreted, a relatively higher concentration of LH is secreted from the

anterior pituitary gland than FSH. Therefore during follicle development, the increased LH

concentration causes cessation of the recruitment of further follicles under the effect of FSH.

The hormonal changes resulting from the production of inhibin cause some of the previously

recruited follicles to undergo atresia.

Activin

Activin is a glycoprotein that is produced within granulosa cells in females and sertoli cells in

the male. Activin is thought to play an almost directly opposite role to that of inhibin and is

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involved in many physiological functions including stimulation of FSH synthesis and other

roles including cell proliferation, cell differentiation, apoptosis and homeostasis. 

The target tissue for activin in the male is the epididymis where it enhances spermatogenesis

via increased FSH secretion. Activin also enhances the effect of LH on the testes. 

In the female activin has an effect on the anterior pituitary gland, specifically on gonadotroph

cells, resulting in increased FSH secretion. The increased concentrations of activin results in

increased FSH binding on the female follicle and FSH-induced aromatisation (increased

synthesis of oestrogens). Activin also enhances the action of LH in the ovary. 

Prostaglandin F2α

Prostaglanin is a C2O fatty acid and is produced within the uterine endometrium and vesicular

glands. Estradiol stimulates prostaglandin synthesis while progesterone inhibits it. The target

tissue in the female is the corpus luteum, uterine myometrium and ovulatory follicles. In the

female PGF2α cause luteolysis and can also cause the induction of tone and contractions

within the uterus. It plays an important role in partuition in ruminants. 

If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved

where concentrations of PGF2α remain below a threshold level allowing the corpus luteum to

continue to secrete progesterone and thus maintain pregnancy. There are two main factors

involved in the regulation of uterine secretions of PGF2α; oxytocin secretions from the corpus

luteum and molecules secreted by the developing embryo that facilitate the maternal

recognition of pregnancy. 

Prostaglandin (PGE2)

PGE2 is another form of prostaglandin that is produced by the ovary, uterus and embryonic

membranes. This form of prostaglandin also has other important roles including vasodilation,

smooth muscle relaxation, and inhibition of the release of noradrenaline from sympathetic

nerve terminals.

In females it's target tissue is the cervix (it is a potent cervical dilator), corpus luteum and the

oviduct where it helps induce ovulation and the secretion of progesterone from the corpus

luteum. PGE2 also plays an important role during labour where it aids the softening of the

cervix in animals with a soft-type cervix(equine and human) and aids stimulation of uterine

contractions. It can thus be used to prepare the tract for parturition.

Human Chorionic Gonadotrophin (hCG)

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hCG is a form of glycoprotein that is synthesised within the trophoblast cells of a blastocyst.

hCG is particularly important in primate reproduction where it has a similar effect to LH in

stimulating the continued production of progesterone and oestrogens. This represents part of

the system involved in foetal-maternal communication and pregnancy recognition. Primate

blastocysts therefore produce hCG in relatively high concentrations during the first 3 months

of pregnancy. hCG has also been suggested to play a role in defence of the embryo from the

maternal immune system during the initial stages of pregnancy. In males hCG increases the

growth of the foetal testes.

Equine Chorionic Gonadotrophin (eCG)

eCG is a form of glycoprotein that is produced from chorionic girdle cells. Chorionic tissues

in horses as well as primates also form hormones. eCG is formed in foetal endocrine cells and

is found within the maternal circulation. eCG is thought to play a similar role in horses to

hCG in primates in terms of pregnancy recognition. Foetal production of eCG is highest

between 30-70 days of pregnancy. The primary target of eCG are the ovaries where they

faciliate the formation of the accessory corpora lutea and ensure that progesterone production

is maintained. eCG is also thought to stimulate follicular growth and ovulation in the horse. If

eCG is given to other species it acts in a similar manner to FSH and therefore eCG is often

used to induce super-ovulation in species where a large number of oocytes are required for

embryo transfer.

Placental Lactogen (PL)

Placental lactogen is a form of protein that is produced by the placenta and is chemically

close in composition to growth hormone. The primary target tissue of PL are the mammary

glands where they stimulate the growth of alveoli during pregnancy. 

PL is also referred to as Chorionic Somatomammotropin (CS).

Relaxin

Relaxin is produced mainly by the corpus luteum in most species and in the placenta (main

contributor in the equine) and ovaries throughout pregnancy. During pregnancy, relaxin

prevents the initiation of uterine contractions, together with progesterone. Relaxin

accumulates troughtout pregnancy and is released in lare amounts a few days before partus.

Its target organs are the cervix, vagina, pubic symphesis and related structures. Relaxin is

responsible for the softening and relaxation of connective tissues in the cervix, muscles and

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ligaments in the pelvis prior to parturition. Estradiol priming is required for this. This

relaxation of tissues is performed in conjunction with prostaglandin.

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