Dogs in heat - FAQ.
Q. What are dogs in heat?

A. Heat is a synonym for the estrous cycle. It is during this cycle that bitches may get pregnant.

Q. What are the symptoms of dogs in heat?

A. Bitches tend to bleed from the vagina when they're on heat and swelling of the vulva occur. They also experience increased urination. For small breeds, there is usually not much bleeding so owners may need to pay close attention to their female dog to notice her 1st cycle.

Q. When does a bitch come on heat?

A. Usually, the bitch has her 1st cycle at around 6 months old. Some bitches start earlier and other female dogs later. When owners get a new female dog, they should monitor her and note when she has her 1st cycle. If the bitch is 14 months of age and still has not been in heat yet, owners should get the opinion of a pet clinician.

Q. How long does dogs in heat last?

A. Around 3 weeks or twenty-one days. In some female dogs, the heat only lasts 2 weeks, although in some breeds it may last 4 weeks.

Q. How often are dogs in heat?

A. Most bitches have regular estrous cycles; every 6 to 8 months.

Q. When can bitches pregnant?

A. Female dogs can only get pregnant when on heat. The rule-of-thumb is that the most fertile period is between eleven to fifteen days of the estrous cycle. During that time, female dogs will most likely let any male dogs to mount her and mating occurs.

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Stages of Labor in Cats
Fortunately, over 99% of kitten deliveries occur without complications and/or assistance from their owners. However, when your queen is set to start labor, it is comforting to recognize the symptoms and signs of labor in cats.

24 to 48 hours before labor in cats starts, your queen might seem restless, anxious and she might me extremely affectionate. The queen will be looking for a warm, quiet and dark place. This will become her place to nest and have the litter. You have to know that this nesting behavior can occur up to 72 hours before labor starts. At this stage, just prepare her nesting environment by placing old sheets, blankets, litter, food and water in her room.

The first stage of labor, which usually lasts for about 24 hours, is characterised by restlessness, vocalisation and nesting behavior. She might be panting and purring a lot. However, some normally affectionate queens may show signs of aggression towards their owners as the time of parturition approaches. The queen will often lick her genitals a lot more than usual as well. The best sign that stage two of labor in cats is imminent, is when the queen stops eating for at least 12 hours.



Once stage two of labor in cats starts, kittens are usually produced quite quickly with relatively little abdominal straining but often with a loud scream. The birth of the first kitten takes 30-60 minutes and the interval between delivery of subsequent kittens varies from 5 to 90 minutes. Most queens will sever the umbilical cord, eat the placenta and clean the kittens without requiring any assistance.

Frequently a queen will suckle the first born while parturition continues. Occasionally, the second stage of labor in cats may be divided in two parts, with the queen resting for as long as 12-24 hours between deliveries of her kittens. This usually happens when the queen does not feel safe due to the presence of strange people or other animals.

Stage three of labor, the expulsion of the placentas, usually occurs after each kitten is delivered and the queen usually cuts the cord and consumes the placentas. Placenta contains nutrients that will help the queen producing quality milk. It is rare for placentas to be retained - the administration of oxytocin is advised in such cases. After birth, the kittens will try to suckle quickly. It is important for them to receive the first milk from birth, the colostrum, because it contains important antibodies.

Because she will be breastfeeding for about five - six weeks, the queen will need more water at the beginning to compensate for her loss of fluids. It is a good idea to leave a bowl of water permanently near the delivery room. At the end of the lactation period, usually around 5-6 weeks, the queen's milk production will decrease and she will no longer be interested to breastfeed. It is now the time for you to place a bowl of dry food specifically for the kittens. Around 8 weeks, the kittens can finally be separated from their mother and live their own lives.

Stages of dog pregnancy

The development of a new individual requires the transfer of male gametes to the female genital tract for fertilization of the female gametes. Dog pregnancy stages involve spermatozoa, which have been concentrated and stored in the epididymis, gradually change from oxidative (aerobic) to glycolytic (anaerobic) metabolism as they progress through the epididymis. In this state, spermatozoa are in a situation of reduced metabolism. Mature sperm are only able to metabolize a special sugar, fructose, within the reproductive tract.

Sperm are ejaculated usually directly into the cervix and uterus. The movement of sperm through the cervix is aided by estrogen-induced changes in cervical mucus, which result in the formation of channels that facilitate movement of sperm. This has been particularly emphasized in primates, in which the thinning of mucus occurs just before ovulation, a factor that can be used to predict the time of ovulation.

The first studies in dog pregnancy stages emphasized on sperm transportation. It is now known that sperm undergoing so-called fast transport are not involved in fertilization; in fact, they are damaged by the rapid transport. Sperm need to undergo changes within the female genital tract that are a prerequisite for fertilization; the process is called capacitation. One of the effects of capacitation is the removal of glycoproteins from the sperm cell surface.

The glycoproteins, perhaps added for protective purposes, interfer with fertilization. This change allows sperm to undergo the acrosome reaction when they come in contact with oocytes. The acrosome reaction involves the release of hydrolytic enzymes from the acrosomal cap; this may be important for penetration of the sperm through the granulosa and zona pellucida to the oocyte plasma membrane.

Acrosin, a proteolytic enzyme plays a role in dog pregnancy stages as well. It digests the acellular coating around the oocyte. Both enzymatic events allow the sperm to penetrate to the oocyte. The acrosome reaction also changes the surface of the sperm, which allows it to fuse with the oocyte. The acrosomal reaction results in tail movements that feature a flagellar beat that tends to drive sperm in a forward direction.

Because of the changes that spermatozoa must undergo within the female reproductive tract before fertilization, the deposition of sperm before ovulation is the preferred timing for producing maximal fertility. Females are usually sexually receptive for at least 24 hours before ovulation and, in the natural setting, insemination usually occurs a number of hours before the occurrence of ovulation.

An interesting finding in the mare is her ability to distinguish fertilize from unfertilized oocytes; unfertilized oocytes from previous cycles are retained within the oviduct, whereas recently fertilized oocytes (embryos) move through the oviduct of the uterus. It is likely that all dogs recognize pregnancy by the presence of an embryo(s) at the early oviductal pregnancy stage. However, this recognition does not necessarily result in prolongation of the corpus luteum and the continued production of progesterone, which is essential for the maintenance of pregnancy in dogs.

Pheromones are chemical compounds that allow communication among dogs through the olfactory system. When sexual behavior is affected, the compounds are called dog appeasing pheromones. Pheromones arise from several tissue sources; the most prominent ones for dogs are sebaceous glands, the reproductive tract, and the urinary tract.

Dog appeasing pheromones are important for the attraction of the male to the female at the time of sexual receptivity. Sexual attractiveness of the female evolves from the pheromones that she elicits on a limited, cyclical basis in association with estrus.

The classic way for males to delineate their territory has been for them to mark the area with urine. In general, dog appeasing pheromones that affect sexual behavior tend to have a musk type of odor.

Some of the first experiments that demonstrated the potency of males odors to influence reproductive behavior were done in mice. One syndrome, called the Whitten effect, involved the synchronization of estrus in female mice through the sudden introduction of a male. The effect of the pheromones in this case is to stimulate the synthesis and release of gonadotropins.

Appeasing pheromones in dogs can account for some of the effect of the male. More recent studies, however, have shown that sight of the male by the female as well as physical contact are important factors that influence gonadotropin secretion and thus ovarian activity.

Dog labor stages

Three stages of labor exist in dogs. Stage 1 is characterized by nesting behavior, restlessness, shivering, and anorexia. Bitches usually pant. The cervix dilates during stage 1. No external signs of uterine or abdominal contractions exist. However, uterine contractions can be documented using external pressure transducers (tocodynamometers) that are strapped around the belly. During dog pregnancy, uterine contractions are slow and tonic in nature. During stage 1 of parturition, uterine contractions increase in frequency, duration and strength. These changes are coincident with the decline in pregesterone concentrations, the decline in rectal temperature, and the change of the behavior of the bitch. As determined by the changes in rectal temperature and change in the dam's behavior, stage 1 normally lasts for 6 to 12 hours. As determined by the change in uterine contractions until the delivery of the first pup, the duration of stage 1 was reported to be 12 hours in average.

Stage 2 is characterized by obvious abdominal contractions, passage of amnionic fluid and deliver of the puppy. Rectal temperature rises to normal. Stage 2 is usually accomplished in 3 to 6 hours. There may be intermittent, active abdominal straining for several hours before the birth of the first neonate. Constant, unrelenting straining is not normal. Usually less than 1 hour passes between the delivery of subsequent puppies.

The placenta is normally passed within 5 to 15 minutes of birth of each neonate. This is stage 3. The dam removes the amniotic membranes and cleans the neonate, severing the umbilical cord and eating the placenta. If the dam fails to remove the fetal membranes from the neonate's face, the owner should. Cleaning the neonate is important maternal behavior necessary for bonding between the dam and her offspring; thus the dam should be encouraged to do it. All placentas should be passed within 4 to 6 hours. If the owner is attending, the umbilical cord should be clamped and cut about 1 cm from the body wall. If bleeding occurs, the cord can be ligated.

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Artificial insemination (AI) in dogs is used primarily when natural breeding cannot be accomplished. Transporting semen, rather than live animals, to distant geographic locations is a great advantage of AI over natural service. AI is also used when behavioral problems, such as partner preference, or physical problems, such as vaginal prolapse, prevent copulation of the desired pair of dogs.

Some dog breeders prefer AI because they believe that the risks of breeding trauma is minimized and that the stud is less likely to be exposed to infectious diseases carried by the bitch. In addition, a single ejaculate with sufficient numbers of spermatozoa can be divided and used to inseminate several bitches. Although the number of viable spermatozoa necessary to maintain conception rates and litter size has not been determined for dogs, 150 to 200 X 10 000 000 viable spermatozoa is considered the minimum desirable number for intravaginal insemination. However, pregnancies produced by intrauterine insemination have been achieved under ideal conditions with as few as 20 X 10 000 000 fresh spermatozoa.

Several factors determine the success of artificial insemination in dogs, including the reproductive health of the animals, the quality of the semen, the timing and the number of inseminations, and the technical skills of the person performing the insemination. First and foremost is the reproductive health of the male and female. Normal dogs are expected to be willing and able to breed. However, various causes of reluctance to breed may make dog artificial insemination necessary. On the other hand, the problem that necessitated AI may also adversely affect fertility.

Second, the timing of insemination in dogs is critical. Animals may be brought in for AI because the usual behavioral signs of estrus are not manifested or recognized. In such cases, estrus may be identified by exfoliative vaginal cytology and ovulation can be estimated by serum luteinizing hormone (LH) or progesterone concentrations. Ideally, several inseminations are planned for a particular estrous cycle, because it has been shown that conception rates and litter size are better if bitches are bred two or three times than if the are bred once. If only two inseminations are included in the stud fee, which is common practice, the second insemination would ideally occur 48 or more hours after the first insemination during the fertile period.

Infertility in dogs | Canine Infertility

Normal seminal quality, normal desire to breed (libido), and normal ability to mate are all necessary for normal fertility in males. Therefore, the diagnostic approach to infertility must investigate all three of these factors. The diagnostic approach begins with a complete history-taking and physical examination. The history-taking should assess the male's past breeding performance, breeding management, fertility of the females to which he has been bred, and current or previous health problems.

Dogs achieving pregnancy rates of less than 75% when bred to apparently normal females using proper breeding management should probably be evaluated for subfertility. Pregnancy rates of 84.4% ± 12.4% have been reported for privately owned, fertile stud dogs in which two matings/estrus were done. Better than 90% pregnancy rates are achieved in well-managed commercial breeding colonies, but these rates stem from the fact that individual dogs with lower rates are likely to be promptly culled from such colonies.

Assessment of the male's libido and mating ability can help narrow the differential diagnoses. A normal male may appear to lack libido if he is not in his established territory; if he is less dominant than the female or another male in the immediate vicinity; if he is inexperienced or frightened; or if he prefers a different partner. Some normal males show no interest until the female is actually in estrus, as opposed to proestrus. Dogs that are accustomed to semen collection may no longer be interested in natural service despite normal arousal and a willingness to ejaculate. Daily ejaculation, especially over a week or two, and ejaculation more often than twice a day are other factors that can diminish the libido of normal male dogs. Such frequent ejaculation does not diminish libido in tomcats. Excessive endogenous or exogenous glucocorticoids, stress, and pain can also cause decreased libido in dogs. Libido also appears to decrease with advancing age.

Some animals may exhibit normal arousal and mount, only to dismount before attempting intromission. It is often difficult to determine whether this behavior is caused by inadequate libido or by inadequate mating ability. This behavior is often exhibited when a vaginal abnormality is encountered and also in some males accustomed to semen collection. Painful conditions often diminish libido, as well as interfere with mating ability. Generally, mating ability is determined by physical, mechanical, and neurologic factors governing mounting, erection, intromission, and ejaculation. Orthopedic disorders of the rear legs, spine, and less commony, the front legs may prevent mounting or intromission but do not usually affect libido and ejaculatory ability. Semen collection and artificial insemination could be used in such animals.

Signs of cat in heat.

The expression "cat in heat" is used by owners to describe the summation of pro-oestrus and oestrus. There is no specific lay terminology fot the rest of the oestrus cycle of the queen. The cat heat symptoms are quite easy to notice. High demand for affection, spending excessive time licking genitals and loud vocalizing are the most common signs of a cat in heat (queen).

Under normal climatic conditions queens are seasonally poly-oestrus. However, under modern housing conditions queens frequently cycle throughout the year. The sequence of events is pro-oestrus followed by oestrus and metoestrus. The queen then goes into a short period of sexual inactivity (dioestrus). This cycle of events is repeated until the end of the breeding season. The last metoestrus of the breeding season is followed by a longer period of sexual inactivity (anoestrus, the non-breeding season) that lasts until the first pro-oestrus of the next period of sexual activity.

On average, domestic cats reach puberty by 6-9 months of age. Sexual activity of free ranging cats is photoperiod-dependant, thus the onset of puberty may be influenced by the time of year that the queen was born with puberty occurring at around 6 months of age (range 5-7 months).

The onset and duration of ovarian activity is linked closely to day length. Ovulation is not spontaneous. It is induced naturally by mating or artificially by stimulation of the genitals and cervix or by hormone administration. However, it may also be induced by proximity to other female cats. The stages of the oestrus cycle cannot be identified using vaginal cytology in the queen.

Pro-oestrus may follow soon after parturition or may be preceded by a period of anoestrus. Queens can become pregnant again whilst still suckling a litter of kittens. The interval between the birth of a litter and the subsequent heat period is variable, but is usually within 4-8 weeks. The interval is dependent on the age at which litters are weaned and, in cats with a non-breeding season, the time of the year when kittens are born. In the Northern Hemisphere, queens cycle between January (irrespective of climatic conditions) and September with peaks of sexual activity in late January/February, May and June, and occasionally in September. Anoestrus generally lasts from late September to late January.

Dogs mating

Traditionally, female owners have their bitches mated twice, 11 and 13 days after the onset of pro-oestrus, to try and ensure that spermatozoa are present in the female reproductive tract at around the time of ovulation. This rule of thumb method is generally successful due to the unusual longevity of dog spermatozoa (up to 7 days) in the female genital tract.

Much time is lost if matings in dogs are ineffectual, thus there is an urgent need to get it right on each occasion. This has led to the development and employment of more scientific dog mating techniques. Such methods are particularly apposite when, as is possible with modern transportation methods, a visit to the stud dog of choice necessitates a protracted and expensive journey.

There is no doubt that many fertility problems result from being arranged at a convenient time rather than on the most appropriate day. If the timing of ovulation can be more precisely determined, fertility rates are likely to increase and the expected whelping date can be predicted more accurately. Thus, if the timing of ovulation can be predicted, conception failures are less likely and the reproductive management of the bitch can be simplified.

Dog on heat signs
The dog heat symptoms occur when bitches are attractive to male dogs. The term "heat" is used by dog owners to describe the summation of pro-oestrus and oestrus. There is no specific lay terminology for the rest of the oestrus cycle of the bitch. Signs of a dog in heat start approximately 9 days of pro-oestrus.

The frequency of dog heat symptoms is primarily determined by the length of anoestrus, which varies from bitch to bitch. The normal inter-oestrus interval is 5-10 months. However, breed variability can be striking, e.g., German Shepherds commonly have an inter-oestrus period of 4-4.5 months and African breeds, such as the Basenji, cycle only once per year.

Dog mating occurs when the bitch is in oestrus. Before mounting a bitch, the dog may go through a relatively prolonged courtship procedure, but often the male will simply briefly apply its tongue on the female dog genitals before mounting. As a result of this attention, the bitch will usually stand firmly with its tail held to one side exposing its vulva.

Penetration in the dog is achieved without erection because of the presence of the os penis. However, once inside the female dog, engorgement of the bulbus glandis occurs and this is accompanied by strong thrusting movements. This results in the ejaculation of the prostatic fluid. Once pelvic thrusting ends, the male will dismount and, and, by lifting one hind leg over the bitch, end up tied to tail to tail to the bitch, locked by the engorged bulbus that makes separation difficult. During the "tie", seminal fluids continue to disperse and this second part is fertile rich.

Many female dogs become pregnant after the briefest of intromissions. The tie can last anything from 5-60 minutes (average 20 minutes) and during this time, the dog and bitch may drag each other around. The tie finally breaks quite spontaneously and some seminal fluid may be seen draining from the female dog's genitals.

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Fertilisation in the queen takes place in the oviducts and 6-8 days post-mating the blastocysts migrate into the uterus. Implantation is thought to occur 11-14 days post-mating.

Pregnancy in cats is most commonly confirmed by abdominal palpation: a series of discrete, firm spherical uterine enlargements can be readily felt by days 17-25 of gestation. Developing kittens can be seen using ultrasonography as early as day 14-15 and foetal heartbeats can be seen from day 22. Foetal skeletons can be visualized on radiographs from day 43 of gestation onwards. No laboratory tests are currently used routinely for the diagnosis of pregnancy in cats, although there has been substancial work on the fecal excretion of progesterone.

The duration of cat pregnancy is 67 days on average (range 62-71). The average number of kittens born alive per litter is 4, with a range of 1-8 live kittens per litter. Approximately 5% of kittens are stillborn; the percentage is higher in Persian cats. On average, 87% of kittens born alive are reared successfully. There is no apparent breed difference in the duration of gestation.

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Signs of labor in dogs and dog labor symptoms.

Classically, dog labor is divided into 3 stages:

1st stage: Cervical relaxation and dilatation;
2nd stage: Production of young
3rd stage: Expulsion of the placenta.

In animals such as dogs that produce more than one foetus, the stages alter as each individual foetus is produced.

During the first stage, which lasts from 4 hours (average) to 36 hours in primigravid bitches, the cervix relaxes and dilates. The bitch becomes more restless and nervous, shivers, pants, may vomit and may tear up bedding material, possibly as a reaction to pain. Weak urine contractions may be apparent.

The second stage is characterized by strong uterine contractions and by visible straining. Between contractions the bitch will lick the vulvar region, especially once the foetal sac ruptures and placental fluid is released. Once the foetal head or pelvis is engaged in the bitch's pelvic girdle, strong abdominal straining is stimulated. The duration of the second stage of the labor in dogs is extremely variable between individuals and between puppies within a single litter. As a rule of thumb, however, no more than 6 hours should be allowed to elapse after the delivery of the first puppies before an investigation is carried out, since a long delay may result in placental separation and death of any remaining viable pups.

During the signs of dog labor, the interval between births is also variable. Second and subsequent puppies are usually produced after no more than 30 minutes of straining. Rest periods of more than 3-4 hours should be regarded as abnormal. It is not uncommon for a large litter to take up to 24 hours to be produced. Bitches that are good mothers will clean and succkle the puppies between successive births and it is better to allow this to occur.

The third stage begins when the foetal membranes are expelled. Puppies may be born with the membranes intact or they may be born simply attached by the umbilical cord with the placenta remaining in the genital tract. In the latter case, the placenta will be expelled separately before, with or after subsequent births. It is personal preference whether a bitch is allowed to eat the placentas or not, although it has been suggested that placental hormones may promote uterine involution and milk production. In the case of large litters, it is probably unwise to let a bitch eat all the placentas.

A bitch relaxing and nursing its puppies contentedly signals the end of whelping in dog labour. Finally, it is generally accepted that the administration of a single dose of oxytocin at the end of parturition, to ensure rapid uterine contraction and the expulsion of any remaining placentas, as a wise precaution. We also recommend this natural balanced real-meat dog food and natural dietary supplement for after-birthing recover.

More info on dog labor.

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Pregnancy in dogs - Dog pregnancy signs - Dog Pregnancy Stages

The average bodyweight gain of a pregnant dog from oestrus to parturition is 36% (range 20-55%), with the increase being most marked in the last third of the dog pregnancy stages. Pregnant dog signs are a change in body shape usually visible by about day 56 of pregnancy and foetal movements may also be noted around this time. The nipples enlarge and mammary development occurs during the second half of pregnancy and a serous secretion may be present shortly before parturition. Bitch owners frequently want to know whether their bitch is pregnant following a planned dog mating, principally out of curiosity, but also so that adequate plans can be made well in advance of the anticipated whelping date.

Abdominal palpation, usually 3-4 weeks post-mating, is used commonly for the diagnosis of pregnancy in dogs. This method is sensitive when executed by an experienced clinician. However, small litter size and lack of knowledge of the conception date can result in false negative diagnoses. Problems are frequently encountered in certain breeds, in fat bitches and in bitches that guard their abdomens.

Radiography can be used to confirm dog pregnancy but the foetal skeletons do not become apparent radio-opaque until day 45. Ultrasound can be used to visualize foetal vesicles from day 16-20 of pregnancy onwards. Foetal heart beats can be seen, using real time ultrasound from day 24-28 of pregnancy onwards.

It is well established that circulating concentrations of progesterone, oestrogen and PRL in pregnant bitches, unmated bitches, and bitches that have failed to become pregnant are very similar. Thus levels of conventional hormones cannot be used to detect pregnancy in dogs. However, it has been shown that acute phase protein levels are significantly elevated from day 28-37 post-mating in pregnant bitches in comparison with non-pregnant animals. Acute phase protein concentrations can be measured by most well-equipped laboratories and this method is remarkably reliable, especially given the fact that acute phase proteins are not dog pregnancy symptoms specific, they can be raised due to stress and in certain disease conditions.

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In temperate climates, many species are seasonal breeders. Sheep, goats and horses are examples of them. This means that periods of sexual activity (the oestrus season) are alternated with periods of sexual inactivity (the anoestrus season). In sheep for instance, sexual activity starts when the day length increases (long-day breeders). The difference in breeding season between species is connected with the differences in gestation length. In temperate and cold climates, the result is that both horses and sheep give birth to their young in spring, a period with sufficient food, giving them the best chance of survival.

The pineal gland is the main regulatory organ in seasonality. Via the eyes and complex neural connections day length is registered in the pineal gland. The pineal gland produces indoleamins of which melatonin is the most important. Melatonin is produced and secreted during the night. When days become shorter, the exposure of the animal to melatonin increases. This has, through not yet fully elucidated connections, a stimulating effect on the GnRH secretion by the hypothalamus in short-day breeders like sheep. In long-day breeders (horse) the increased exposure to melatonin during long nights (short days) inhibits the GnRH release by the hypothalamus. In this way the day length differences are recognized and translated into signals that turn on and off sexual activity. Of the domesticated animals, the cow lost most of its seasonality due to the process of domestication over the centuries.

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Stimuli from the environment are registered by the senses and transmitted to the brain. Sensory perceptions can be of a different nature. The eyes (light or the sight of other animals of the same species), the nose (sex odours), and the tactile senses register information from the environment, and the optic nerve, olfactory nerve and the sensory nerves transmit the message to the brain. The brain is able to translate the information and, if necessary, react by sending a nervous impulse through the nerve fibres to a target organ.

The hormonal system is a regulatory system that sends information by means of chemical messengers. This is a system that is regulated by feedback loops and impulses from the nervous system and several organs.

A hormone can be defined as a chemical substance, produced in a gland or tissue in the body, which evokes a specific reaction in hormone sensitive tissue. The action of the hormonal system can be subdivided according to the way the hormones reach the target cells. Acting in the autocrine way, the producing cell is also the target cell. Acting paracrine, the hormones influence the neighboring cells or organs. Endocrine hormones are transported by the blood, and acting exocrine the hormones are excreted (in the digestive tract, urinary tract, at the skin, etc.) before reaching the target organs. So far, most information is available about the endocrine functions of the hormonal system. During the past decade, the paracrine and autocrine functions have received more attention from researchers, but many aspects are still unknown.

After reaching a target cell the hormone has to provoke a reaction. For this, target cells have hormone-specific receptors. A hormone receptor is a unique molecular structure in or on the cell with a high and specific affinity for a particular hormone. After binding to the receptor the message can be passed on. This message will lead to a cell-specific response, which generally involves activation or inactivation of enzymes in the target cells.

So the hormone receptors have two important functions:
- Recognition of the specific hormone by the target cell;
- translation of the hormonal signal into a cell-specific response.

The effect of an endocrine hormone release can vary with circumstances. The number and type of receptors of a target cell are not fixed. The formation and degradation of receptors is a dynamic process. The function of one hormone in a cell can be the induction or degradation of receptors for another messenger. Furthermore, receptors can be blocked by an excess of hormones. Extra stimulation by a normally highly effective dose of hormones will then cause no further effect. Many pathological conditions in reproduction are caused by derailments at the receptor level. Most receptors need a second messenger to transmit the message. One of the best known second messengers is cyclic AMP.

After binding to the receptor, the hormone activates the adenylate-cyclase-system situated in the cell membrane. ATP is the converted into cyclic AMP. cAMP, the second messenger, in its turn, activates an inactive cAMP-protein-kinase-A that splits up into an active catalytic unit and a regulatory unit. The active catalytic unit of the protein-kinase will stimulate the phosphorylation of a protein synthesis, growth or hormone secretion.

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In mammals, two regulatory systems are involved in the reproductive process. The endocrine system and the nervous system both play a specific role. A subtle interplay between the two systems is essential for the cascade of events that finally has to result in the birth and successful rearing of healthy offspring. This section will give some basic theory about the functioning of the reproductive processes, and give an example of the interplay between the different steps in the process by means of a brief insight into the regulation of reproduction in the cow.

The principle of reproduction in the male shows a pattern similar to that in the female. GnRH from the hypothalamus stimulates the release of FSH and LF (in the male formerly called ICSH=interstitial cell stimulating hormone). FSH acts directly on the seminiferous tubules of the testis (germ cells and Sertoli cells). Here spermatogenesis is stimulated by FSH. Sertoli cells produce inhibin, which has a negative feedback on FSH secretion by the pituitary gland.

LH stimulates the production of testosterone by the Leydig cells. Testosterone influences the Sertoli cells and is necessary for successful spermatogenesis. Furthermore, testosterone induces morphological changes and typical male behaviour. Testosterone has a negative feedback on the LH secretion by suppressing the pulsatile GnRH release from the hypothalamus. So in the male similar control systems, with stimulation and feedback loops, can be found as in female reproduction.

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