Male Reproductive Anatomy
- The male gonads (testes) produce sperm and lie within the scrotum
- Sperm are delivered to the exterior through a system of ducts: epididymis, ductus deferens, ejaculatory duct, and the urethra
- Accessory sex glands:
- Empty their secretions into the ducts during ejaculation
- Include the seminal vesicles, prostate gland, bulbourethral glands
- The Scrotum
- Sac of skin and superficial fascia that hangs outside the abdominopelvic cavity at the root of the penis
- Contains paired testicles separated by a midline septum
- Its external positioning keeps the testes 3deC lower than core body temperature (needed for sperm production)
- Intrascrotal temperature is kept constant by two sets of muscles:
- Dartos - smooth muscle that wrinkles scrotal skin
- Cremaster - bands of skeletal muscle that elevate the testes
- The Testes
- Each testis is surrounded by two tunics:
- The tunica vaginalis, derived from peritoneum
- The tunica albuginea, the fibrous capsule of the testis
- Septa divide the testis into lobules, containing seminiferous tubules
- Seminiferous tubules - produce the sperm
- The straight tubulus rectus conveys sperm to the rete testis
- From the rete testis, the sperm:
- Leave the testis via efferent ductules
- Enter the epididymis
- Surrounding the seminiferous tubules are interstitial cells (cells of Leydig) that produce androgens (testosterone)
- Spermatic cord - encloses PNS and SNS nerve fibers, blood vessels, and lymphatics that supply the testes
- Development Aspects: Descent of the Gonads
- About 2 months before birth and stimulated by testosterone, the testes leave the pelvic cavity and enter the scrotum
- Testes --> passes through inguinal canal of abdominal wall --> Forms spermatic cord which contains?
- Failure to descend--> cryptorchidism-->infertile
- Function of the gubernaculum?
- The Penis
- Consists of an attached root and a free shaft that ends in glans penis
- Prepuce, or foreskin - cuff of skin covering the distal end of the penis
- Erectile tissue - spongy network of connective tissue and smooth muscle riddled with vascular spaces
- Corpus spongiosum - surrounds the urethra and expands to form the glans and bulb of the penis
- Corpora cavernosa - paired dorsal erectile bodies bound by fibrous tunica albuginea
- Crura - proximal end of the penis surrounded by muscle; anchors the penis to the pubic arch
- Genetic Sex Determination
- There are two types of sex chromosomes: X and Y
- Females have two X chromosomes; males have one X and one Y
- A single gene on the Y chromosome, the SRY gene, initiates testes development and determines maleness
- 5th week - gonadal ridges form and paramesonephric (Müllerian) ducts form in females, mesonephric (Wolffian) ducts develop in males
- Male structures begin development in the 7th wk; female in the 8th wk
- External genitalia arise from the same structures in both sexes
- Epididymis
- Its head caps the superior aspect of the testis
- The duct of the epididymis has stereocilia that:
- Absorb testicular fluid
- Pass nutrients to the sperm
- Nonmotile sperm enter, pass through its tubes and become motile
- Upon ejaculation the epididymis contracts, expelling sperm into the ductus deferens
- Ductus Deferens and Ejaculatory Duct
- Runs from epididymis through inguinal canal into the pelvic cavity
- Its terminus expands to form the ampulla and then joins the duct of the seminal vesicle to form the ejaculatory duct
- Propels sperm from the epididymis to the urethra
- Urethra - Conveys both urine and semen (at different times)
- Consists of three regions
- Prostatic - portion surrounded by the prostate
- Membranous - lies in the urogenital diaphragm
- Spongy, or penile - runs through the penis and opens to the outside at the external urethral orifice
- Accessory Glands
- Seminal Vesicles
- Lie on the posterior wall of the bladder and secrete 60% of the volume of semen
- Join the ductus deferens to form the ejaculatory duct
- Sperm and seminal fluid mix in the ejaculatory duct and enter the prostatic urethra during ejaculation
- Prostate Gland
- Doughnut-shaped gland that encircles part of the urethra inferior to the bladder
- Its milky, slightly acid fluid, which contains citrate, enzymes, and prostate-specific antigen (PSA); one-third of the semen volume
- Plays a role in the activation of sperm
- Enters the prostatic urethra during ejaculation
- Bulbourethral Glands (Cowper's Glands)
- Pea-sized glands inferior to the prostate
- Produce thick, clear mucus prior to ejaculation that neutralizes traces of acidic urine in the urethra
Male Sexual Response
- Erection
- Enlargement and stiffening of the penis from engorgement of erectile tissue with blood; induced by sexual stimuli
- During sexual arousal, a PNS reflex promotes the release of nitric oxide; Nitric oxide causes erectile tissue to fill with blood
- Expansion of the corpora cavernosa:
- Compresses their drainage veins
- Retards blood outflow and maintains engorgement
- The corpus spongiosum functions in keeping the urethra open during ejaculation
- Erection can be induced or inhibited solely by emotional or higher mental activity
- Ejaculation the propulsion of semen from the male duct system
- Sympathetic nerves serving the genital organs cause:
- Reproductive ducts and accessory organs to contract and empty their contents
- The bladder sphincter muscle to constrict
- Bulbospongiosus muscles to undergo a rapid series of contractions
- Propulsion of semen from the urethra
- Semen
- Milky white, sticky mixture of sperm and accessory gland secretions
- Provides a transport medium and nutrients (fructose), protects and activates sperm, and facilitates their movement
- Prostaglandins in semen:
- Decrease the viscosity of mucus in the cervix
- Stimulate reverse peristalsis in the uterus
- Facilitate the movement of sperm through the female reproductive tract
- The hormone relaxin enhances sperm motility
- The relative alkalinity of semen neutralizes the acid environment found in the male urethra and female vagina
- Seminalplasmin - antibiotic chemical that destroys certain bacteria
- Clotting factors coagulate semen immediately after ejaculation, then fibrinolysin liquefies the sticky mass
- Only 2-5 ml of semen are ejaculated, but it contains 50-130 million sperm/ml
- Each cell has two sets of chromosomes (one maternal, one paternal) and is said to be diploid (2n chromosomal number)
- Humans have 23 pairs of homologous chromosomes
- Gametes only have 23 chromosomes and are said to be haploid (n chromosomal number)
- Gamete formation is by meiosis, in which the number of chromosomes is halved (from 2n to n)
- Meiosis
- Two nuclear divisions, meiosis I and meiosis II, halve the number of chromosomes in the four daughter cells
- Chromosomes replicate prior to meiosis I
- Meiosis I
- Homologous pairs of chromosomes undergo synapsis and form tetrads with their homologous partners
- Crossover, the exchange of genetic material among tetrads, occurs during synapsis
- Tetrads line up at the spindle equator during metaphase I
- In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to opposite ends of the cell
- At the end of meiosis I each daughter cell has:
- Two copies of either a maternal or paternal chromosome (Independent Assortement)
- A 2n amount of DNA and haploid number of chromosomes
- Meiosis II
- Mirrors mitosis except that chromosomes are not replicated before it begins
- Meiosis accomplishes two tasks:
- It reduces the chromosome number by half (2n to n)
- It introduces genetic variability
- Spermatogenesis
- The sequence of events that produces sperm in the seminiferous tubules of the testes
- Cells making up the walls of seminiferous tubules are in various stages of cell division
- These spermatogenic cells give rise to sperm in a series of events
- Mitosis of spermatogonia, forming spermatocytes
- Meiosis forms spermatids from spermatocytes
- Spermiogenesis - spermatids form sperm
- Spermatogonia - outermost cells in contact with the epithelial basal lamina
- Spermatogenesis begins at puberty as each mitotic division of spermatogonia results in type A or type B daughter cells
- Type A cells remain at the basement membrane and maintain the germ line
- Type B cells move toward the lumen and become primary spermatocytes
- Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes
- Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids
- Spermatids are small round cells seen close to the lumen of the tubule
- Spermatids to Sperm
- Late in spermatogenesis, spermatids are haploid but nonmotile
- Spermiogenesis - spermatids lose excess cytoplasm and form a tail, becoming sperm
- Head - contains DNA and has a helmetlike acrosome containing hydrolytic enzymes that allow sperm to penetrate and enter egg
- Midpiece - contains mitochondria spiraled around the tail
- Tail - a typical flagellum produced by a centriole
- Sustentacular Cells (Sertoli Cells)
- Cells that surrounds developing cells
- They are bound together with tight junctions forming an unbroken layer with the seminiferous tubule, dividing it into two compartments
- Basal compartment -contains spermatogonia & primary spermatocytes
- Adluminal compartment -contains meiotic cells and tubule lumen
- Their tight junctions form a blood-testis barrier
- This prevents sperm antigens from escaping through the basal lamina into the blood
- Since sperm are not formed until puberty, they are absent during thymic education
- Spermatogonia are recognized as "self" and are influenced by bloodborne chemical messengers that prompt spermatogenesis
- Spermatocytes and spermatids are enclosed in sustentacular cells
- Deliver nutrients to dividing cells
- Move them along to the lumen
- Secrete testicular fluid that provides transport medium for sperm
- Produce chemical mediators that help regulate spermatogenesis
Hormonal Regulation of Testicular Function
- The hypothalamus releases gonadotropin-releasing hormone (GnRH)
- GnRH stimulates the anterior pituitary to secrete FSH and LH
- FSH causes sustentacular cells to release androgen-binding protein (ABP)
- LH stimulates interstitial cells to release testosterone
- ABP binding of testosterone enhances spermatogenesis
- Feedback inhibition on the hypothalamus and pituitary results from:
- Rising levels of testosterone
- Increased inhibin
- Mechanism and Effects of Testosterone Activity
- Testosterone is synthesized from cholesterol
- It must be transformed to exert its effects on some target cells
- Prostate - it is converted into dihydrotestosterone (DHT) before it can bind within the nucleus
- Neurons - it is converted into estrogen to bring about stimulatory effects
- Testosterone targets all accessory organs and its deficiency causes these organs to atrophy
- Male hormones make their appearance at puberty and induce changes in nonreproductive organs, including
- Appearance of pubic, axillary, and facial hair
- Enhanced growth of the chest and deepening of the voice
- Skin thickens and becomes oily
- Bones grow and increase in density
- Skeletal muscles increase in size and mass
- Testosterone is the basis of libido in both males and females
References
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