HORMONAL CONTROL OF SPERMATOGENESIS IN FARM ANIMALS

-       The hormones control the reproduction even at the embryonic stage. In the developing embryo the gonads remain in different.

-       Differentiation of gonads into the testis in male occur earlier compare to the differentiation of gonad into ovary in female.

-       The primitive testis apparently secrets androgen like substance which is responsible for,

(1)          The development of mesonephric tubules and ducts into permanent genital duct system of the male.

(2)          The formation of the male’s external genitalia and

(3)          The degeneration of the paramesonephric duct system.

ü Normal reproductive activity in male animals comprises the production of semen containing normal spermatozoa in the adequate numbers, together with the desire and the ability to mate.

o  These sexual functions are under the intrinsic control through the hormones and the central nervous system but environmental factors such as temperature, length of day light, nutrition, change of surrounding and systematic disease may influence reproductive performance.

o  The same pituitary gonads tropic hormones occur as in the female and they act on testis as follows:

o  Follical stimulating hormone promotes development of the epithelium of the seminiferous tubules and there by it controls spermatogenesis.

o  Luteinizing hormone, or interstitial cell  stimulating hormone, promotes growth and function of the interstitial or leydig cells and thus the control the secretion of the male sex hormones, testosterone.

o  Chemically testosterone is steroid in nature it is closely related to estrogen and progesterone in females and is excreted as androsteron.

o  It causes development of outward male characteristic as well as promoting growth and the function of accessory sexual organs, by its influence on the accessory glands it affects the composition of the semen.

o  Therefore, the functions of the testicals are spermatogenesis and endocrine, since both are under pituitary control there is a close relationship between them. 

o  LH is required for the spermatogenesis because it is required for the production of the testosterone hormone.

o  However, testosterone alone is not adequate for completion of spermatogenesis and a block on spermatogenesis occurs at meiosis.

o  FSH is required for initial spermatogenesis: FSH is also important for completion of meiosis and for final morphological differentiation of the spermatid to spermatozoa through its influence on the activity of sertoli cells.

o  FSH also playing role in the release of spermatids from the syncytium of spermatids surrounding the sertoli cells.

o  There is some evidence that a third gonadotropin (prolactin) also plays an important role in the process of spermatogenesis, prolactin facilitates the inter action of LH with its receptors located on leydig cells.

o  The sertoli cells provide nutrition and regulate the spermatogenesis.

^o    Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived.

^o    Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two spermatids or young spermatozoa.

^o    These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa.

^o    Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis.

^o    In mammals it occurs in the male testes and epididymis in a stepwise fashion, and for humans takes approximately 64 days.

^o    Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction.

^o    It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age.

^o    The entire process can be broken up into several distinct stages, each corresponding to a particular type of cell.

§  Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways:

ü Maintain the environment necessary for development and maturation via the blood-testis barrier

ü Secrete substances initiating meiosis

ü Secrete supporting testicular fluid

ü Secrete Androgen-Binding Protein, which concentrates testosterone in close proximity to the developing gametes

ü Testosterone is needed in very high quantities for maintenance of the reproductive tract, and ABP allows a much higher level of fertility

ü Secrete hormones affecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, Inhibin.

ü Phagocytose residual cytoplasm left over from spermiogenesis.

ü They release Antimullerian hormone which prevents formation of the Mullerian Duct / Oviduct.

ü Protect spermatids from the immune system of the male.

Hormonal control of spermatogenesis

§  Hormonal control of spermatogenesis varies among species.

§  Initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamus, pituitary gland and Leydig cells.

§  If the pituitary gland is removed, spermatogenesis can still be initiated by follicle stimulating hormoneand testosterone.

§  Follicle stimulating hormone stimulates both the production of androgen binding protein by Sertoli cells, and the formation of the blood-testis barrier. 

§  Androgen binding protein is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis, which can be 20-50 times higher than the concentration found in blood.

§  Follicle stimulating hormone may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis.

§  However, increasing the levels of follicle stimulating hormone will increase the production of spermatozoa by preventing the apoptosis of type A spermatogonia.

§  The hormone inhibin acts to decrease the levels of follicle stimulating hormone.

§  Studies from rodent models suggest that gonadotropin hormones (both LH and FSH) support the process of spermatogenesis by suppressing the proapoptotic signals and therefore promote spermatogenic cell survival. 

§  The Sertoli cells themselves mediate parts of spermatogenesis though hormone production. They are capable of producing the hormones estradiol and inhibin.

§  The Leydig cells are also capable of producing estradiol in addition to their main product testosterone.

§  In mammals, pituitary control of spermatogenesis varies with age. In the rat, before puberty FSH is highly active whereas LH is not; after puberty it seems that LH alone is able to support spermatogenesis.

Hormonal regulation of spermatogenesis in experimental animals:

§  In the rat, the roles of follicle stimulating hormone (FSH) and luteinizing hormone (LH) vary with the age of the animal.

§  FSH is effective in the prepubertal rat in the support of spermatogonial multiplication.

§  LH is active in the hypophysectomized adult.

§  FSH acts by binding specifically to receptor sites on cellular membranes of the seminiferous epithelium inducing biochemical changes. The development of a system which limits the response to FSH according to age is suggested.

§  Spermatogenesis proceeds only to primary spermatocytes. Morphological maturation of Sertoli cells occurs as long as LH-like activity exceeded FSH-like activity.

§  In hypogonadotropic hypogonadism, FSH has been shown to be inactive on germ cells while LH allows resumption of meiosis after hypophysectomy.

Hormone	Secretion From	Function
GnRH	Hypothalamus	§  Control release of LH and FSH.
FSH	Anterior pituitary gland	§  Stimulates maturation of seminiferous tubules and spermatogenesis
LH (Luteinizing hormone)	Anterior pituitary gland	§  Produced by anterior pituitary increases testosterone production by Leydig cells
Testosterone	Leydig cells	§  Necessary for normal sperm development. §  It activates genes in Sertoli cells, which promote differentiation of spermatogonia.
Activin	Sertoli Cells	§  Enhances FSH biosynthesis and secretion
Inhibin	Sertoli Cells	§  Regulates FSH synthesis and inhibits FSH secretion.

Testicular hypoplasia

Definition :Testicular hypoplasia implies an incomplete development of the germinal epithelium of the seminiferous tubules, due to inadequate numbers of germinal cells within the testis. Lack of germinal cells may arise throughpartial or complete failure of the germinal cells to develop in the yolk sac, failure to migrate to the genital ridge, failure to multiply in the developing gonad, or widespread degeneration of embryonic germinal cells within the primitive gonad.

 General information: Mild cases may exhibit moderate oligospermia or poor sperm morphology, but severe cases may be aspermic. A hereditary form of hypoplasia exists in Swedish Highland cattle affecting the left testis more commonly than the right. Formerly, in the UK, many cases were detected when bulls were licensed at 10 months of age; however, the abandoning of such licensing means that the frequency of animals with relative hypoplasia is likely to increase.

            A high incidence of hypoplasia occurs in the Welsh Mountain pony, in which the right testis is most commonly affected, and in which an inherited aetiology is probable.

            Sporadic cases of hypoplasia occur in all species, occasionally, but not often, with a clear familial predisposition.

 Klinefelter’s syndrome (karyotype XXY): is a sporadic cause of testicular hypoplasia in bulls, and has been reported in rams, boars and dogs. It is also particularly associated with male tortoiseshell and calico cats.

·       The spermatogonia of such animals fail to develop,

·       Seminiferous tubules are virtually devoid of spermatogenic cells.

·       The semen of such animals is therefore aspermic,

·       The Leydig cells being unaffected,

·       Libido is normal.

Diagnosis :

·       By measurement of scrotal circumference, a value below acceptable limits for the species and breed being diagnostic.

·       Palpation of the testes reveals one or both to be small and flabby.

·       Semen analysis may reveal aspermic or oligospermic ejaculates, sometimeswith markedly abnormal morphology or motility characteristics of such sperm as are present.

·       By contrast, libido is generally normal and, for this reason, the condition may escape the owner’s attention until the failure to achieve satisfactory pregnancy rates is noticed.

Treatment: Because of the probable inherited basis of testicular hypoplasia, attempting to breed from an affected animal should be avoided. Attempts at treatment with exogenous hormones are invariably unsuccessful, so castration and (for meat animals) slaughter for recovery of the carcass value should be recommended.

Fig.  Testicular hypoplasia in a ram. The scrotal

circumference of 22 cm was well below the 30–35 cm

expected for a ram during the breeding season.

Physiology Of Male Reproductive System

ü Good reproductive performance of a bull is necessary to obtain a high percent calf crop when natural service is used for breeding.

ü A bull must be fertile, capable and willing to mate a large number of cows during a short breeding season for optimum production.

ü A basic knowledge of the reproductive tract is beneficial for improved management.

ü An understanding of the bull's reproductive system will also help the producer better understand breeding soundness examinations, reproductive problems and breeding impairments.

Anatomy

ü The reproductive tract of the bull consists of the testicles, secondary sex organs, and three accessory sex glands.

ü These organs work in concert for formation, maturation and transport of spermatozoa, which are eventually deposited in the female reproductive tract.

ü The secondary sex organs are the epididymis, vas deferens and penis.

ü The three accessory sex glands include the seminal vesicles, prostate and bulbourethral gland (Cowper's gland).

This basic anatomy is illustrated in Figure 1.

Figure 1. Drawing of the reproductive tract of the bull

Testicle

ü The testicle is located outside the body cavity in the scrotum and has two vital functions  producing the spermatozoa, and producing the male hormone, testosterone.

ü Location of the testicles exterior to the body cavity is essential for normal sperm formation, which occurs only at 4 degrees to 5 degrees below body temperature.

ü The scrotum provides physical protection to the testicle and helps regulate the temperature for optimum spermatozoa development. This regulation is done by coordination of three structures: a temperature-sensitive layer of muscle (tunica dartos) located in the    

   walls of the scrotum, which relaxes when hot and contracts when   

        cold; the external cremaster muscle within the spermatic cord,      

             which controls the proximity of the testicle to the body by                       

             lengthening or shortening depending on environmental  

             temperature; and a counter-current temperature exchange

    regulated by a blood flow process known as the pampiniform

    plexus, which is a coil of testicular veins that

ü provide an effective mechanism for cooling arterial blood entering the testicle and transferring its heat to the venous blood leaving the testicle.

ü One or both testicles occasionally fail to descend into the scrotum during embryological development and are retained in the body cavity. This condition is known as chryptorchidism. Hormone production by chryptorchid males is near normal and the male develops and behaves like a normal male, but will generally be subfertile. This condition is genetically inherited, therefore such males should not be used for breeding.

ü The testicle contains many long, tiny, coiled tubes known as seminiferou tubules, within which the sperm are formed and begin to mature. Scattered throughout the loose connective tissue surrounding the seminiferous tubules are many highly specialized cells, the interstitial cells of Leydig,  that produce testosterone.

ü There are hundreds of individual seminiferous tubules in the body of the testicle which unite with one another to form a few dozen tubules that exit from the testicle and pass into the epididymis.

Epididymis

ü The epididymis is a compact, flat, elongated structure closely attached to one side of the testicle.

ü  It is divided into three regions, the head, body and tail. The many tubules entered the head of the epididymis from the testicle unite to form a single tubule some 130 to 160 feet in length. This tubule is convoluted and packed into the 6- to 8-inch epididymis.

ü Four major functions occur in the epididymis, including the transport of the developing sperm cells from the testicle to the vas deferens; the concentration of the sperm by absorption of surplus fluids; the maturation of the developing spermatozoa; and the storage of viable sperm cells in the epididymis tail. If sexual activity is slowed, resorption of sperm cells from the epididymis tail occurs.

ü The epididymis serves as an outlet for all the sperm produced in the testicle and any blockage of this tube will cause sterility. Temporary blockage due to swelling following an injury or infection (epididymitis) will result in short-term infertility. If the swelling or infection results in formation of scar tissue in the tubule, it may permanently block the passage of sperm. If blockage occurs in both epididymides, the bull will no longer be useful as a breeder.

ü  Surgical removal of the tail of the epididymis (epididectomy) is frequently used as a means of sterilization for teaser (Gomer) bulls for estrus detection.

ü  Epididectomized bulls will still service cows in the usual manner, but will not deposit sperm in the female reproductive tract.

Vas deferens

ü The vas deferens, also known as ductus deferens, emerges from the tail of the epididymis as a straight tubule and passes as part of the spermatic cord through the inguinal ring into the body cavity.

ü Spermatozoa are transported further along the reproductive tract to the pelvic region through the vas deferens by contraction of the smooth muscle tissue surrounding this tubule during ejaculation.

ü Bulls may also be sterilized by a vasectomy in which a section of the vas deferens is removed so that sperm cannot pass to the outside of the body.

Urethra

ü The two vas deferens eventually unite into a single tube, the urethra, which is the channel passing through the penis. The urethra in the male serves as a    

          common passageway for semen from the reproductive tract and urine from    

          the urinary tract.

Accessory glands

ü Two of the accessory glands are found in the general region where the vas deferens unite to become the urethra. Secretions from these glands make up most of the liquid portion of the semen.

ü In addition, the secretions activate the sperm to become motile. The seminal vesicles consist of two lobes about 4 to 5 inches long, each connected to the urethra by a duct.

ü The prostate gland is located at the neck of the urinary bladder where it empties into the urethra. The prostate is relatively small in the bull, as compared to other species, and does not produce a very large volume secretion.

ü The third accessory gland, the Cowper's glands, are small, firm glands located on either side of the urethra. The clear secretion that often drips from the penis during sexual excitement prior to service is largely produced by these glands and serves to flush and cleanse the urethra of any urine residue that may be harmful to spermatozoa.

ü One of the accessory glands may occasionally become infected, resulting in semen samples that are yellow and cloudy and which contain puss cells.

ü It is not uncommon in bulls for the seminal vesicles to be so affected (seminal vesiculitis). Antibiotic treatment is sometimes necessary, but time will generally correct the problem.

Penis

ü The sigmoid flexure is an anatomical structure that provides a means by which the penis is held inside the sheath except during time of service.

ü Strong retractor muscles hold the penis in the "S" shaped configuration. Occasionally these muscles are too weak to function properly and a portion of the penis and sheath lining protrude at all times.

ü This exposes the male to the danger of injury and this characteristic should be avoided when selecting a herd bull.

ü The penis is the organ of insemination. Spongy-type material within the penis is filled with blood during sexual arousal, resulting in erection of the organ.

ü The end of the penis is the glans penis and is richly supplied nerves, which are stimulated during copulation to induce ejaculation. Impairments of the glans penis may exist (Figure 2) and should be detected during a fertility exam.

    Figure 2 :-  Sketches show abnormalities of    

                    Sperm cells and penis

              

Regulation of male hormones

ü The normal functions of male reproduction are largely controlled by hormones that are secreted from the endocrine glands.

ü  The testicle functions as an endocrine gland because of its production of the male hormone, testosterone, by the intersticial cells.

ü Testosterone has several major functions:

v It is largely responsible for development and maintenance of the male Reproductive tract.

v  It causes the development and maintenance of the secondary sex characteristics associated with masculinity, such as the crest and heavily muscled shoulders of a bull.

v It is a major factor in the normal sex drive and behavior of the male.

v It increases muscular and skeletal growth.

v It is essential for normal sperm formation.

ü The testicle is, in turn, under the influence of hormones produced by other glands in the body.

ü The same gonadotropic hormones that regulate ovarian functions in the cow also regulate testicular functions in the bull.

ü  Luteinizing hormone (LH) and follicle stimulating hormone (FSH) appear to be misnamed as pertaining to male reproduction, yet carry out several important male functions.

ü  LH and FSH are released from the pituitary gland and cause the testicle to secrete testosterone, which then acts on the germ cell lining of the seminiferous tubules to stimulate formation of primordial sperm cells.

ü The maturation of spermatids into fully developed sperm cells requires the presence of FSH.

ü  Normal functioning of the male accessory glands requires testosterone. Not only is hormone production of the testicle regulated by hormones released by the anterior pituitary, but the reverse is also true.

ü The level of testosterone in the blood regulates the secretion of gonadotropic hormones from the anterior pituitary via a feedback system.

ü A proper balance of all hormones is vital to successful reproductive functions.