Male Reproductive System

Sexual differentiation timepoints

• 5-6 weeks embryogenesis

• 7 weeks

• 7-12 weeks

• 7-8 months

5-6 weeks embryogenesis

• Sexually indifferent

  • Gonad

  • 2 immature duct systems (wolffian → male. and Müllerian ducts → female)

7 weeks

• SRY protein

  • Transcription factor

  • Promotes the development of gonad into teste

    • Testosterone: development of male sexual organs (reproductive tract from Wolffian ducts)

    • Müllerian inhibiting substance (MIS): inhibits development of female sexual organs

7-12 weeks

• Ovaries: begin oogenesis

• Genitalia develop

7-8 months

• Testes: descend into scrotum

• Ovaries: all primary oocytes (prophase I of meiosis) and primordial follicles

Primary sex organs

• Gonads

  • Testes

    • Production of gametes

    • Synthesis of hormones

Secondary sex organs

• Accessory sex organs

• Internal reproductive organs:

  • Epididymis

  • Vas (ductus) deferens

  • Urethra

  • Seminal vesicles

  • Prostate gland

  • Bulbourethral glands

• External reproductive organs

  • Scrotum

  • Penis

Scrotum

• Thin layer of skin overlaying the darts muscle

  • Elevates the testes (wrinkling)

• Raphe:

  • Raised thickening of skin; divides scrotum into two internal scrotal cavities

• Cremaster muscle:

  • Deep layer

  • Internal and external fascia layers

  • Contraction elevates the testes

  • Sexual arousal, cold temperatures

Development of spermatozoa requires temperatures…

• 2 degrees (F) lower than normal body temperature

Testes

• Tunica albuginea: outer fibrous capsule

• Fibrous partitions (continus with TA), divides testis into lobules

• Seminiferous tubules: sight of sperm production; highly coiled individual tubules (around 800) which empty into common duct system

• Rete testis: channels that direct spermatozoa out of testis and into the channels of the epididymis

Cell types of the testes

• Sertoli cells (nurse cells): form wall of the tubules

  • Key role: support spermatogenesis

• Leydig cells: surround seminiferous tubules

  • Key role: androgen production (mostly testosterone small amount of DHT)

• Gametes at various stages of development → key role: propagation of the species

  • Spermatogonia (stem cell)

  • Spermatocytes

  • Spermatids

  • Spermatozoa (sperm)

Hypothalamic-Pituitary-Gonadal Axis

• Testosterone production by Leydig cells is under regulation of the hypothalamic-pituitary axis

• The hypothalamic- pituitary axis also regulates the process of spermatogenesis

HPG axis → GnRH

• Gonadotropin releasing hormone

  • Released in pulsatile fashion at onset of puberty

HPG axis → FSH

• Follicle stimulating hormone

  • Receptors on Sertoli cells

  • Alter Sertoli cells to assist in spermatogenesis

HPG axis → LH

• Luteinizing hormone

  • Receptors on Leydig cells

  • Stimulate testosterone production

HPG axis → negative feedback

• Inhibin released from Sertoli cells - inhibits release of FSH

• High levels of testosterone can inhibit release of GnRH and LH

Biosynthesis of testosterone

• Cholesterol precursor

• Pregnenolone formation in Leydig cells

  • LH receptors on Leydig cells

  • LH stimulates conversion of cholesterol to pregnenolone

• Both testosterone and DHT exert androgenic effects via androgen receptor (AR)

• DHT binds the androgen receptor with much greater affinity than testosterone and is there 50X more potent

Target tissues of androgens

• Androgenic: effects of development of internal and external male genitalia spermatogenesis, and libido

• Anabolic: growth promotion of somatic tissues

Androgens are important for…

• Sexual differentiation

• Development/maintenance of male phenotype

• Androgens play a role in libido in both men and women

Androgens → increase at puberty

• Secondary sexual characteristic

• Linear growth

• Onset of spermatogenesis

Anabolic steroids

• Exogenous androgens

• Increased skeletal muscle mass

• Decreased adipose

• Decreased sperm production and testicular volume (inhibit HPG)

• Acne

• Liver tumors

• Depression

• Hypertension

Spermatogenesis

• A continual process from the onset of puberty and throughout lifespan of the male

• It takes approximately 64 days for one full cycle of spermatogenesis to complete

  • Around 2 million spermatogonia begin process each day

Anatomy of spermatozoa

• Head

  • Location of nucleus

  • Acrosome: packet of enzymes necessary for fertilization of ova

• Midpiece → mitochondria arranged in spiral around microtubules

  • ATP for tail movement

• Tail → flagellum

  • Whip like motion to propel sperm

Maturation of spermatozoa

• Immature spermatozoa (lack locomotion and ability to fertilize) detach from nurse cells and move along lumen (via villa) of seminiferous tables to epididymis

Epididymis

• Includes head, body, and tail (23 ft long)

• Storage and maturation of sperm (3 weeks)

• Peristaltic smooth muscle movements and cilia

• Recycle function for damaged/unused sperm

Capacitation

• Develop mobility and ability to fertilize

• Requires mixing with seminal fluid secretions and secretions from the female uterus

Spermatic cord

• Extends from the abdominopelvic cavity to the testes (inguinal canal)

• Layers of fascia and muscle surrounding: blood vessels, nerves, lymphatics, ductus deferens

Vas deferens (ductus deferens)

• Tube (18 inches long) carries sperm and mixes it with seminal secretions

• Begins at the tail of the epididymis

• Thick layers of smooth muscle control movement via peristaltic waves

• Travels the spermatic cord

• Branches posterior to bladder and curves inferiorly behind bladder

• Ampulla: enlargement of the lumen that occurs prior to prostate (sperm can be stored here)

• The two ductus deferens converge (with the duct of the seminal gland) to form the short ejaculatory duct

• This duct penetrates the prostate and empties into the (prostatic) urethra

Accessory glands

• Seminal vesicles

• Prostate

• Bulbourethral gland (Cowper’s gland)

Seminal vesicles

• Paired glands at posterior surface of bladder

• Alkaline secretions contain:

  • Fructose → used to generate ATP

  • Prostaglandins → influence smooth muscle contractions

  • Clotting factors → fibrinogen/activators

Prostate

• Glandular tissue surrounded by smooth muscle

• Secretions include:

  • Antibiotics

  • Anti-coagulates → break down fibrin

Bulbourethral glands (Cowper’s gland)

• Paired glands at base of penis

• Thick alkaline, mucus secretion (buffer)

• Lubricate glans of penis

What makes up semen

• Spermatozoa + secretions from accessory glands (spermatozoa is less than 5% of total volume of semen)

General functions of accessory gland secretions

• Capacitation

• Provide nutrients

• Provide a medium for movement

• Buffers to counteract acidity of urethra/vagina

Parts of the penis

• The root

• The body

• The glans

• Erectile tissue

• Dorsal vein, dorsal artery and dorsal nerve runs along the dorsal side of penis

The root of the penis

• Internal

• Fixed end of the penis which attaches it to the body wall

The body of the penis

• Mobile tubular portion

• Skin and darts muscle continuation of scrotum

The glans of the penis

• Extended distal end surrounding the urethral orifice

• Prepuce: foreskin, surround and cover the glans

  • Sebaceous glands secrete smegma (oily mixture and collected dead cells)

Erectile tissue in the penis

• Consists of interconnected vascular channels that have walls composed of smooth muscle and elastic fibers

  • Corpus cavernosa; two lateral columns of erectile tissue which each surround a central artery

  • Corpus spongiosum; column of article tissue that surrounds penile urethra

Flaccidity

• (Non-erect state) is maintained through constriction of arteries to the penis and contraction of the muscular partitions of the erectile tissue

• Smooth muscle is actively contracting

• Relax smooth muscle for erection → remove Ca2+

Erection

• Regulated by the parasympathetic nervous system

• Parasympathetic postganglionic neurons of the penis contains the enzyme (neural) nitric oxide synthase (nNOS)

• Activation of parasympathetic nerves - increased production and release of nitric oxide

• Nitric oxide diffuses into smooth muscle of corpus cavernosa and corpus spongiosum

  • Initiates signaling cascade which decreases levels of intracellular calcium

  • Smooth muscle relaxes

• Also diffuse into penile arteries - vasodilation (increase local blood flow to penis)

Emission

• Movement of semen into prostatic urethra from the ejaculatory duct

  • Mediated by the sympathetic nervous system: smooth muscle contraction in response to norepinephrine release (alpha1 receptors)

    • Ampulla of ductus deferens: delivery of sperm

    • Seminal vesicles: addition of secretions

    • Prostate: addition of secretions

    • Internal urethra sphincter (prevent retrograde flow of sperm into bladder)

Ejaculation

• Expulsion of semen into the urethra

  • Triggered by the movement of semen into the urethra at the base of the penis

  • (Most of the mechanics not well understood) increased firing of efferents from spinal cord stimulate involuntary rhythmic contractions of striated musculature of the perineum (from coccyx to pubic symphysis) to help forcefully expel semen from urethra

  • “Point of no return” - ejaculation is inevitable

  • Tightly linked to orgasm

Sexual response cycle

• Similar events in both males and females

• Excitement

• Plateau

• Orgasms

• Resolution

Excitement

• Increased heart rate and muscle tension

• Lubricating secretions

• Increased blood flow: penile, clitoral, vestibular bulb erecetion

Plateau

• Amplification of excitement phase and increases in blood flow

• Muscle spasms

• Emission in males

Orgasm

• Involuntary muscle contraction

• Vaginal contractions

• Ejaculation in males

• Forceful relaxation of sexual tension

Resolution

• Fatigue and return to normal functioning

• Male specific refractory period: time which another erection cannot be initiated

Fertilization

• Fusion of a secondary oocyte and a sperm to create a zygote

• At ovulation: oocyte surrounded by a layer of follicle cells known as the corona radiata (granulosa cells)

• Enzymes in the acrosome of the sperm break down connections between cells of the corona radiata (requires enzymes with dozens of sperm)

• A single sperm makes contact with sperm receptors on the plasma membrane of the oocyte

  • Membranes of sperm and oocyte fuse (directed by enzymes)

Activation of the oocyte

• Fusion triggers Na+ influx: depolarization

• Release of Ca2+ stores from the SER

• Inactivation of sperm receptors

• Completion of meiosis II

• Creation and then fusion of male and female pronuclei