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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.

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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)===

<br />Vagina: slight mucous secretion, paleness, exfoliation

<br />Vagina: slight mucous secretion, paleness, exfoliation

<br />Cervix: closure, formation of the mucous plug

<br />Cervix: closure, formation of the mucous plug

<br />Uterus: stimulates uterine gland secretions, sensitization of the endometrium to oxytocin, decreases uterine motility, immunosuppression, inhibition of PGF2a and PGE2

<br />Uterus: stimulates uterine gland secretions, sensitization of the endometrium to oxytocin, decreases uterine motility, immunosuppression, inhibition of PGF_2a  and PGE₂

<br />Fallopian tube: increased secretion, decreased motility

<br />Fallopian tube: increased secretion, decreased motility

<br />Mammary gland: stimulates lobulo-alveolar development

<br />Mammary gland: stimulates lobulo-alveolar development

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 [[Oestrous_Cycle_-_Anatomy_%26_Physiology#Oestrus|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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===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===

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. 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_Regression_-_Anatomy_%26_Physiology|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.

Normally the concentration of PGF_2α in arterial blood is relatively low due to extensive metabolism by PGF_2α-dehydrogenase (in especially the lungs). These levels are below the threshold required to cause luteolysis as PGF_2α production in early gestation is low.

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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 [[Ovary_-_Anatomy_%26_Physiology#Arterial_Supply|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.

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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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 [[Maternal_Recognition_of_Pregnancy_-_Anatomy_%26_Physiology|pregnancy recognition]] molecules inhibit the secretion of PGF_2α from the endometrium ensuring that luteolysis cannot occur.

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.

===Prostaglandin (PGE₂)===

===Prostaglandin (PGE₂)===

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In females it's target tissue is the [[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 and aids stimulation of uterine contractions.

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)===

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

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

===Relaxin===

===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.

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 - Review]]

[[Category:To Do - Review]]

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

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