Reproductive Hormones Overview - Anatomy & Physiology

Introduction

Reproductive hormones often have multiple roles and operate via negative feedback systems. The information below will provide the main reproductive hormones in domestic species and their functions.

Please note that due to the complexity of the interactions of some of the hormones noted below, it has not been possible to fully explain the effects of the hormone on this page. Therefore this reproductive hormones overview should be utilised alongside more detailed information on specific physiology such as the reproduction pages which can be found within the WikiAnatomy & Physiology section on the top of the bar on the left.

Reproductive Hormones

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.

GnRH-producing neurons are stimulated into production in response to spontaneous rhythms and by sensory impulses from sensory inputs derived from the external environment. Alterations in the internal conditions of the body can also result in altered GnRH production. For example in some species such as the sheep, there is seasonal sexual activity and the cerebral cortex, hypothalamus, pituitary and testes interact to regulate functions further along the signalling chain.

In females when the oestrogen concentration prior to ovulation reaches a certain threshold, large quantities of GnRH are released in the form of a surge. This results in a corresponding peak in LH that stimulates ovulation. In females this surge centre is often called the preovulatory centre. In males this surge centre becomes inactivated during fetal life due to the brain maturation effects of estradiol (see section below) being able to pass through the blood brain barrier in males, please see the reproductive development of the brain for more details. In males there are between 4-12 GnRH peaks per day. Plasma concentrations of LH peak approximately 10mins post GnRH surge.

Although the hypothalamus via GnRH stimulates the secretion of LH and FSH, it cannot regulate LH and FSH independantly. Therefore another hormone produced from the developing ovarian follicle in the female and sertoli cells in the male acts as a negative feedback mechanism for FSH. Sex hormones also alter the level of production of GnRH from the hypothalamus via a negative feedback system. High concentrations of progesterone or testosterone will reduce the secretion of GnRH and also therefore the secretion of LH and FSH.

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 (see earlier section).

As shown previously, in males there are between 4 to 12 GnRH pulses per day and this therefore means that LH also peaks throughout the day. During these peaks, the production and secretion of testosterone increases. Testosterone secretion also is pulsatile.

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.

In the male FSH also stimulates the secretion of inhibin which has a negative feedback directly to the anterior pituitary. Although GnRH is released in a pulsatile fashion and the other gonadotropic hormone LH is therefore also pulsatile, FSH concentrations do not fluctuate as much as that of LH. This is because of the added regulatory feedback mechanism of inhibin within the regulatory pathways for FSH secretion.

Prolactin (PRL)

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.

Prolactin secretion is regulated by the hypothalamus which produces several neurohormones that affect prolactin concentrations. The most important within this is dopamine (or prolactin inhibitory hormone, PRL-IH) which exerts a totally dominant inhibitory action on prolactin synthesis. The hypothalamic regulation of prolactin secretion is via signals from the central nervous system. Prolactin synthesis is increased when the mother is suckling via a reflex stimulation of the teats. This stimulation reflex reduces the secretion of dopamine and increases the hormone prolactin releasing hormone (PRL-RH). Once prolactin binds to it's target receptors within the mammary gland cells, it activates an intracellular tyrosine kinase. When this occurs in the developing animal this binding can also cause the differentiation of mammary epithelial cells during pregnancy. The half-life of prolactin is approximately 20mins.

Estradiol can also have an effect on the prolactin producing cells within the anterior pituitary and is responsible for increased concentrations of prolactin in females undergoing puberty and may also contribute to the increased concentrations during late pregnancy.

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

Oestrogens have a number of functions related to reproduction and other areas of physiology. In relation to the reproductive role of oestrogens, they stimulate follicular growth and maturation, induce the female to begin displaying oestrous behaviour to facilitate mating, prepare the external genitalia for copulation and create favourable conditions for the development of fertilised egg cells. Oestrogens also contribute to the growth and development of mammary tissue and prepare the uterus for parturition.

Where oestrogens stimulate growth of follicles in the ovaries, oestrogens secreted from the ovary in the follicular phase (proestrous and oestrous) lead to hypertrophy of the epithelium and the endometrium. Secretory glands within the uterus enlarge and secretion is initiated leading to thickening of tissues. The blood vessels supplying the uterus and external genitalia dilate and blood flow to these areas increases significantly. Oedema occurs within the uterus and surrounding connective tissues. Oestrogen also causes increased uterine muscle tone. In the cervix oestrogens stimulate increased mucus secretion and the vaginal epithelium becomes keratinised.

In males the target tissue is the brain where it causes maturation of the brain during development. This maturation process ensures the appropriate development of male sexual behaviours. E₂ in the male also inhibits long bone growth.

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 in the corpus luteum. More detailed information regarding corpus luteum formation and regression please use the links. Progesterone prepares the uterus for reception of fertilised 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.

The concentration of progesterone increases after ovulation increasing the growth of glands found in the endometrium resulting in increased secretion. These secretions include mucin, carbohydrates and specific proteins that are designed for nourishment of the embryo prior to implantation. Progesterone also stimulates the growth of the endometrium and stabilises smooth muscle cells to ensure that they do not contract during foetal development. Once near term, the concentration of progesterone decreases, altering the ratio between progesterone and oestrogen. This stimulates myometrial activity and prepares the uterus for parturition.

Progesterone During Pregnancy

During pregnancy the plasma concentration of progesterone is maintained at an elevated level. Progesterone also inhibits secretion of GnRH and also prevents the ovulation of follicles during the luteal phase and during pregnancy. The exception to this rule is the mare in which the progesterone concentration falls during the later stages of pregnancy.

It is possible to use the relative concentration of progesterone as an aid to pregnancy diagnosis, for example in cattle. However, for a definitive diagnosis a high level of progesterone is required on two separate samples due to the overlap between the luteal phase and pregnancy.

Testosterone (T)

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 cells in the anterior pituitary, via the "portal system" to secrete two intermediate hormones within the male sex hormone cycle; Luteinizing hormone (LH) and Follicle-Stimulating Hormone (FSH).

The secretion of GnRH is pulsatile and can vary greatly throughout the day and/or year, and therefore the secretion of LH and FSH are also pulsatile (although the plasma concentration of FSH does not fluctuate as much as LH due to the effect of Inhibin, see below). The activity of GnRH neuroendocrine cells is determined by spontaneous rhythms and by sensory impulses. Cycles such as seasonal sexual activity are controlled by this pulsatile system. In male animals there are generally 4 to 12 GnRH pulses per day.

Testosterone Regulation

When LH binds to the Leydig cells, it stimulates the cellular messenger cAMP to activate protein kinase A. Protein kinase A undergoes a series of phosphorylations that in turn activate a series of enzymes that synthesis testosterone from the cholesterol base molecule. A portion of the testosterone produced in the Leydig cells diffuses into the Sertoli cells that are positioned adjacent to the Leydig cells in the testes but seperated by a basal lamina. This secreted testosterone is converted to to the female sex hormone estradiol in the Sertoli cell and as with the testosterone, a proportion diffuses into the blood, becoming part of the negative feedback system for LH.

Testosterone inhibits the secretion of GnRH from the hypothalamus and therefore secretion of LH from the pituitary gland. If the testes are removed via castration, blood concentrations of LH and FSH will increase as there is only limited negative feedback.

Effects of Male Sex Hormones

Testosterone plays a crucial role in the development of male sex organs during fetal growth where increased production of testosterone causes penis growth and development of accessory sex glands during puberty. Testosterone also affects a number of other characteristics of the male, often called the "secondary sex characteristics". Testosterone is able to bind to receptors in the cytosol of cells in the same manner as other steroid hormones and these hormone-receptor complexes are then able to bind to DNA in the nucleus resulting in alterations in the level of transcription of specific genes.

Testosterone has a number of anabolic effects stimulating the development and growth of the skeleton and skeletal muscles. Muscle masses show a general increase and in certain body regions such as the neck of stallions or bulls there is obvious hypertrophy. Testosterone also alters behaviour in terms of increasing the degree of sex drive and as a result of the action in several areas of the brain, behaviour can become more aggressive. The larynx of males also enlarges during puberty and the vocal cords lengthen resulting in a deeper and stronger voice.

Testosterone also causes an increase in the level of pheromones to be secreted by glands in the skin which attract and evoke sexual behaviour in females. Glands use in scent marking and territorial marking are also activated by testosterone. In certain species, tusks, antlers and horns are also stimulated to develop.

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 anterior pituitary, specifically the gonadotroph 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 stimulates FSH secretion. 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. Inhibin in the female can also be diminished by GnRH and enhanced by insulin-like growth factor-1 (IGF-1).

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 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.

A further non-reproductive role of activin is it's role in skin lesions where it is thought to stimulate keratinocytes.

Prostaglandin F2α

Prostaglanin is a C₂O fatty acid and is produced within the uterine endometrium and vesicular glands. Prostaglandins are lipid soluable and are therefore able to diffuse into the ovarian artery via veins that would otherwise take the PGF_2α away from the uterine horn. The target tissue in the female is the corpus luteum, uterine myometrium and ovulatory follicles. In the female prostaglandin can cause luteolysis and also cause the induction of tone and contractions within the uterus and ovulation.

During the latter stages of the luteal phase the corpus luteum begins to regress under the effect of PGF_2α. Normally the concentration of PGF_2α in arterial blood is relatively low due to extensive metabolism via the lungs. However there is a large concentration gradient from the uterine vein to the ovarian artery resulting in higher concentrations of PGF_2α than in systemic circulation. If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved where concentrations of PGF_2α remain below a threshold level allowing the corpus luteum to continue to secrete prostaglandin. There are two main factors involved in the regulation of uterine secretions of PGF_2α; oxytocin secretions from the corpus luteum and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy.

Oxytocin secretion via the corpus luteum stimulates endometrial production of PGF_2α and by the end of the luteal phase the concentration of oxytocin and the number of oxytocin recptors within the endometrium allow the production of enough PGF_2α to breach the threshold level and cause luteolysis. During pregnancy the embryonically produced pregnancy recognition molecules inhibit the secretion of PGF_2α from the endometrium ensuring that luteolysis cannot occur.

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 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 and aids stimulation of uterine contractions.

Human Chorionic Gonadotrophin (hCG)

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.

As hCG is only produced by embryonic cells, the presence of this hormone within maternal blood can be used for pregnancy confirmation.

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 in the placenta and ovaries throughout pregnancy. During pregnancy relaxin prevents the initiation of uterine contractions, together with progesterone. Relaxin is also responsible for the softening of connective tissues in the cervix and ligaments in the pelvis prior to parturition. This relaxation of tissues via relaxin is performed in conjunction with prostaglandin.

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