The Anatomy & Physiology Of Sperm Production In Boars -

submaxillary gland (located below the jaw)androgen is converted to the 5-α androgenform and when aerosolized, induces thestanding response in female pigs. Thishormone has been synthesized and ismarketed as SOA (Sex Odor Aerosol,Intervet, Millsboro, DE) for stimulatingestrus in female swine when sprayed towardthe snout.Testicular DescentDuring the fetal period the testes of the malebegin development inside the abdomen andthen at approximately 60 days of gestation,are gradually pulled out of the abdominalcavity and into the scrotum (Figure 2). Theyare pulled through an opening called theinguinal canal, which allows passage ofthe testes and formation of the spermaticcord. As the male develops, the canalbecomes smaller, allowing enough room foronly the spermatic cord. Failure of the testesto descend can cause sterility. Failure of oneof the testicles to descend is called unilateralcryptorchidism and can lead to a male that isnormal in appearance but with reducedspermatogenic capability. The reduction inspermatogenic activity occurs since spermproduction by the testes must occur in thescrotum where temperatures are lower thanbody temperature. Testosterone levels arenot affected in these males and except forreduced sperm production, all behaviors andgrowth are normal. In the case where bothtestes fail to descend into the scrotum(bilateral cryptorchidism), these males arecompletely sterile even though testosteronelevels are normal and they appear to act likefertile males. One other abnormalityassociated with testicular descent is thecommon problem of scrotal hernia. Thisoccurs when some of the contents of theabdominal cavity pass through the inguinalcanal. This problem is thought to begenetically linked, and is often observed incastrated males.In barrows thisphenomenon may be due to improperpressure applied to the supportive structuresof the testes during castration.Boar MaturityPhysiological maturation in boars after birthis an ongoing process. Between 1-2 monthsof age, mounting behavior is first observedin young boars. At three months of age thereis a second period of germ cell division(future sperm producing cells) and a rapidincrease in the testes to body weight ratio.What this indicates is that this period and theperiod during fetal development when germcells also increased may be important fordetermining sperm production potential. Itfollows then, that factors that influence boarphysiology at these times could influencefertility potential. At about four months ofage, sperm first appear in the seminiferoustubules and erection can be accomplished inthe immature boar. At 5 ½ months of age,puberty begins and sperm now appear in theejaculate. Over the next 6-18 months, thetestes increase in size and both semenconcentration and ejaculate volumescontinue to increase. By 18 months of ageno appreciable improvements in fertility areobserved and the boar is considered fullymature.Sperm ProductionOnce puberty occurs, spermatogenesis is acontinual process. Inside the testicle, presperm cells, called spermatogonia, begin tomature near the outside wall of theseminferous tubes (Figure 6). As the cellmatures, it moves closer to the center of thetube where it will be released into the spermpassageway (lumen). During maturationinside the Sertoli cell, the sperm cellchanges shape from nearly a round shape tothe elongated sperm head with a tail (Figure4). Within any Sertoli cell, there are manysperm cells in different stages ofdevelopment. The time period required to4

change from a round cell on the outer wallof the tube to its release into theseminiferous tubule as a sperm with a tailtakes approximately 34-36 days. Theproduction of sperm cells in the testes is notsynchronized within the tubules so thatsperm cells can be produced for ejaculationon a daily basis. Therefore along the entirelength of the seminiferous tubules there aresegments that contain different stages ofsperm cell development.Sperm Maturation & StorageSperm leave the testicle and enter theepididymis. As sperm leave the testicle, theyenter the head of the epididymis. The spermcells are very concentrated and in this formare not motile. In this portion of theepididymis, the head of the spermincorporates proteins into the membrane,which are thought to be essential forfertilizing the egg. As they are moved downthe length of the epididymis by fluid andsmooth muscle contractions, they enter intothe mid-piece where they now acquire thecapability for tail movement. However,although they are capable of movement,they are still too concentrated to permitmotion. As the sperm are moved into the tailof the epididymis, additional proteins areadded which are important for spermfertility. Sperm are stored immotile in thetail region in concentrated form. The entireduration of the trip through the epididymisrequires 12-14 days. The sperm willacquire full motility and fertilizingcapability when diluted with seminal plasmain the ejaculate. Therefore, whenconsidering the total time needed for asperm cell to begin development until itappears in the ejaculate, requiresapproximately 45 days.Thermoregulation of the TestesThe process of spermatogenesis is highlydependent upon temperature and isespeciallysensitivetoelevatedtemperatures. Therefore the role of thescrotum is to regulate testes temperature tobe 3.5 F (2 C) lower than bodytemperature. The scrotum has both structuralfeatures and responsive mechanisms toregulate testes temperature. The structuralsystemincludeslowamountsofsubcutaneous fat in order to retain less heat.One of the more intricate mechanisms forheat loss is the intertwining arrangement ofthe arterial and venous blood vessels thatenter and leave the testes. In thisarrangement, warmed arterial blood enteringthe testes is cooled by the returning venousblood leaving the testes. This allows thecooling of 102 F arterial blood to 98 Fbefore it enter the testes. In the scrotum, theresponse mechanisms involve heat sensorsthat induce body heat loss through boarpanting. The scrotum also contains asurrounding muscle called the tunica dartos.In cold weather this muscle contracts andwrinkles the surface of the scrotum tominimize surface area heat loss and in warmweather relaxes and increases surface area.Another muscle called the cremaster, isfound in the spermatic cord and contracts orrelaxes in order to pull the testis closer to thebody in cold weather or let them hangfurther away in hot weather.Accessory GlandsThe accessory glands are important foradding seminal plasma to the sperm cells.Recently much attention has been directedtoward the importance and function of thesefluids. With AI, this component is highlydiluted in the semen extension process andtherefore its effect has been diminished.The prostate gland is the first gland to meetthe pelvic urethra. It is responsible forflushing out urine and any bacteria in thetract before entry of sperm into the urethra.The vesicular gland produces most of thevolume, energy sources, buffers and ions.5

The bulbourethral glands produce the gelatinplug. The gelatin plug comes out in manyfractions over time during ejaculation butpredominantly at the end of ejaculation. Thisis used to seal the large volume of ejaculate(150-500 cc) in the female uterus followingnatural service. The plug sets up in thecervix and may remain intact for a variablelength of time. When collecting semen fromthe boar for AI, the first part of the ejaculateis discarded, and since the gel plug isrepresented in many phases, boar semen istypically filtered during the collectionprocess.In general, the accessory glands areresponsible for providing volume to diluteand stimulate motility, provide a fluidmedium for ejaculation, energy in the formof citric acid and fructose, and pH buffering.Raw semen has a pH of 7.3-7.9, isapproximately 94-98% water, and containsnumerous organic and inorganic molecules.Other more uncertain roles of seminalplasma may involve its ability toimmunosuppress the female after breedingin order to prevent sperm from beingdestroyed in the female tract beforefertilization.The Boar PenisThe penis contains the penile urethra and isa common passageway for both semen andurine. The boar’s penis is composed oflimited erectile tissue and when sexualstimulation occurs, shows limited increase inlength and width. However, the boar peniswhen extended is quite long. When the boaris not sexually stimulated, the muscle iscontracted and forms an S-shaped bend inthe penis. This S-bend keeps the long boarpenis inside the boar’s body and preventsextension of the penis out of the prepuce.Once the boar is excited, neural stimulationcauses the retractor muscle to relax and theS-bend unfolds, and the penis extends. Theboar penis exits the male’s body through thepreputial opening. One unique feature of theboar is the presence of a preputial pouch ordiverticulum. This pouch is located justabove the prepuce and accumulates urineand sloughed cells. This can become quiteodorous. The pouch fluid is importantbecause it could potentially contaminate theejaculate. The fluid should therefore bemanually massaged to empty beforeejaculation.The extension of the penis causestension in the fibroelastic tissues of the boarpenis and causes twisting of the free end ofthe penis to form a corkscrew shape. Thecorkscrew shape is perfectly formed tomatch the patterns of pads inside thefemale’s cervix. Upon erection andintromission of the penis into the cervix, thelocking of the penis into the cervical pads isassociated with pressure. This pressurestimulus is needed to induce the ejaculationreflex in the boar.Semen Ejaculation & CollectionSexual stimulation (5-10 min restraint) ofthe boar prior to ejaculation has been shownto dramatically increase the concentrationand volume of the boar ejaculate. Sexualstimulationinitiatessmoothmusclecontractions in the tail of the epididymis andthe vas deferens. The boar usually takes 715 minutes to ejaculate the 150-500 mlvolume.Factors Influencing Semen QualityBreeds/LinesAlthough there have been reports of breeddifferencesinejaculatevolume,concentration, motility and even percent oflive/dead cells between European breeds ofswine (Duroc, Yorkshire, and Hampshire),the results appear inconclusive. Perhaps theclearest example of the effects of breed onboar fertility is illustrated when comparingChinese Meishan boars with conventionalbreeds of European origin. For example, the6

Meishan breed has 70% motile sperm inthe tail of the epididymis at 98 days of agecompared to 150 days of age for Europeanbreeds. However, despite the early pubertyfor the Meishan, the mature paired testesweight (269 vs. 541 g) and daily spermproduction (6 vs. 12 billion) are almost halfof European breeds.Testis SizeThe size of the testes is moderately heritableand has been related to early puberty in bothmale and female siblings and offspring. Theweight of the testes at puberty, and in theadult boar, is related to sperm output andeven testosterone levels. However, therelationship of testes length or width (thatwhich could be measured in the live animal)to spermatogenic potential remains unclear.Testis size has also been reported to changedue to season and is somewhat larger in coolweather and smaller in warm weather.However, testis measures in live boars ofsimilar age and weight tend not to be closelyrelated to fertility.Boar AgeThe boar continues to increase testes sizeand spermatogenic output after puberty, andit has been reported that boars less than 9months of age have both lower ejaculatevolumes and concentrations compared toboars 18 months of age or older. Therefore,in order to maximize fertility, boars less than1 year of age should be collected no morethan one time per week and boars over 1year of age can be collected up to two timesper week. There will obviously come a timein a boar’s reproductive life when fertility isdiminished by advanced age. This age is notwell established, since there has been a highrate of boar turnover with rapid geneticimprovement. The result of this is that manyboars are only in stud for a period 2-4 yearsbefore culling. However, it has beenobserved that in older aged boars, there arehigher incidences of sperm abnormalities,lower pregnancy rates and litter sizes.Daily Sperm ProductionDaily sperm production is not constant. Inone study, daily sperm production over aseven-month period of time fluctuatedbetween 12 and 14.5 billion cells producedper day.Sperm StorageSperm that are stored in the tail of theepididymis are fertile for several weeks.Older or excess sperm in the tail of theepididymis are lost through minor musclecontractions, which allow sperm to beeliminated with urine. For mature boars,collection of sperm twice a day forconsecutive days allows collection of 5billion sperm per ejaculate. When boars arerested and collected only twice a week 50billion sperm can be collected per ejaculate.Boars that are collected once every twoweeks have up to 100 billion sperm in theejaculate. However, with long periods ofsexual rest, an increase in the number ofdegenerating and non-fertile sperm isobserved. It is reported that sperm first losetheir ability to fertilize an egg, and then theylose their motility, and lastly the areobserved to disintegrate.Sperm AbnormalitiesSperm abnormalities in the ejaculatetypically take 14 days to appear due to thetime required for sperm that are formed atthe time of the stress or injury to travel tothe tail of the epididymis where they areejaculated. After this 14-day delay, thelength of time and degree to whichabnormalities appear depends upon theseverity and length of the stress and whichstages of sperm cell development weresusceptible to injury.The presence of cytoplasmic droplets onsperm is frequently observed. The droplet7

originates from separation of the sperm cellfrom the Sertoli cell. During the separationprocess, some of the cytoplasm naturallyremains attached to the sperm cell. As thesperm cell is moved along the seminiferoustubule and into the epididymis, the dropletmoves from the proximal position (close tothe head) further down the tail (distal) andthen falls off. However, in some cases ofstress, the ejaculate contains higher amountsof these droplets. It appears that the stressmay induce a sperm cell membrane defect orsomehow alters the normal maturationfunction of epididymis.Stress & EnvironmentSocialLibido and ejaculate volumes are lower inboars raised in physical isolation from othermales or females.LightingSemen quality appears not to be greatlyinfluenced by the effects of supplementallight during short days. However, increasingor decreasing light length in the oppositeseason can advance the age of puberty.Perhaps one of the best-studied stresses thataffect male fertility is temperature. Lowtemperatures have not been found to causegreat problems for boar fertility except whentemperatures dip below -10 C (14 F). Thisoccurs since boars will increase feed intakein order to keep body temperature elevated.This has the effect of allowing too muchheat to be produced in the body, which hasbeen shown to reduce fertility.Elevated temperatures are much more of aconcern. Temperatures above 85 F reduceboth sperm production and motility ofejaculated sperm for up to 2 months.Temperatures above 95 F for 3 days haveincreased abnormalities in ejaculatesbeginning 2-6 weeks after the stress. Thereason for the delay and length of theappearance of abnormalities depends uponthe 14-day transport in the epididymis andthe sperm cell stages during the 34-36 daydevelopment period. Heat stressed boarsalso have higher levels of basic proteins intheir ejaculates and these proteins areassociated with reduced sperm life ofextendedsemenatcoolstoragetemperatures.SeasonThe wild pig is a seasonal breeder so it is notunreasonable to assume that the domesticpig still may retain some seasonal responses.Wild boars (European) typically breed fromJanuary to May, and the female wild pig isanestrus from June to December. In thedomestic boar, sperm production appears tobe highest between September to Februarycompared to March through August. Thedifferences between the wild and thedomesticated boar may be a reflection of thedifferencesinavailablefeedstuffs,environmental housing, and management.Temperature8

Figure 1. Boar Reproductive Tract. (From Scientific Farm Animal Production, Taylor & Field)Figure 2. Boar Reproductive Tract (from Anatomy and Physiology of Farm Animals, Frandson).9

Figure 3.HypothalamusPituitaryTestesFigure 4.Male Hormone PathwaysFeedback System Functional by 1 Month of AgeEstrogenAccessory Sex GlandsMale TractBehavior10

Figure 5. Cross-section of Testicle (from Reproduction in Farm Animals, Hafez)Figure 6. Seminiferous Tubule (from Anatomy and Physiology of Farm Animals, Frandson, andApplied Animal Reproduction, Beardon and Fuquay)11

highly convoluted and densely packed tubes within the testes. All of the separate sperm producing tubes however, eventually converge into a single collection tube in the center of the testicle. The central sperm collection tube (rete testes, Figure 5) exits the boar testicle at the bottom and enters the storage and maturation tube, called the