WikiVet English - User contributions [en]

Praziquantel

2017-04-01T15:53:17Z

Lboc:

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This is the only currently used drug in the pyrazinoisoquinolines. It has unsurpassed activity against [[cestodes]] and has activity against some [[trematodes]], these include schistosomes but not the liver flukes.

It works by causing an influx of Ca2+ across the parasite tegument. This results in muscular spasms and the cestodes loose their position in the intestine and are moved away by peristalsis. This leads to the animals body attacking the cestodes weakened tegumental defence with antibodies, immune cells and lytic enzymes. The dead tapeworm is therefore mostly digested by the time it arrives in the faeces. Owners of pets with tapeworms often feel that this drug doesn't work as they don't see any passed tapeworms. Where in fact the drug has worked but has left no visible evidence to the owner of its working.

Hoof - Anatomy & Physiology

2015-10-15T15:40:02Z

Lboc:

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==Introduction==
The hoof is defined from a physiologic perspective as the modified skin [[Skin - Anatomy & Physiology#Epidermis|(epidermis)]] covering the tip of the digit and all enclosed structures. The hoof provides protection to the distal limb and is formed by keratinisation of the epithelial layer and modification of the underlying dermis. The keratin in the [[Skin - Anatomy & Physiology#Epidermis|epidermis]], when thickened and cornified, is referred to as [[Horn - Anatomy & Physiology|horn]]. Horn makes up the outer surface if the hoof and is particularly resistant to mechanical and chemical damage.

Each epidermal region of the hoof is associated with a dermal region (corium). The corium are connected to the underlying structures by the subcutis. As the foot can be divided into 5 segments – Wall, coronary, periople, sole, and frog – there are 5 corresponding underlying corium.
 

The origin of the hoof is as a form of protection to the distal phalanx and stems from local modifications of the epidermis, [[Skin - Anatomy & Physiology#Dermis|dermis]] and subcutaneous layers. There is great variation in the shape and types of hooves across different species and this is often due to the multi-role nature of the hoof. In some species the hoof may play an important role in non-locomotion roles such as digging or being used as a weapon. The hoof also acts as a shock-absorber in that the hoof acts to reduce the concussion of foot impact on the structures of the distal limb. It has also been suggested in some species that the elastic nature of components of the hoof may aid the return of blood to the heart. The following information relates primarily to the horse hoof.

==Five Segments of the Hoof==
===Wall===
[[File:Lateral_view_horse_foot.jpg|thumb|right|150px|]]
The wall of the hoof is the element of the hoof that is visible. The wall forms the medial, lateral and dorsal aspect of the hoof and it can be further divided into the toe, quarters and heels. At the heel the walls reflect back on themselves at a point called the angles and in doing so forms the bars. The bars, although moving cranially, gradually fade along the edge of the frog and never actually meet.
The wall of the hoof is widest at the distal aspect of the hoof, i.e. at the toe. The wall of the hoof decreases in width laterally and medially (around the quarters of the hoof). In most domestic species the wall of the hoof is between 5 - 10mm thick and consists of three layers. There is an outer layer of thin but dense horn that is shiny and acts to reduce evaporation from the horn and therefore prevent dehydration of the other layers of the horn – the periople. There is an intermediate layer which represents the main structure of the wall and is composed of amorphous horn reinforced with many tubular shaped horn rods. Finally, there is an inner laminar layer where there are interdigitating laminae of horn and dermal laminae which ensure the hoof itself is firmly anchored to the distal phalanx. The bond between the horn and the underlying dermis is a living bond that gradually allows the wall of the hoof to slide in a distal direction towards the ground where the distal border is worn away via contact with the ground. Attached by the subcutis to the periosteum of the distal phalanx, the laminar corium suspends the entire weight of the animal by the distal phalanx within the hoof capsule via microscopic interdigitations with the epidermal laminae of the hoof wall. Damage to the vasculature of the laminar corium can result in compromises in the integrity of the interdigitations.

The hooves in newborn foals are bilaterally symmetrical. Over a period of just a few months, forces exerted on the hoof during locomotion cause a visible difference between the right and left, as well as front and hind hooves. Thus, isolated specimens of equine feet can be distinguished as follows:

Front vs hind:
Front: The angle between the toe and the ground is approximately 45 degrees. The sole is circular in shape.
Hind: The angle between the toe and the ground is 50-55 degrees. The sole is oval in shape.

Right vs left:
Quarters (lateral and medial walls) are steeper on the medial side of the hoof.
Assessment of the external anatomy can be a used as an important part of a lameness examination.

===Periople===
The periople is a small band of soft tissue found over the proximal surface of the wall of the hoof. The periople represents the junction between the wall and the skin of the limb. The periople is responsible for the production of the outer layer of the wall (above). The periople segment extends around the proximal edge of the wall and in horses covers the bulb of the heel and part of the frog. In ruminants it forms part of the join between both hooves of the foot. The perioplic corium, proximal to the coronary band and continuous with the skin, it produces the thin, shiny layer (stratum externa) of the hoof wall.

===Coronary Segment===
[[image: X-section through hoof.jpg|thumb|200px|right|A X-section through a typical hoof. ©Rachael Wallace2008]]
The wall of the hoof is created at the coronary dermis and grows in a distal direction from the coronary dermis. The coronary dermis is studded with many papillae which are directed towards the ground in the direction of growth. The epidermis covering these papillae produce horn tubules which are embedded into amorphous inter-tubular horn. This inter-tubular horn is created by the spaces between the papillae within the coronary dermis. The combination of both of these horn types ensures the horn has sufficient strength.

The pigmentation of the hoof is derived from melanocytes found in the coronary epidermis. Any pigmentation in the hoof will be most pronounced in the outer part of the hoof wall as the deeper layers of the hoof usually contain fewer melanocytes. It is this unpigmented element of the hoof that forms the 'white line' in the sole of hoofs and is particularly important in horses as a landmark for shoeing.

The coronary corium is responsible for the growth of the bulk of the tubular and non-tubular horn that make up the hoof wall. This wall glides distally at a rate of 5-6mm a month and by forming epidermal laminae itself it interdigitates with the underlying dermal laminae. Neither of these laminae are pigmented so when the epidermal laminae appear on the solar surface, a non-pigmented region known as the white line appears. The white line is used as important landmark in farriery as structures central to the line will be dermal and so vascular and sensitive. Interruptions in the coronary corium can result in defective hoof wall growth.

===Sole Segment===
[[image: Plantar hoof aspect.jpg|thumb|175px|left|A view of the solar surface of an equine hoof. The wall has been removed on the right to show the underlying dermis. ©Rachael Wallace2008]]
The sole is the area distal to the bars and apex of the frog enclosed by the hoof wall. The area where the bars and wall enclose it is known as the angle of the sole. Since the sole is slightly concave, the majority of the horse's weight is transferred through the margin of the sole.

The sole represents the part of the foot in contact with the ground and its composition differs between species. The keratin found in the sole is formed from the epidermis on the underside of the third phalanx and can grow to a thickness of around 10mm in domestic species. The keratin found on the sole is much more easily worn down or abraded than that of the wall of the hoof. The equine sole has a central frog structure whilst ruminants and pigs have a bulb structure to the sole.

The solar corium is the dermal layer underlying the solar surface that produces a superficially flaky epidermis. Sufficient solar depth is necessary to protect the underlying soft tissue and bony structures.

===Frog-Bulb Segment===
The frog is a wedge-shaped structure which sits between the bars and has an apex facing distally, with 2 crura flanking a central sulcus. Between the crus and bar of each half of the sole lies the collateral sulcus. Opposite the apex, the frog expands forming the bulbs of the heel. The frog is a mass of keratinized stratified squamous epithelium, which is softer than other parts of the hoof due to its increased water content. Usually, the frog contributes to the weightbearing surface where it functions as a shock absorber. Apocrine glands within the corium of the frog produce secretions on the surface. The frog ensures that the wall of the hoof is forced outwards when weight is put on the limb thus ensuring that the 'hoof mechanism' functions correctly and ensuring circulatory flow around the hoof and back towards the heart.
The ruminant/pig 'bulb' provides the hoof with the caudal and mid-hoof contact area with the ground and is chiefly involved in weight bearing. The bulb inserts into the V-shaped sole. The bulb is made of relatively soft material, mainly inter-tubular horn and is of a considerable thickness.

The frog corium overlies the digital cushion and generates the specialised soft epidermal tissues of the frog.

==Deeper structures of the foot==
[[File:Horse_hoof_wild_bare_sagittal.jpg|thumb|200px|right|Saggital section through horse hoof.]]
Enclosed within the hoof capsule are the bony structures – the [[Phalanges_-_Horse_Anatomy#Distal_Phalanx|distal phalanx]], distal end of the [[Phalanges_-_Horse_Anatomy#Middle_Phalanx|middle phalanx]], the [[Joints_and_Ligaments_-_Horse_Anatomy#Distal_Interphalangeal_.28Coffin.29_Joint|distal interphalangeal joint]], and the [[Phalanges_-_Horse_Anatomy#Distal_Sesamoid_.28Navicular.29_Bone|distal sesamoid bone (navicular)]]. There are also soft tissue structures including ligaments, cartilage, the digital cushion and the insertions of the [[Tendons_-_Horse_Anatomy#Thoracic_Limb|common digital extensor tendon]] and the [[Tendons_-_Horse_Anatomy#Flexorsdeep|digital flexor tendon]].

===Ungual (collateral) cartilages===
The ungual cartilages are extensions of the [[Phalanges_-_Horse_Anatomy#Distal_Phalanx|distal phalanx]] (Plll) that extend caudally and dorsally from the medial and lateral margins of the [[Phalanges_-_Horse_Anatomy#Distal_Phalanx|distal phalanx]], curving inwards towards each other in the heel region. The cartilages extend just beyond the confines of the hoof capsule making them palpable just above the coronary band at the lateral and medial edges of the foot. The ungual cartilages can ossify resulting in ‘side bones’ which have the potential for fracturing. The cartilages can also become infected resulting in the condition known as ‘quittor’.

The cartilages are securely attached to the other internal structures of the foot by a series of ligaments that extend from the medial and lateral cartilages to the distal and middle phalanx, the [[Phalanges_-_Horse_Anatomy#Distal_Sesamoid_.28Navicular.29_Bone|distal sesamoid bone (navicular)]], and the digital cushion.

===Collateral ligaments.===
The distal interphalangeal joint is enclosed within the hoof capsule. It is stabilised by the medial and lateral collateral ligaments which form part of the joint capsule, connecting the distal end of Pll with the proximal edge of the [[Phalanges_-_Horse_Anatomy#Distal_Phalanx|distal phalanx]].

===Annular ligaments===
The annular ligament has its origins on the medial and lateral surfaces of distal Pl. It is the most superficial structure in the region, lying just beneath the skin and fusing with the [[Tendons_-_Horse_Anatomy#Flexorsdeep|digital flexor tendon]] where it enters the hoof capsule. Once inside the hoof the annular ligament merges with the fibrous attachments of the ungual cartilages and digital cushion, and continues with the [[Tendons_-_Horse_Anatomy#Flexorsdeep|digital flexor tendon]] down to its insertion onto the [[Phalanges_-_Horse_Anatomy#Distal_Phalanx|distal phalanx]].

===Sesamoidean ligaments===
The dorsal border of the [[Phalanges_-_Horse_Anatomy#Distal_Sesamoid_.28Navicular.29_Bone|distal sesamoid bone (navicular)]] is held securely to the palmar/plantar surface of the [[Phalanges_-_Horse_Anatomy#Distal_Phalanx|distal phalanx]] by the distal sesamoidean ligament and to the proximal phalanx via the proximal interphalangeal collateral ligaments by means of a pair of medial and lateral collateral sesamoidean ligaments.

===Navicular bursa===
The [[Phalanges_-_Horse_Anatomy#Distal_Sesamoid_.28Navicular.29_Bone|distal sesamoid bone (navicular)]] lies between the middle and distal phalanges and the deep digital flexor tendon. Associated with it is a fluid-filled sac that reduces friction between the bone and the [[Tendons_-_Horse_Anatomy#Flexorsdeep|digital flexor tendon]] that lies over the top of it— the navicular bursa. Inflammation in the region is involved in navicular disease which is a common cause of lameness.

===Digital Cushion===
The digital cushion is the internal tissue deep to the frog. It lies between the ungual cartilages and is comprised of collagenous, elastic tissue infiltrated by adipose tissue. At the bulbs of the heel, it is subcutaneous and is soft and loose in texture. It has connection with the digital annular ligament and, at the apex to the deep digital flexor tendon at its point of insertion on the distal phalanx. It acts as one of the major shock absorbers of the foot. When the limb is weight bearing, the increase in pressure and change in shape of the digital cushion and the frog compress the veins in the foot aiding venous return.

===Blood supply to the digit===
The main vessels supplying the digit in the forelimb are the medial and lateral palmar digital aa, both of which arise from the median a. In the digit of the hind limb the medial and lateral digital aa. are a continuation of the metatarsal a. and are also contributed to by the medial and lateral plantar aa. which branch from the sapheneous a.
The digital arteries give rise to numerous branches forming rich networks for the vascular tissues. Many anastomoses occur. The terminal branches of the main vessels finally enter a bony canal in the distal phalanx. Venous drainage is similar with the most distal vessels being the medial and lateral palmar/plantar digital veins. The compressive action of the hoof on the soft tissues within during locomotion generates an important function promoting venous return.

Normal equine digital vasculature anatomy can be divided into five major areas of perfusion:
1. Coronary plexus
2. Dorsal lamellar plexus
3. Circumflex vessels
4. Terminal arch
5. Heel perfusion.
Loss of perfusion to the lamella vessels, circumflex vessels, and terminal arch indicates a poor prognosis without aggressive therapy.

===Microcirculation in the dermal laminae===
Numerous arteriovenous anastomoses occur which are of a somewhat unusual type. Under normal circumstances these are closed and as a result circulation within the capillary beds of the dermal laminae occurs. Certain systemic pathologies may result in opening of these AV anastomoses resulting in ischaemia of the laminae. This in turn results in the hoof wall separating from the distal phalanx producing the disease termed “laminitis”, which can be either acute or chronic.

===Innervation of the equine digit===
The digit of the forelimb is innervated by the medial and lateral digital nerves. The medial digital n. is a continuation of the median n. and the lateral digital n. is derived from both median and ulnar nerves. These run on the palmar aspect of the digit in close proximity to the main arteries and veins. They give rise to several dorsal branches which supply dorsally located areas.

The pelvic limb digit is innervated on the dorsal aspect by the common digital nn. Derived from the fibular n. The plantar aspect is innervated by the medial and lateral digital nn which originate from the tibial n.
In addition to their normal importance in supplying innervation to the sensitive tissues of the equine digit these nerves are also of considerable clinical importance as they are utilized for the procedure termed diagnostic nerve blocks.

==Species variation==

===Ruminant Hoof===
The ruminant hoof, although resembling the equine hoof in some characteristics, differs from the equine hoof in several ways. In the ruminant hoof there are two separate main digits and the wall of the hoof is bent to form a border. Also the bulb of the heel covers the entire caudal surface of the hoof and most of the plantar surface, leaving only a small area of sole visible. In ruminants the interdigitating lamellae are smaller and less well developed than in equids.
 
 
The hooves of the main digits curve medially towards each other. The lateral digit carries more weight than the medial digit, and is larger. On the abaxial wall, the distal border makes contact with the ground along its entire length, whereas, on the axial wall, only does so toward the toe. The thickness of the wall increases towards the apex and the plantar surface. The horn of the hoof generally grows at a rate of 5 mm per month, and in cattle allowed to move freely, growth should equal wear. In intensively kept cattle, growth exceeds wear, and foot trimming is required to maintain optimal shape and angle. The optimal angle of the toe from the ground is 50 degrees. Where horn overgrowth occurs, the coffin joint is gradually overextended and the deep flexor tendon tensed. This results in greater weight being placed over the caudal part of the hoof and can cause pain and lameness.
 
 
'''Dewclaws''' are present in most ruminants but do not make contact with the ground. They consist of wall and bulb and have no practical importance.

See the [[Bovine Lower Limb - Anatomy & Physiology|bovine lower limb]] for further detail.

===Porcine Hoof===
[[File:Pig cracked hooves.JPG|thumb|right|150px|Pig Hooves and a cracked hoof wall]]
The hooves of pigs are principally similar to those of ruminants, however the wall is straight, not bent medially at the toe, and they have a soft bulb that is well distanced from the wall and sole. The hooves of the accessory digits are of the same structure as the principal digits, but only bear weight on soft ground.

Hoof trimming in pigs is rarely required due to the short lifespan of the farmed pig.

{{Learning
|flashcards = [[Hoof flashcards - Anatomy & Physiology|Hoof Flashcards]]
|full text = [http://www.cabi.org/cabdirect/FullTextPDF/2005/20053192986.pdf ''' The growth and adaptive capabilities of the hoof wall and sole: functional changes in response to stress.''' Bowker, R. M.; American Association of Equine Practitioners (AAEP), Lexington, USA, Proceedings of the 49th Annual Convention of the American Association of Equine Practitioners, New Orleans, Louisiana, USA, 21-25 November 2003, 2003, pp 146-168, 32 ref.] [http://www.cabi.org/cabdirect/FullTextPDF/2005/20053192988.pdf '''Contrasting structural morphologies of "good" and "bad" footed horses.''' Bowker, R. M.; American Association of Equine Practitioners (AAEP), Lexington, USA, Proceedings of the 49th Annual Convention of the American Association of Equine Practitioners, New Orleans, Louisiana, USA, 21-25 November 2003, 2003, pp 186-209, 73 ref.]
|OVAM = [http://www.onlineveterinaryanatomy.net/content/white-line-bovine-hoof White Line of Bovine Hoof] [http://www.onlineveterinaryanatomy.net/content/hoof-wall-bovine-distal-limb Bovine Hoof Wall]
}}

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[[Category:Integumentary System - Anatomy & Physiology]]
[[Category:A&P Done]]

Reproductive Hormones Overview - Anatomy & Physiology

2015-05-01T19:55:13Z

Lboc: Spelling

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==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.
 
 
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_System_Development#Sexual_Differentiation_of_the_Brain|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 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)===
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).
 
 
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 [[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.
 
 
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 [[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.
 
 
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.
 
'''Effects on reproductive organs:'''
 Vagina: slight mucous secretion, hyperaemia, oedema
 Cervix: relaxation, liquification of mucous plug (causing the bull string)
 Uterus: stimulates uterine gland development, sensitization of the endometrium to oxytocin, immune activation (local), leucocyte infiltration, secretion of PGF2a and PGE2
 Fallopian tube: increased motility and cilia activity
 Mammary gland: stimulates mammary duct development
 Corpus luteum: Luteolytic (bovine and ovine) but luteotrophic (equine and porcine)
 
 
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.
 
 
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 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.
 
'''Effects on reproductive organs:'''
 Vagina: slight mucous secretion, paleness, exfoliation
 Cervix: closure, formation of the mucous plug
 Uterus: stimulates uterine gland secretions, sensitization of the endometrium to oxytocin, decreases uterine motility, immunosuppression, inhibition of PGF2a and PGE2
 Fallopian tube: increased secretion, decreased motility
 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.
====Progesterone During 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.
 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.
 
 
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_and_Spermiation_-_Anatomy_%26_Physiology|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).
 
 
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_-_Anatomy_%26_Physiology|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 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.
 
 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. 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 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_Regression_-_Anatomy_%26_Physiology|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_Regression_-_Anatomy_%26_Physiology|corpus luteum]] and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy.
 
 
Oxytocin secretion via the [[Corpus_Luteum_Regression_-_Anatomy_%26_Physiology|corpus luteum]] stimulates endometrial production of PGF2α 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 PGF2α 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 PGF2α from the endometrium ensuring that luteolysis cannot occur.
 
 
Normally the concentration of PGF2α in arterial blood is relatively low due to extensive metabolism by PGF2α-dehydrogenase (in especially the lungs). These levels are below the threshold required to cause luteolysis as PGF2α production in early gestation is low.
 
 
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 PGF2α production increases luteolysis will occur as PGF2α Is able to reach its target in the ovary before being metabolized in systemic circulation.
 
 
Horses and pigs do not poses this countercurrent mechanism. In these spp. the [PGF2α-dehydrogenase] in systemic circulation is much lower in order to induce luteolysis when Prostaglandin concentration rises.

===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_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)===
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.
 
 
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 [[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 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 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.

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[[Category:To Do - Review]]
[[Category:Reproductive System - Anatomy & Physiology]]