28. ANATOMY OF MALE REPRODUCTIVE SYSTEM

28. ANATOMY OF MALE REPRODUCTIVE SYSTEM



STRUCTURES


Functions of the Male Reproductive System

1. The testes produce sperm and the male sex hormone testosterone.

2. The ducts transport, store, and assist in maturation of sperm.

3. The accessory sex glands secrete most of the liquid portion of semen.

4. The penis contains the urethra, a passageway for ejaculation of semen and excretion of urine


A. EXTERNAL REPRODUCTIVE ORGAN

1. Penis - delivers sperm into the female reproductive tract
2. Scrotum - supports the testes.


B. INTERNAL REPRODUCTIVE ORGANS

1. Testes
2. Accessory glands
3. Accessory ducts


Penis

• The penis contains the urethra 

•  is a passageway for 

1. the ejaculation of semen and
2.  the excretion of urine . 

It is cylindrical in shape and 

•  consists of 

         a. a body, 
         b.  glans penis, and
         c. a root. 

 
The body of the penis

•  is composed of three cylindrical masses of tissue, 
• each surrounded by fibrous tissue called the tunica albuginea . 

• The two dorsolateral masses are called the corpora cavernosa penis 
• corpora = main bodies;
• cavernosa = hollow). 

 
Corpus spongiosum penis

• The smaller midventral mass, 
• contains the spongy urethra and
• keeps it open during ejaculation. 

Erectile tissue

• Skin and a subcutaneous layer enclose all three masses, which consist of erectile tissue. 
•  is composed of numerous blood sinuses (vascular spaces)
• lined by endothelial cells and
•  surrounded by smooth muscle and elastic connective tissue.

Glans penis

• The distal end of the corpus spongiosum penis is a slightly

         enlarged, 
         acorn-shaped region called the glans penis; 

• its margin is the corona. 
• The distal urethra enlarges within the glans penis 
• and forms a terminal slitlike opening, the external urethral orifice.

Prepuce  or foreskin.

• Covering the glans in an uncircumcised penis is the loosely fitting prepuce (PRE¯ -poos), or foreskin.

The root of the penis 

           is the attached portion (proximal portion).

 
It consists of 


1.the bulb of the penis, 

• the expanded portion of the base of the corpus spongiosum penis, and

 

2. the crura of the penis 

• (singular is crus resembling a leg), 
• the two separated and tapered portions of the corpora cavernosa penis. 

The bulb of the penis 

• is attached to the inferior surface of the deep muscles of the perineum and

•  is enclosed by the bulbospongiosus muscle,

                                 - a muscle that aids ejaculation. 


Crura of the penis 

• Each crus of the penis bends laterally away from the bulb of the penis to attach to the ischial and inferior pubic rami 
• and is surrounded by the ischiocavernosus muscle . 

• The weight of the penis is supported by two ligaments that are continuous with the fascia of the penis.

 (1) The fundiform ligament arises from the inferior part of the linea alba. 

(2) The suspensory ligament of the penis arises from the pubic symphysis.



Scrotum

• (SKRO¯ -tum bag), 
• the supporting structure for the testes, 
• consists of loose skin and underlying subcutaneous layer that hangs from the root (attached portion) of the penis. 

Externally, 

• the scrotum looks like a single pouch of skin

• separated into lateral portions by a median ridge called the raphe (RA-fe¯ =seam). 

Internally, 

• the scrotal septum divides the scrotum into two sacs, each containing a single testis . 
• The septum is made up of a subcutaneous layer and muscle tissue called the dartos muscle (DAR-to¯s = skinned), 
• which is composed of bundles of smooth muscle fibers

• The dartos muscle is also found in the subcutaneous layer of the scrotum. 

Cremaster muscle

• (kre¯-MAS-ter suspender)
• Associated with each testis in the scrotum is the cremaster muscle , 
• a series of small bands of skeletal muscle that descend as an extension of the internal oblique muscle through the spermatic cord to surround the testes.

 In response to cold temperatures, the cremaster and dartos muscles contract.

• Contraction of the cremaster muscles moves the testes closer to the body, where they can absorb body heat. 

• Contraction of the dartos muscle causes the scrotum to become tight (wrinkled in appearance), which reduces heat loss. 

• Exposure to warmth reverses these actions.

• The location of the scrotum and the contraction of its muscle fibers regulate the temperature of the testes. 

• Normal sperm production requires a temperature about 2–3 C below core body temperature. This lowered temperature is maintained within the scrotum because it is outside the pelvic cavity.

TESTES

The testes (TES-te¯z), or testicles

•  are paired 
• oval glands in the scrotum 
• measuring about 5 cm (2 in.) long and 2.5 cm (1 in.) in diameter . 
• Each testis (singular) has a mass of 10–15 grams. 

• The testes develop near the kidneys, in the posterior portion of the abdomen, and 
• By third fetal month , it starts it descend
• they usually begin their descent into the scrotum through the inguinal canals during the latter half of the seventh month of fetal development.

• Inguinal canal  is a passageway in the anterior abdominal wall leading to scrotum 

COVERINGS OF TESTES

 1. Tunica vaginalis (tunica sheath)

• serous membrane 
• which is derived from the peritoneum and
•  forms during the descent of the testes,
•  partially covers the testes.

•  A collection of serous fluid in the tunica vaginalis is called a hydrocele (HI¯- dro¯-se¯l; hydro- water; -kele hernia). 
• It may be caused by injury to the testes or inflammation of the epididymis. 
• Usually, no treatment is required. 

2. The tunica albuginea

• Internal to the tunica vaginalis 
• is a white fibrous capsule composed of dense irregular connective tissue,
•  (al -bu¯-JIN-e¯ -a; albu- white); 
• It is a continuation of membrane of abdomino - pelvic cavity.

• it extends inward, forming septa that divide the testis into a series of internal compartments called lobules. 

• Each of the 200–300 lobules contains one to three tightly coiled tubules, the seminiferous tubules (semin- seed; fer- to carry), where sperm are produced.

SEMINIFEROUS TUBULES

Spermatogenesis

• The process by which the seminiferous tubules of the

testes produce sperm is called spermatogenesis (sper -ma-to¯-

JEN-e-sis; genesis beginning process or production).

• Tightly coiled tubules in each testes

• The seminiferous tubules contain two types of cells: 

1. GERMINAL TISSUE 

•     in the lining of walls of seminiferous tubules
• contain 2 types of cells

           a. spermatogenic cells -  the sperm-forming cells, and

  

           b.  sustentacular cells - provide nourishment for germinal sperm 

Sertoli cells - a type of sustentacular cell -  which have several functions in supporting spermatogenesis . 

• Embedded among the spermatogenic cells in the seminiferous tubules are large Sertoli cells or sustentacular cells  (sus -ten-TAK-u¯ -lar), 
• which extend from the basement membrane to the lumen of the tubule. 

Sertoli cells 

• support and protect developing spermatogenic cells in several ways. 

1. They nourish spermatocytes, spermatids, and sperm; 
2. phagocytize excess spermatid cytoplasm as development proceeds; and 
3. control movements of spermatogenic cells and the release of sperm into the lumen of the seminiferous tubule.
4.  They also produce fluid for sperm transport, 
5.  secrete the hormone inhibin, and
6.  regulate the effects of testosterone and FSH (follicle-stimulating hormone).

• Internal to the basement membrane and spermatogonia, tight junctions join neighboring Sertoli cells to one another. 

• These junctions form an obstruction known as the blood–testis barrier because substances must first pass through the Sertoli cells before they can reach the developing sperm.

•  By isolating the developing gametes from the blood, the blood–testis barrier prevents an immune response against the spermatogenic cell’s surface antigens, which are recognized as “foreign” by the immune system. 
• The blood–testis barrier does not include spermatogonia

2. LEYDIG CELLS

• In the spaces between adjacent seminiferous tubules are clusters of cells called Leydig (interstitial) cells 

• Interstitial endocrinocytes
• clustres of endocrine cells in between seminiferous tubules.

• secrete male sex hormone / androgen - testosterone.

•  An androgen is a hormone that promotes the development of masculine characteristics. 
• Testosterone also promotes a man’s libido (sexual drive).

Spermatogonia

• Stem cells called spermatogonia (sper - ma-to¯-GO¯ -ne¯ -a; -gonia offspring; singular is spermatogonium)

• develop from primordial germ cells (primordi- primitive or early form) that arise from the yolk sac 
• and enter the testes during the fifth week of development. 

• In the embryonic testes, the primordial germ cells differentiate into spermatogonia, which remain dormant during childhood and actively begin producing sperm at puberty. 

• Toward the lumen of the seminiferous tubule are layers of progressively more mature cells.
•  In order of advancing maturity, 

                 these are 

                      a. primary spermatocytes, 

                      b. secondary spermatocytes, 

                      c. spermatids, and 

                      d. sperm cells. 

• After a sperm cell, or spermatozoon (sper -ma-to¯ -ZO¯ -on; -zoon life) has formed, it is released into the lumen of the seminiferous tubule. (The plural terms are sperm and spermatozoa.)

Reproductive System Ducts / accessory ducts in Males

Sperm produced in the testes are carried to the point of ejaculation from penis by a system of ducts.

1. Duct of testes
2. Epididymus
3. Ductus deferens
4. Ejaculatory duct
5. Urethra


1. Ducts of the Testis

• Pressure generated by the fluid secreted by Sertoli cells pushes sperm and fluid along the lumen of seminiferous tubules and then into a series of very short ducts called straight tubules / tubule recti. 
•  The straight tubules lead to a network of ducts in the testis called the rete testis (RE¯ -te¯ network). 
• From the rete testis, sperm move into a series of coiled efferent ducts in the epididymis that empty into a single tube called the ductus epididymis.

2. Epididymis


The epididymis (ep -i-DID-i-mis; epi- above or over; -didymis
testis) 

• is a comma-shaped organ 
• about 4 cm (1.5 in.) long 
• that lies along the posterior border of each testis.

The plural is epididymides (ep -i-did-I¯M-i-de¯s). 




seminiferous tubules 
         
                      ↓

   becomes straight - tubule recti 

                    ↓
opens into tubules - network of tube - Rete testes

                    ↓    

drain into 15 - 20 tubes called efferent ducts

                      ↓    

efferent duct extend upward into a convoluted mass of tube 

that forms a crescent shape along its side

                        ↓

this coiled tube is called epididymus


Each epididymis

• consists mostly of the tightly coiled ductus epididymis.

• The ductus epididymis would measure about 6 m (20 ft) in length if it were uncoiled. 

• It is lined with pseudostratified  columnar epithelium and 

• encircled by layers of smooth muscle.


• The free surfaces of the columnar cells contain stereocilia,



• which despite their name are long, branching microvilli (not

cilia) that increase the surface area for the reabsorption of degenerated sperm. 
• Connective tissue around the muscle layer attaches the loops of the ductus epididymis and carries blood vessels and nerves.

Epididymus has 

                  1. head

                  2. body

                  3. tail

• The efferent ducts from the testis join the ductus epididymis at the larger, superior portion of the ep
• ididymis called the head.

• The body is the narrow midportion of the epididymis, and 

• the tail is the smaller, inferior portion.
• At its distal end, the tail of the epididymis continues as the ductus (vas) deferens

Functions

1. store sperm until maturity which remain viable here for up to several months. 

• Any stored sperm that are not ejaculated by that time are eventually reabsorbed.

2. passage way of sperm

3. propel sperm to next duct system , into the ductus (vas)

deferens during sexual arousal by peristaltic contraction of its

smooth muscle.

sperm maturation,

• the process by which sperm acquire motility and the ability to fertilize an ovum. 
• This occurs over a period of about 14 days.

3. Ductus deferens

• Within the tail of the epididymis, the ductus epididymis becomes less convoluted, and its diameter increases. 

• Beyond this point, the duct is known as the ductus deferens or vas deferens.
•  which is about 45 cm (18 in.) long, 

• ascends along the posterior border of the epididymis through the spermatic cord and 
• then enters the pelvic cavity. 
• There it loops over the ureter and 
• passes over the side and down the posterior surface of the urinary bladder . 

ampulla

• The dilated terminal portion of the ductus deferens is the
•  (am-PUL-la little jar). 

The mucosa of the ductus deferens consists of 

• pseudostratified columnar epithelium
• and lamina propria (areolar connective tissue). 

The muscularis is composed of three layers of smooth muscle;

•  the inner and outer layers are longitudinal, 
• and the middle layer is circular.

Functions

1.   conveys sperm during sexual arousal from the epididymis toward the urethra by peristaltic contractions of its muscular coat. 

2. Like the epididymis, the ductus deferens also can store sperm for several months.

• Any stored sperm that are not ejaculated by that time are eventually reabsorbed.

Spermatic Cord

•  is a supporting structure of the male reproductive system that ascends out of the scrotum .
• It consists of the 

                   a. ductus (vas) deferens as it ascends through the scrotum, 
                   b. the testicular artery, 
                   c. veins that drain the testes 
                   d. and carry testosterone into circulation (the pampiniform plexus), 
                   e. autonomic nerves, 
                   f. lymphatic vessels, and 
                   g. the cremaster muscle. 


The term varicocele (VAR-i-ko¯ -se¯l; varico- varicose; -kele hernia)

• refers to a swelling in the scrotum due to a dilation of the veins that drain the testes. 
• It is usually more apparent when the person is standing and typically does not require treatment.

Inguinal canal

• The spermatic cord and ilioinguinal nerve pass through the inguinal canal (IN-gwin-al groin), 
• an oblique passageway in the anterior abdominal wall just superior and parallel to the medial half of the inguinal ligament. 
• The canal, which is about 4–5 cm (about 2 in.) long, 
• originates at the deep (abdominal) inguinal ring, a slitlike opening in the aponeurosis of the transversus abdominis muscle; 
• the canal ends at the superficial (subcutaneous) inguinal ring , a somewhat triangular opening in the aponeurosis of the external oblique muscle. 
• In females, the round ligament of the uterus and ilioinguinal nerve pass through the inguinal canal.

4. Ejaculatory Ducts

• (e-JAK-u¯ -la-to¯ -re¯; ejacul- to expel)

• Each ejaculatory duct  is about 2 cm (1 in.) long and 
• is formed by the union of the duct from the seminal vesicle and the ampulla of the ductus (vas) deferens. 

•  form just superior to the base (superior portion) of the prostate and 
• pass inferiorly and anteriorly through the prostate. 
• They terminate in the prostatic urethra, where they eject sperm and seminal vesicle secretions just before the release of semen from the urethra to the exterior.

5. Urethra

• In males, the urethra is the shared terminal duct of the reproductive and urinary systems; 
• it serves as a passageway for both semen and urine. 
• About 20 cm (8 in.) long, 
• it passes through the

                         a. prostate,
                         b.  the deep muscles of the perineum, and
                         c.  the penis, and

•  is subdivided into three parts . 

1. The prostatic urethra 

• is 2–3 cm (1 in.) long 
• and passes through the prostate. 

2. The membranous urethra

• As this duct continues inferiorly, it passes through the deep muscles of the perineum, where it is known as the membranous urethra. 
•  is about 1 cm (0.5 in.) in length. 

3. The spongy urethra

• As this duct passes through the corpus spongiosum of the penis, it is known as the spongy (penile) urethra, 
• which is about 15–20 cm (6–8 in.) long.  
• ends at the external urethral orifice.

Accessory Sex Glands

1. Seminal vesicle
2. Prostate
3. Bulbo-urethral gland

• The ducts of the male reproductive system store and transport sperm cells,
•  but the accessory sex glands secrete most of the liquid portion of semen. 

Seminal Vesicles

• The paired seminal vesicles (VES-i-kuls) or seminal glands
•  are convoluted pouchlike structures, 
• about 5 cm (2 in.) in length, 
• lying posterior to the base of the urinary bladder and 
• anterior to the rectum .

Through the seminal vesicle ducts

•  they secrete an alkaline, viscous fluid that contains 

                       1. fructose (a monosaccharide sugar), 
                       2. prostaglandins, and
                       3. clotting proteins that are different from those in blood. 

• The alkaline nature of the seminal fluid helps to neutralize the acidic environment of the male urethra and female reproductive tract that otherwise would inactivate and kill sperm. 

• The fructose is used for ATP production by sperm. 

• Prostaglandins contribute to sperm motility and viability
• and may stimulate smooth muscle contractions within the female reproductive tract. 

• The clotting proteins help semen coagulate after ejaculation. 

• Fluid secreted by the seminal vesicles normally

                              - constitutes about 60% of the volume of semen.


Prostate

•  (PROS-ta¯ t)
•  is a single, doughnut-shaped gland
• about the size of a golf ball. 
• It measures about 

                   - 4 cm (1.6 in.) from side to side, 
                   - about 3 cm (1.2 in.) from top to bottom, and
                   - about 2 cm (0.8 in.) from front to back. 

• It is inferior to the urinary bladder and surrounds the prostatic urethra . 

• The prostate slowly increases in size from birth to puberty.
•  It then expands rapidly until about age 30, after which time its size typically remains stable until about age 45, when further enlargement may occur.

• The prostate secretes a milky, slightly acidic fluid (pH about 6.5) that contains several substances.

 (1) Citric acid in prostatic fluid is used by sperm for ATP production via the Krebs cycle.

(2) Several proteolytic enzymes, such as prostate-specific antigen (PSA), pepsinogen, lysozyme, amylase, and hyaluronidase,
eventually break down the clotting proteins from the seminal
vesicles. 

(3) The function of the acid phosphatase secreted by the prostate is unknown. 
(4) Seminalplasmin in prostatic fluid is an antibiotic that can destroy bacteria. 

• Seminalplasmin may help decrease the number of naturally occurring bacteria in semen and in the lower female reproductive tract. 
• Secretions of the prostate enter the prostatic urethra through many prostatic ducts.

Prostatic secretions 

• make up about 25% of the volume of semen and 
• contribute to sperm motility and viability.

Bulbourethral Glands

• The paired bulbourethral glands (bul -bo¯-u¯-RE--thral), or Cowper’s glands, 
• about the size of peas. 
• They are located inferior to the prostate on either side of the membranous urethra within the deep muscles of the perineum, and
•  their ducts open into the spongy urethra 

 During sexual arousal, 

• the bulbourethral glands secrete an alkaline fluid into the urethra that protects the passing sperm by neutralizing acids from urine in the urethra. 
• They also secrete mucus that lubricates the end of the penis and the lining of the urethra, decreasing the number of sperm damaged during ejaculation

Semen

• Semen ( seed) is a mixture of sperm and seminal fluid, a liquid that consists of the secretions of the seminiferous tubules, seminal vesicles, prostate, and bulbourethral glands. 

• The volume of semen in a typical ejaculation is 2.5–5 milliliter (mL), 

                  - with 50–150 million sperm per mL. 

• When the number falls below 20 million/mL, the male is likely to be infertile. 

• A very large number of sperm is required for successful fertilization because only a tiny fraction ever reaches the secondary oocyte.

• Despite the slight acidity of prostatic fluid, semen has a slightly alkaline pH of 7.2–7.7 due to the higher pH and larger volume of fluid from the seminal vesicles. 

•  The prostatic secretion gives semen a milky appearance, and 
• fluids from the seminal vesicles and bulbourethral glands give it a sticky consistency.

• Seminal fluid provides sperm with a transportation medium, nutrients, and protection from the hostile acidic environment of the male’s urethra and the female’s vagina.

• Once ejaculated, liquid semen coagulates within 5 minutes due to the presence of clotting proteins from the seminal vesicles.
• The functional role of semen coagulation is not known, 
•  but the proteins involved are different from those that cause blood coagulation. 

• After about 10 to 20 minutes, semen reliquefies because prostate-specific antigen (PSA) and other proteolytic enzymes produced by the prostate break down the clot. 

• Abnormal or delayed liquefaction of clotted semen may cause complete or partial immobilization of sperm, thereby inhibiting their movement through the cervix of the uterus. 

• The presence of blood in semen is called hemospermia (he¯ mo¯-SPER-me¯-a; hemo- blood; -sperma seed). 
• In most cases, it is caused by inflammation of the blood vessels lining the seminal vesicles; 
• it is usually treated with antibiotics.

Spermatogenesis

In humans, 

• spermatogenesis takes 65–75 days. 
• It begins with the spermatogonia, which contain the diploid (2n) number of chromosomes . 

• Spermatogonia are types of stem cells; 
• when they undergo mitosis, some spermatogonia remain near the basement membrane of the seminiferous tubule in an undifferentiated state to serve as a reservoir of cells for future cell division and subsequent sperm production. 

•  The rest of the spermatogonia lose contact with the basement membrane,
• squeeze through the tight junctions of the blood–testis barrier,
• undergo developmental changes, 
• and differentiate into primary spermatocytes (SPER-ma-to¯-sı¯tz ). 

• Primary spermatocytes, like spermatogonia, are diploid (2n); that is, they have 46 chromosomes.

• Shortly after it forms, each primary spermatocyte replicates its DNA 
• and then meiosis begins . 

• In meiosis I, homologous pairs of chromosomes line up at the metaphase plate,
• and crossing-over occurs. 

• Then, the meiotic spindle pulls one (duplicated) chromosome of each pair to an opposite pole of the dividing cell. 
• The two cells formed by meiosis I are called secondary spermatocytes. 

• Each secondary spermatocyte has 23 chromosomes, the haploid number (n). 
• Each chromosome within a secondary spermatocyte, however, is made up of two chromatids (two copies of the DNA) still attached by a centromere. 
• No replication of DNA occurs in the secondary spermatocytes.

• In meiosis II, the chromosomes line up in single file along the metaphase plate, 
• and the two chromatids of each chromosome separate. 

• The four haploid cells resulting from meiosis II are called spermatids. 
• A single primary spermatocyte therefore produces four spermatids via two rounds of cell division (meiosis I and meiosis II). 

•  As spermatogenic cells proliferate, they fail to complete cytoplasmic separation (cytokinesis). 
• The cells remain in contact via cytoplasmic bridges through their entire development .

•  This pattern of development most likely accounts for the synchronized production of sperm in any given area of seminiferous tubule. 
• It may also have survival value in that half of the sperm contain an X chromosome and half contain a Y chromosome.
•  The larger X chromosome may carry genes needed for spermatogenesis that are lacking on the smaller Y chromosome.

• The final stage of spermatogenesis, spermiogenesis (sper - me¯-o¯-JEN-e-sis), is the development of haploid spermatids into sperm.
•  No cell division occurs in spermiogenesis; 
• each spermatid becomes a single sperm cell. 

• During this process, spherical spermatids transform into elongated, slender sperm. 
• An acrosome  forms atop the nucleus, which condenses and elongates, 
• a flagellum develops, 
• and mitochondria multiply. 
• Sertoli cells dispose of the excess cytoplasm that sloughs off. 
• Finally, sperm are released from their connections to Sertoli cells, an event known as spermiation.

•  Sperm then enter the lumen of the seminiferous tubule. 
• Fluid secreted by Sertoli cells pushes sperm along their way, toward the ducts of the testes. 
• At this point, sperm are not yet able to swim.

Sperm

• Each day about 300 million sperm complete the process of spermatogenesis.

• A sperm is about 60 m long 
• and contains several structures that are highly adapted for reaching and penetrating a secondary oocyte . 

The major parts of a sperm are 

• the head 
• and the tail.

HEAD

•  The flattened, pointed head of the sperm 
• is about 4–5 m long. 
• It contains a nucleus with 23 highly condensed chromosomes.

• Covering the anterior two-thirds of the nucleus is the acrosome (acro- atop; -some body), 
• a caplike vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte to bring about fertilization. 
• Among the enzymes are 

                   hyaluronidase 

                   and proteases. 

TAIL

The tail of a sperm 

• is subdivided into four parts: 

 1. neck,

 2. middle piece,

 3. principal piece,

 4. and end piece. 

• The neck is the constricted region just behind the head that contains centrioles. 
• The centrioles form the microtubules that comprise the remainder of the tail. 

• The middle piece contains mitochondria arranged in a spiral, which provide the energy (ATP) for locomotion of sperm to the site of fertilization and for sperm metabolism. 

• The principal piece is the longest portion of the tail, and

•  the end piece is the terminal, tapering portion of the tail. 

• Once ejaculated, most sperm do not survive more than 48 hours within the female reproductive tract.

Hormonal Control of the Testes

Although the initiating factors are unknown,

•  at puberty certain hypothalamic neurosecretory cells increase their secretion of gonadotropin-releasing hormone (GnRH). 
• This hormone in turn stimulates gonadotrophs in the anterior pituitary to increase their secretion of the two gonadotropins, 

1. luteinizing hormone (LH) and
2. follicle-stimulating hormone (FSH). 

1. LH 

• stimulates Leydig cells, which are located between seminiferous tubules, to secrete the hormone testosterone (tes TOSte- ro¯ n). 

• This steroid hormone is synthesized from cholesterol in the testes and is the principal androgen. 
• It is lipid-soluble and 
• readily diffuses out of Leydig cells into the interstitial fluid and then into blood. 

• Via negative feedback, testosterone suppresses secretion of LH by anterior pituitary gonadotrophs and suppresses secretion of GnRH by hypothalamic neurosecretory cells.
•  In some target cells, such as those in the external genitals and prostate, 

                        - the enzyme 5 alpha-reductase converts testosterone to another androgen called dihydrotestosterone (DHT).

2. FSH 

• acts indirectly to stimulate spermatogenesis . 
• FSH and testosterone act synergistically on the Sertoli cells to stimulate secretion of androgen-binding protein (ABP)  into the lumen of the seminiferous tubules and into the interstitial fluid around the spermatogenic cells. 

• ABP binds to testosterone, keeping its concentration high. 

• Testosterone stimulates  the final steps of spermatogenesis in the seminiferous tubules.
• Once the degree of spermatogenesis required for male reproductive functions has been achieved, Sertoli cells release inhibin, a protein hormone named for its role in inhibiting FSH secretion by the anterior pituitary . 

• If spermatogenesis is proceeding too slowly, less inhibin is released, 
• which permits more FSH secretion 
• and an increased rate of spermatogenesis.

Testosterone and dihydrotestosterone 

• both bind to the same androgen receptors, which are found within the nuclei of target cells. 
• The hormone–receptor complex regulates gene expression, turning some genes on and others off. 
•  Because of these changes, the androgens produce several effects:

• Prenatal development.

 Before birth,

• testosterone stimulates the male pattern of development of reproductive system ducts and the descent of the testes. 
• Dihydrotestosterone stimulates development of the external genitals . 
• Testosterone also is converted in the brain to estrogens (feminizing hormones), which may play a role in the development of certain regions of the brain in males.

• Development of male sexual characteristics.

At puberty,

• testosterone and dihydrotestosterone bring about development and enlargement of the male sex organs and the development of masculine secondary sexual characteristics.

• Secondary sex characteristics are traits that distinguish males and females but do not have a direct role in reproduction.

• These include 

1. muscular and skeletal growth that resultsin wide shoulders and narrow hips; 

2. facial and chest hair (within hereditary limits) and more hair on other parts of the body; 

3. thickening of the skin; 

4. increased sebaceous (oil) glandsecretion; 

5. and enlargement of the larynx and consequent deepening of the voice.

• Development of sexual function. 

• Androgens contribute to male sexual behavior and spermatogenesis and to sex drive (libido) in both males and females. the adrenal cortex is the main source of androgens in females.

• Stimulation of anabolism. 

• Androgens are anabolic hormones; that is, they stimulate protein synthesis. 
•  This effect is obvious in the heavier muscle and bone mass of most men as compared to women.

• A negative feedback system regulates testosterone production 
• When testosterone concentration in the blood increases to a certain level, it inhibits the release of GnRH by cells in the hypothalamus. 

• As a result, there is less GnRH in the portal blood that flows from the hypothalamus to the anterior pituitary.

• Gonadotrophs in the anterior pituitary then release less LH, so the concentration of LH in systemic blood falls. 

• With less stimulation by LH, the Leydig cells in the testes secrete less testosterone,
• and there is a return to homeostasis. 

• If the testosterone concentration in the blood falls too low, however, GnRH is again released by the hypothalamus and stimulates secretion of LH by the anterior pituitary.
•  LH in turn stimulates testosterone production by the testes.

ERECTION

Upon sexual stimulation (visual, tactile, auditory, olfactory,

or imagined), 

• parasympathetic fibers from the sacral portion of the spinal cord 
• initiate and maintain an erection, the enlargement and stiffening of the penis. 

• The parasympathetic fibers release and cause local production of nitric oxide (NO). 
• The NO causes 

  A. smooth muscle in the walls of arterioles supplying erectile tissue to relax, which allows these blood vessels to dilate.

• This in turn causes large amounts of blood to enter the erectile tissue of the penis. 

 B. causes the smooth muscle within the erectile tissue to relax, 

• resulting in widening of the blood sinuses.

• The combination of increased blood flow and widening of the blood sinuses results in an erection. 
• Expansion of the blood sinuses also compresses the veins that drain the penis; 
• the slowing of blood outflow helps to maintain the erection.

The term priapism (PRI¯-a-pizm) refers to 

• a persistent and usually painful erection of the penis that does not involve sexual desire or excitement. 
• The condition may last up to several hours
• and is accompanied by pain and tenderness. 
• It results from abnormalities of blood vessels and nerves, 
• usually in response to medication used to produce erections in males who otherwise cannot attain them. 

Other causes include 

• a spinal cord disorder,
• leukemia, sickle-cell disease, 
• or a pelvic tumor.

EJACULATION

Ejaculation (e¯-jak-u¯-LA¯ -shun; ejectus- to throw out), 

• the powerful release of semen from the urethra to the exterior, 
• is a sympathetic reflex coordinated by the lumbar portion of the spinal cord. 

• As part of the reflex, the smooth muscle sphincter at the base of the urinary bladder closes, preventing urine from being expelled during ejaculation, and semen from entering the urinary bladder.

 Even before ejaculation occurs, 

•  peristaltic contractions in the epididymis, ductus (vas) deferens, seminal vesicles, ejaculatory ducts, and prostate propel semen into the penile portion of the urethra (spongy urethra). 

Typically, 

this leads to

emission (e--MISH-un), the discharge of a small volume of semen before ejaculation. 

Emission may also occur during sleep (nocturnal emission). 

The musculature of the penis (bulbospongiosus, ischiocavernosus, and superficial transverse perineus muscles), which is supplied by the pudendal nerve, 

• also contracts at ejaculation 

Once sexual stimulation of the penis has ended, 

• the arterioles supplying the erectile tissue of the penis constrict and the smooth muscle within erectile tissue contracts, making the blood sinuses smaller. 
• This relieves pressure on the veins supplying the penis and allows the blood to drain through them. 
• Consequently, the penis returns to its flaccid (relaxed) state.

Circumcision

Circumcision ( to cut around) is a surgical procedure in which part of or the entire prepuce is removed. 

It is usually performed 

      - just after delivery,

      - 3 to 4 days after birth, 

      -or on the eighth day as part of a Jewish religious rite. 

• Although most health-care professionals find no medical justification for circumcision, some feel that it has benefits, such as a

1. lower risk of urinary tract infections, 
2. protection against penile cancer,
3. and possibly a lower risk for sexually transmitted diseases. 

Indeed,

studies in several African villages have found lower rates of HIV infection among circumcised men. 

A premature ejaculation 

• is ejaculation that occurs too early, 
• for example, during foreplay or upon or shortly after penetration. 

It is usually caused by

•  anxiety, 
• other psychological causes, 
• or an unusually sensitive foreskin or glans penis. 

For most males, 

• premature ejaculation can be overcome by 

1. various techniques (such as squeezing the penis between the glans penis and shaft as ejaculation approaches), 
2. behavioral therapy, 
3. or medication.