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==Introduction==

==Introduction==

Reproductive hormones often have multiple roles and operate via [[Negative_Feedback_-_Anatomy_%26_Physiology|negative feedback systems]]. The information below will provide the main reproductive hormones in domestic species and their functions.

Reproductive hormones often have multiple roles and operate via [[Negative_Feedback_-_Anatomy_%26_Physiology|negative feedback systems]]. The information below will provide the main reproductive hormones in domestic species and their functions.

==Reproductive Hormones==

==Reproductive Hormones==

===Gonadotropin Releasing Hormone (GnRH)===

===Gonadotropin Releasing Hormone (GnRH)===

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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.

Although the hypothalamus via GnRH stimulates the secretion of LH and FSH, it cannot regulate LH and FSH independently. 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)===

===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).  

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 [[Follicles_-_Anatomy_%26_Physiology#Primary_Follicle|'''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).  

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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.

Prolactin secretion is regulated by the hypothalamus which produces several neurohormones that affect prolactin concentrations. The most important within this is [[Neurotransmitters_-_Anatomy_%26_Physiology#Biogenic_Amines|'''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.

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===Estradiol (E₂)===

===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.

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 [[Follicles_-_Anatomy_%26_Physiology#General_Structure|'''granulosa cells''']], the [[Placenta_-_Anatomy_%26_Physiology|placenta]] and the Zona reticularis of the [[Adrenal_Glands_-_Anatomy_%26_Physiology#Adrenal_Cortex|adrenal cortex]]. In males in it is produced in sertoli cells found in the testes. Estradiol is synthesised from cholestrol.

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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.

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.

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Where oestrogens stimulate growth of follicles in the ovaries, oestrogens secreted from the ovary in the [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Follicular_Phase|follicular phase]] ([[Oestrous_Cycle_-_Anatomy_%26_Physiology#Pro-Oestrus|proestrous]] and [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Oestrus|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.

Where oestrogens stimulate growth of follicles in the ovaries, oestrogens secreted from the ovary in the '''[[Oestrous_Cycle_-_Anatomy_%26_Physiology#Follicular_Phase|follicular phase]]''' ('''[[Oestrous_Cycle_-_Anatomy_%26_Physiology#Pro-Oestrus|proestrous]]''' and '''[[Oestrous_Cycle_-_Anatomy_%26_Physiology#Oestrus|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.

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===Progesterone (P₄)===

===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_-_Anatomy_%26_Physiology|corpus luteum]]. More detailed information regarding corpus luteum [[Corpus_Luteum_Formation_-_Anatomy_%26_Physiology|formation]] and [[Corpus_Luteum_Regression_-_Anatomy_%26_Physiology|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.

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_-_Anatomy_%26_Physiology|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 [[Corpus_Luteum_Formation_-_Anatomy_%26_Physiology|formation]] and [[Corpus_Luteum_Regression_-_Anatomy_%26_Physiology|regression]] 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.  

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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.

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====

====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 [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Luteal_Phase|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.  

During pregnancy the plasma concentration of progesterone is maintained at an elevated level. Progesterone also inhibits secretion of FSH and LH (negative feedback at hypothalamic level  by  inhibiting GnRH) and thus also prevents the [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Oestrus|ovulation]] of follicles during the [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Luteal_Phase|luteal phase]] and during pregnancy. In most domestic species the corpus luteum  persists for the entire length of gestation.

<br />The exception to this rule is the mare in which the progesterone concentration falls during the later stages of pregnancy.  This is due to the regression of the corpus luteum around day 180 of the 330-340 day gestation period.

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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).  

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).  

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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.

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====

====Effects of Male Sex Hormones====

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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 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_-_Anatomy_%26_Physiology|larynx]] of males also enlarges during puberty and the vocal cords lengthen resulting in a deeper and stronger voice.

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===Inhibin===

===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.  

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).

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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).

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.

<br />Inhibin in the female can also be diminished by GnRH and enhanced by insulin-like growth factor-1 (IGF-1).

===Activin===

===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 secretion and other roles including cell proliferation, cell differentiation, apoptosis and homeostasis.

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.

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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.

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.

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

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

===Prostaglandin F2α===

===Prostaglandin F_2α===

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

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_Regression_-_Anatomy_%26_Physiology|corpus luteum]], uterine myometrium and ovulatory follicles. In the female PGF_2α cause luteolysis and can also cause the induction of tone and contractions within the uterus. It plays an important role in partuition in ruminants.

Oxytocin secretion via the  [[Corpus_Luteum_Regression_-_Anatomy_%26_Physiology|corpus luteum]] stimulates endometrial production of PGF_2α and by the end of the [[Oestrous_Cycle_-_Anatomy_%26_Physiology|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 [[Maternal_Recognition_of_Pregnancy_-_Anatomy_%26_Physiology|pregnancy recognition]] molecules inhibit the secretion of PGF_2α from the endometrium ensuring that luteolysis cannot occur.

The  [[Ovary_-_Anatomy_%26_Physiology#Arterial_Supply|ovarian artery]] is wrapped around the uterine vein. This creates a countercurrent mechanism by which the lipid soluable prostaglandins are able to diffuse from the uterine vein into the [[Ovary_-_Anatomy_%26_Physiology#Arterial_Supply|ovarian artery]]. During the latter stages of the [[Oestrous_Cycle_-_Anatomy_%26_Physiology|luteal phase]] as PGF_2α production increases luteolysis will occur as PGF_2α Is able to reach its target in the ovary before being metabolized in systemic circulation.

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During the latter stages of the luteal phase the corpus luteum begins to regress under the effect of PGF_2α.

Horses and pigs do not poses this countercurrent mechanism. In these spp. the [PGF_2α-dehydrogenase] in systemic circulation is much lower in order to induce luteolysis when Prostaglandin concentration rises.

During the second half of the [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Luteal_Phase|luteal phase]] of the reproductive cycle prostaglandins are synthesised. These prostaglandins are lipid soluable and are therefore able to diffuse through cells walls into the ovarian artery. The overall concentrations of prostaglandins within the blood are reasonably low as the hormone is broken down within the lungs.

===Prostaglandin (PGE₂)===

===Prostaglandin (PGE₂)===

PGE₂ is another form of prostaglandin that is produced by the ovary, uterus and embryonic membranes. It's target tissue in females is the corpus luteum and the oviduct where it helps induce ovulation and the secretion of progesterone from the corpus luteum.  

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_Regression_-_Anatomy_%26_Physiology|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)===

===Human Chorionic Gonadotrophin (hCG)===

hCG is a form of glycoprotein that is synthesised within the trophoblast cells of a [[Blastocyst_Embryonic_Development_-_Anatomy_%26_Physiology|blastocyst]]. In females the target tissue is the ovary where it results in the ovarian production of progesterone. In males hCG increases the growth of the foetal testes.

hCG is a form of glycoprotein that is synthesised within the trophoblast cells of a [[Blastocyst_Embryonic_Development_-_Anatomy_%26_Physiology|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 [[Maternal_Recognition_of_Pregnancy_-_Anatomy_%26_Physiology|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)===

===Equine Chorionic Gonadotrophin (eCG)===

eCG is a form of glycoprotein that is produced from chorionic girdle cells. In females the primary target organ are the ovaries where they faciliate the formation of the accessory corpora lutea.

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 [[Maternal_Recognition_of_Pregnancy_-_Anatomy_%26_Physiology|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 (PL)===

Placental lactogen is a form of protein that is produced by the placenta. It's primary target tissue are the mammary glands where they stimulate milk production.

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.

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 ligaments in the pelvis prior to parturition. Estradiol priming is required for this. This relaxation of tissues via relaxin is performed in conjunction with prostaglandin.

[[Category:To Do - A&P]]

[[Category:To Do - Review]]

[[Category:Reproductive System - Anatomy & Physiology]]

[[Category:Reproductive System - Anatomy & Physiology]]