Male Reproductive System Overview

The Two Sexes

• Sexual reproduction is biparental
• Gametes (sex cells) are produced by each parent: egg (ovum) and sperm (spermatozoon)
• A zygote (fertilized egg) has a combination of both parent's genes
• Copulatory organs include the penis which introduces the gametes into the female reproductive tract and the vagina, which receives the sperm

Overview of the Reproductive System

• Primary sex organs (gonads) are testes and ovaries
• Secondary sex organs in males comprise a system of ducts, glands, and the penis to deliver sperm cells
• Secondary sex organs in females comprise uterine tubes, the uterus, and the vagina
• External genitalia are located in the perineum
• Internal genitalia are located mainly in the pelvic cavity, except for the testes
• Secondary sex characteristics are features that further distinguish the sexes and play a role in mate attraction, developing at puberty
• Secondary sex characteristics common to both sexes include pubic and axillary hair, scent glands, and the pitch of the voice
• Male secondary sex characteristics include facial hair, coarse and visible hair on the torso and limbs, and a relatively muscular physique
• Female secondary sex characteristics include the distribution of body fat, breast enlargement, and relatively hairless skin

Chromosomal Sex Determination

• Human cells contain 23 pairs of chromosomes
• There are 22 pairs of autosomes, and 1 pair of sex chromosomes. Males are XY and females are XX
• Males produce half Y-carrying sperm and half X-carrying sperm
• All eggs carry the X chromosome

Development of Reproductive Tracts

• Gonads begin to develop at 5 or 6 weeks as gonadal ridges
• There are 2 sets of ducts: mesonephric ducts which develop into the male reproductive system and paramesonephric ducts which develop into the female reproductive tract
• The SRY gene encodes for the protein testes-determining factor (TDF)
• TDF stimulates testes to secrete testosterone at 8 to 9 weeks and stimulates mesonephric ducts to develop into the male anatomy. Testes also secrete Müllerian-inhibiting factor
• Estrogen levels are always high in pregnancy

Development of External Genitalia

• The similarity of external genitalia in both sexes: the genital tubercle becomes the glans of the penis or glans clitoris
• In males, the pair of urogenital folds encloses the urethra, forming the penis or labia minora in females
• Paired labioscrotal folds become either the scrotum or labia majora
• By week 12, male or female genitalia are distinctly formed
• Organs that develop from the same embryonic structure are homologous
• The penis is homologous to the clitoris, and the scrotum is homologous to the labia majora

Descent of Testis

• Gonads migrate into the pelvic cavity (ovaries) or scrotum (testes)
• The gubernaculum is a connective tissue cord that extends from the gonad to the floor of the pelvic cavity
• The vaginal process is fold of the peritoneum that extends into the scrotum
• The inguinal canal is the pathway of low resistance through the groin and is created by the gubernaculum and vaginal process
• The descent of the testes begins as early as 6 weeks

Male Reproductive System

• The male reproductive system comprises the urinary bladder, pubic symphysis, root of penis, ductus (vas) deferens, shaft of penis, corpus cavernosum, corpus spongiosum, epididymis, glans of penis, prepuce, testis and scrotum

Scrotum and Spermatic Cord

• The scrotum is a pouch of skin, muscle, and fibrous connective tissue containing the testes
• The spermatic cord is a bundle of fibrous connective tissue containing the ductus deferens, blood and lymphatic vessels, and testicular nerve
• Sperm cannot be produced at 37° C and must be held at about 35° C

Spermatic Cord

• 3 mechanisms to regulate testicle temperature:
  • Cremaster muscle: strips of internal abdominal oblique muscle. Contracts when cold, draws testes upward toward body
  • Dartos muscle: subcutaneous layer of smooth muscle. Contracts when it's cold, wrinkling the scrotum
  • Pampiniform plexus: extensive network of veins from the testes that surround the testicular artery and spermatic cord
• Pampiniform plexus acts as a countercurrent heat exchanger and removes heat from descending arterial blood

Countercurrent Heat Exchanger

• Arterial blood cools as it descends, while venous blood carries away heat, ascending to the pelvic cavity
• It helps to maintain the temperature required for sperm production

Testis and Associated Structures

• The testes (testicles) are endocrine and exocrine glands that produce sex hormones and sperm
• The tunica vaginalis and white fibrous capsule (tunica albuginea) covers it
• The testis consists of 250 to 300 wedge-shaped lobules comprising seminiferous tubules for sperm generation
• The rete testis is located on the posterior side of the testis and collects sperm from seminiferous tubules
• Efferent ductules collect sperm from the rete testes and transport them to the epididymis
• The duct of the epididymis (head, body, and tail) is the site of sperm maturation and storage and allows sperm to be fertile for 40 to 60 days
• If sperm is not ejaculated, it disintegrates, and the epididymis reabsorbs it

Histology of Testis

• Interstitial (Leydig) cells between tubules produce testosterone
• Sustentacular (Sertoli) cells are in between germ cells, protecting the germ cells, providing nutrients, waste removal, growth factors, and other needs
• Sertoli cells are part of the blood-testes barrier

Male Duct System

• The ductus (vas) deferens is a muscular tube that is 45 cm long
• It runs from the scrotum through the inguinal canal to the posterior surface of the bladder
• The duct widens behind the bladder to form the ampulla and comprises a thick wall of smooth muscle well innervated by sympathetic nerve fibers
• The ejaculatory duct is a 2 cm duct formed from the ductus deferens and seminal vesicle, which passing through the prostate to empty into the urethra

Male Urethra

• The 18 cm male urethra is shared by the reproductive and urinary systems
• It has 3 regions: prostatic, membranous, and spongy (penile) urethra

Accessory Glands

• There are 3 sets of glands in the male reproductive system, which include the seminal vesicles, the prostate gland and Bulbourethral (cowper) glands
• The seminal vesicles are a pair of glands posterior to the bladder, that empty into the ejaculatory duct and forms 60% of semen
• The prostate gland is a thin milky secretion which make up 30% of the semen
• Bulbourethral (cowper) glands produce a clear slippery fluid that lubricates the head of the penis in preparation for intercourse and is alkaline

Prostate Diseases

• Benign prostatic hyperplasia (BPH) is a noncancerous enlargement of the prostate that compresses the urethra and obstructs urine flow and promotes bladder and kidney infections
• Prostate cancer is the second most common cancer in men that goes unnoticed until it causes pain
• Prostate cancer can metastasize to nearby lymph nodes and then to the lungs and other organs
• It is diagnosed from elevated levels of prostate-specific antigen and acid phosphatase in the blood and can be detected by a digital rectal exam (DRE)

Anatomy of the Penis

• The penis serves to deposit semen in the vagina
• Erectile tissue fills with blood during sexual arousal
• The corpus spongiosum encloses the spongy (penile) urethra, and the distal end enlarges and forms the glans and the proximal end ends as a dilated bulb
• The two corpora cavernosa are dorsal and contain a deep artery, and each arm called a crus as it attaches the penis to the pubic arch
• Blood sinuses (lacunae) are separated by trabeculae

Puberty

• The reproductive system remains dormant for several years after birth
• Puberty, which is the first few years of adolescence, until the first menstrual period in girls or the first ejaculation of viable sperm in boys, typically starts around age 12 in girls and age 13 in boys
• During puberty there is enlargement of secondary sexual organs
• Testosterone stimulates a burst of generalized body growth
• Puberty is also when erythropoiesis, basal metabolic rate, and appetite increase
• Pubic hair, scent and sebaceous glands develop
• Puberty Stimulates sperm production and libido (sex drive)
• Adolescence is the period from the onset of gonadotropin secretion and reproductive development until a person attains full adult height

Hormones and Endocrine Control

• GnRH from the hypothalamus stimulates the anterior pituitary to secrete FSH and LH
• FSH stimulates sustentacular cells to secrete androgen-binding protein (ABP)
• LH stimulates interstitial cells to secrete testosterone (anrogen)
• In the presence of ABP, testosterone stimulates spermatogenesis
• Testosterone also stimulates the libido and the development of secondary sex organs and characteristics
• Testosterone has negative feedback effects that reduce GnRH secretion and pituitary sensitivity to GnRH.
• Sustentacular cells also secrete inhibin, which selectively inhibits FSH secretion and thus reduces sperm production without reducing testosterone secretion

Aging and Sexual Function

• There is a decline in testosterone secretion during aging
• Peak testosterone secretion is at 7 mg/day at age 20 then declines to 1/5 of that by age 80, related to the decline in the number of interstitial cells & sustentacular cells
• Climacteric (male menopause) is caused by a decline in testosterone and inhibin after age 50 (andropause)
• Climacteric results in reduced libido and sperm quality/quantity, erectile dysfunction, decrease in muscle and bone mass, and an increase in fat
• Erectile dysfunction (impotence), the inability to produce or maintain an erection sufficient for intercourse, impacts 20% of men in their 60s and 50% of those in their 80s

Spermatogenesis and Meiosis

• Spermatogenesis is the process of sperm production in seminiferous tubules
• Involves 3 principal events: remodeling of large germ cells into small, mobile sperm cells with flagella, creation of haploid cells, and the shuffling of gens via recombination

Spermatogenesis

• Spermatogenesis is the cross section of seminiferous tubules and requires secondary spermatocytes, sustenacular cells, blood-testis barrier, tight junctions, and a basement membrane

Spermiogenesis

• Spermiogenesis changes that transform spermatids into spermatozoa
• This involved discarding excess cytoplasm and growing tails

Spermatozoon

• The head is pear-shaped: 4 to 5 microns long structure containing the nucleus, acrosome, and basal body of the tail flagella
• The acrosome is an enzyme cap that contains egg penetrating enzymes
• The basal body is where flagellum attaches
• The tail is divided into 3 regions: midpiece where mitochondria around axoneme of the flagella enables flagellar movement ; the axoneme itself, surrounded by a sheath of supporting fibers; and the endpiece which is a very narrow tip of flagella

Semen (Seminal Fluid)

• Semen is the fluid expelled during orgasm
• There are 2-5 mL of fluid expelled during ejaculation containing 60% seminal vesicle fluid, 30% prostatic fluid, and 10% sperm and spermatic duct secretions
• Normal sperm count is 50-120 million/mL, and lower than 20 to 25 million/mL = infertility
• Seminal vesicles contribute viscous yellowish fluid including fructose and other carbohydrates, citrate, prostaglandins, and proseminogelin
• The prostate produces a thin, milky white fluid including calcium, citrate, and phosphate ions and includes a clotting enzyme, and Protein-hydrolyzing enzyme serine protease (PSA)

Anatomy of Male Sexual Response

• The internal pudendal (penile) artery enters the root of the penis and divides in two
• The dorsal artery supplies blood to the skin, fascia, and corpus spongiosum
• The deep artery travels through the core of the corpus cavernosa and dilation of the deep artery fills lacunae causing an erection
• Many anastomoses unite deep and dorsal arteries and the Deep dorsal vein drains blood from penis

Anatomy of Male Sexual Response

• The glans has an abundance of tactile, pressure, and temperature receptors
• The dorsal nerve of penis and internal pudendal nerves lead to integrating center in sacral spinal cord
• Both autonomic and somatic motor fibers carry impulses from integrating center to penis

Male Sexual Response

• Intercourse divided into 4 recognizable phases of excitement, plateau, orgasm, and resolution
• Sexual dysfunction treatments are often developed that correspond to each phase of intercourse
• Sexual intercourse is also known as coitus, coition, or copulation

Excitement and Plateau

• The Excitement phase is initiated by a broad spectrum of erotic stimuli which results in vasocongestion, myotonia and increased HR, BP & respiratory rate
• Bulbourethral glands secrete their fluid
• Erection of the penis occurs due to parasympathetic triggering of nitric oxide (NO) secretion, which dilates deep arteries
• The Plateau phase is when respiratory rate, heart rate, and blood pressure stay increased
• Vasocongestion and myontonia become more pronounced
• Lasts for a few seconds or a few minutes before orgasm

Orgasm and Ejaculation

• Orgasm (climax) is a short, intense reaction resulting in semen discharge lasting 3-15 seconds and elevating heart rate, blood pressure, and breathing
• Ejaculation occurs in two stages: Emission is when the sympathetic nervous system stimulates peristalsis , propels and semen in the urethra activates somatic and sympathetic reflexes that stimulate muscular contractions that lead to expulsion
• There is a resolution phase where the body returns to a pre-excitement state
• Internal pudendal artery constricts, reducing blood flow to penis
• Penis becomes soft and flaccid
• Cardiovascular and respiratory responses return to normal
• The refractory period is a period following resolution in which it is usually impossible for a male to attain another erection or orgasm (10mins to a few hrs)

How Viagra (Sildenafil citrate) Prolongs Erection

• Treatments for erectile dysfunction include Viagra, Levitra, and Cialis which are all phosphodiesterase inhibitors
• Sexual stimulation triggers NO secretion, which activates cGMP, which increases blood flow into erectile tissue
• Drugs like Viagra slow down the breakdown of cGMP, prolonging duration of erection

Sexually Transmitted Diseases

• Sexually Transmitted Diseases (STDs) have an incubation period in which the pathogen multiplies with no symptoms and a communicable period
• Bacterial STDs include: Chlamydia (may cause urethral discharge and testicular pain), Gonorrhea (pain and pus discharge and may result in sterility from pelvic inflammatory disease) and Syphilis (hard lesions (chancres) at site of infection- disappearance of chancres ends first stage. second stage is widespread pink rash and neurosyphilis is third stage with cardiovascular damage and brain lesions)
• Viral STDs include: Genital herpes (most common STD in US), Genital warts (warts on perineal region, cervix, anus), and Hepatitis B and C (inflammatory liver disease)