• Male reproductive functions have 3 major parts:
  • Spermatogenesis → formation of sperm.
  • Male sexual act.
  • Hormonal regulation of male reproductive functions.
• Male sex hormones also affect:
  • Accessory sexual organs.
  • Cellular metabolism.
  • Growth.
  • Other body functions.
• Fig. 81.1A shows the main parts of the male reproductive system, while Fig. 81.1B shows the testis and epididymis in more detail.
• Each testis contains up to 900 coiled seminiferous tubules.
  • Each tubule is more than 0.5 meter long.
  • Sperm are formed here.
• After formation, sperm pass into the epididymis.
  • Epididymis is a highly coiled tube about 6 meters long.
• The epididymis continues into the vas deferens.
  • Near the prostate, the vas deferens enlarges to form the ampulla of the vas deferens.
• There are 2 seminal vesicles, one on each side of the prostate.
  • They empty their secretions into the prostatic end of the ampulla.
• Contents from the ampulla + seminal vesicles enter the ejaculatory duct.
  • The ejaculatory duct passes through the prostate gland.
  • It then opens into the internal/prostatic urethra.
• Prostatic ducts also empty their secretions into this pathway and finally into the prostatic urethra.
• The urethra is the final passage connecting the reproductive tract to the exterior.
• The urethra receives mucus mainly from:
  • Many small urethral glands along its length.
  • Paired bulbourethral (Cowper) glands, located near the beginning of the urethra.

KEY CONCEPT

Sperm pathway:
Seminiferous tubules → Epididymis → Vas deferens → Ampulla → Ejaculatory duct → Urethra → Exterior

Accessory secretions join this pathway:
Seminal vesicles + Prostate + Urethral/Cowper glands

Conceptual Examples

• Sperm formation → occurs in the seminiferous tubules.
• Sperm transport/storage pathway → seminiferous tubules → epididymis → vas deferens.
• Before entering the urethra → sperm-containing fluid receives secretions from the seminal vesicles and prostate.
• Final exit pathway → urethra carries the contents to the exterior.

Physiological Anatomy of the Male Sexual Organs

• Fig. 81.1A shows the main parts of the male reproductive system, while Fig. 81.1B shows the testis and epididymis in more detail.
• Each testis contains up to 900 coiled seminiferous tubules.
  • Each tubule is more than 0.5 meter long.
  • Sperm are formed in these tubules.
• After formation, sperm pass into the epididymis.
  • The epididymis is another highly coiled tube, about 6 meters long.
• The epididymis continues into the vas deferens.
• Just before the vas deferens enters the prostate gland, it becomes enlarged to form the ampulla of the vas deferens.
• There are two seminal vesicles, one on each side of the prostate.
  • They empty into the prostatic end of the ampulla.
• Contents from the ampulla + seminal vesicles enter the ejaculatory duct.
• The ejaculatory duct:
  • Passes through the prostate gland.
  • Opens into the internal urethra.
• Prostatic ducts also empty prostatic secretions into this pathway and then into the prostatic urethra.
• The urethra is the final passage carrying contents from the reproductive tract to the exterior.
• The urethra receives mucus from:
  • Numerous small urethral glands along its length.
  • Mainly the paired bulbourethral (Cowper) glands near the beginning of the urethra.

KEY CONCEPT

Sperm pathway:
Seminiferous tubules → Epididymis → Vas deferens → Ampulla → Ejaculatory duct → Urethra → Exterior

Accessory secretions join the pathway from:
Seminal vesicles + Prostate + Urethral/Cowper glands

Conceptual Examples

• Where are sperm formed? → Seminiferous tubules.
• Where do sperm go next? → Epididymis.
• Which tube carries sperm from the epididymis? → Vas deferens.
• Where do seminal vesicle secretions join? → Near the ampulla/ejaculatory duct.
• Final passage to the exterior? → Urethra.

SPERMATOGENESIS

• During embryonic development, primordial germ cells migrate into the testes.
• These cells become immature germ cells called spermatogonia.
• Spermatogonia lie in 2–3 layers along the inner surface of the seminiferous tubules (Fig. 81.2A).
• At puberty, spermatogonia begin repeated mitotic divisions.
• They continuously:
  Proliferate → Differentiate through definite stages → Form sperm (Fig. 81.2B).

STEPS OF SPERMATOGENESIS

• Spermatogenesis occurs in the seminiferous tubules during active sexual life.
• It is stimulated by gonadotropic hormones from the anterior pituitary.
• It begins at an average age of about 13 years.
• It continues through most of life but decreases markedly in old age.
• In the first stage, spermatogonia move between the Sertoli cells toward the central lumen of the seminiferous tubule.
• Sertoli cells are large cells with extensive cytoplasm that surround and support the developing germ cells as they move toward the lumen.

KEY CONCEPT

Primordial germ cell → Spermatogonium → Mitotic proliferation + differentiation → Sperm

Site: Seminiferous tubules
Starts: Puberty (~13 years)
Stimulus: Anterior pituitary gonadotropic hormones
Direction of development: Outer part of tubule → Between Sertoli cells → Central lumen

Conceptual Examples

• Before puberty: Spermatogonia are present, but active sperm production has not yet begun.
• At puberty: Gonadotropic stimulation activates spermatogonia → sperm formation starts.
• Think of Sertoli cells as supporting cells: developing germ cells remain surrounded by them while moving toward the tubular lumen.

Meiosis

• Spermatogonia that cross into the Sertoli cell layer enlarge and develop into primary spermatocytes (Fig. 81.3).
• Each primary spermatocyte undergoes meiosis and forms 2 secondary spermatocytes.
• After a few days, the secondary spermatocytes divide again to form spermatids.
• Spermatids are then modified into mature spermatozoa (sperm).
• During meiosis, the chromosome number is reduced:
  • Spermatocyte = 46 chromosomes (23 pairs)
  • Spermatid = 23 chromosomes
• Therefore, each sperm carries only half of the father’s genetic information needed by the future fetus.
• At fertilization:
  • Half of the genetic material comes from the father’s sperm.
  • Half comes from the mother’s oocyte.
• The complete process from spermatogonium → spermatozoon takes about 74 days.

Sex Chromosomes

• Each spermatogonium contains 23 pairs of chromosomes.
• One pair determines the sex of the offspring:
  • X chromosome → female chromosome.
  • Y chromosome → male chromosome.
• During meiosis, X and Y chromosomes separate into different sperm:
  • Some sperm carry X chromosome.
  • Other sperm carry Y chromosome.
• Therefore, the type of sperm that fertilizes the ovum determines the sex of the offspring.

KEY CONCEPT

Spermatogonium → Primary spermatocyte → 2 Secondary spermatocytes → Spermatids → Spermatozoa

46 chromosomes → Meiosis → 23 chromosomes in each sperm

X-bearing sperm → female offspring
Y-bearing sperm → male offspring

Total spermatogenesis time ≈ 74 days

Conceptual Examples

• Sperm with X + Ovum with X → XX → Female
• Sperm with Y + Ovum with X → XY → Male
• So, the father’s sperm determines whether the offspring is XX or XY.

Formation of Sperm

• Newly formed spermatids initially look like ordinary epithelial-like cells.
• They soon differentiate and elongate to become mature spermatozoa (sperm).
• Each sperm has 2 main parts: head and tail (Fig. 81.4).
• The head contains:
  • A tightly condensed nucleus.
  • Only a thin layer of cytoplasm and cell membrane around it.
• The anterior two-thirds of the head is covered by a thick cap called the acrosome.
  • It is formed mainly from the Golgi apparatus.
  • It contains enzymes similar to lysosomal enzymes.
• Important acrosomal enzymes include:
  • Hyaluronidase → digests proteoglycan filaments of tissues.
  • Proteolytic enzymes → digest proteins.
  • These enzymes help the sperm enter and fertilize the ovum.
• The sperm tail is called the flagellum and has 3 major components:
  • Axoneme → central skeleton made of 11 microtubules.
  • Thin cell membrane → surrounds the axoneme.
  • Mitochondria → surround the proximal part of the axoneme and form the body of the tail.
• The back-and-forth movement of the flagellum gives the sperm its motility.
• This movement occurs by rhythmic longitudinal sliding between the tubules of the axoneme.
• Energy for movement is supplied as ATP, produced by the mitochondria in the body of the tail.
• Normal sperm move through fluid at about 1–4 mm/min, helping them travel through the female genital tract toward the ovum.

KEY CONCEPT

Spermatid → Differentiation + elongation → Spermatozoon

Head: Nucleus + acrosome → helps in fertilization
Tail: Axoneme + mitochondria → produces movement
Mitochondria → ATP → Flagellar movement → Sperm motility

Conceptual Examples

• Acrosome = penetration part → its enzymes help sperm enter the ovum.
• Mitochondria = energy source → produce ATP for tail movement.
• Flagellum = movement part → propels sperm toward the ovum.

Hormonal Factors That Stimulate Spermatogenesis

• Several hormones are essential for normal sperm formation.

1. Testosterone
   • Secreted by Leydig cells in the interstitial tissue of the testis (Fig. 81.2).
   • Essential for growth and division of testicular germ cells, the early step of sperm formation.
2. Luteinizing hormone (LH)
   • Secreted by the anterior pituitary.
   • Stimulates Leydig cells → testosterone secretion.
3. Follicle-stimulating hormone (FSH)
   • Also secreted by the anterior pituitary.
   • Stimulates Sertoli cells.
   • Without FSH stimulation, spermatids cannot convert into sperm.
   • This conversion is called spermiogenesis.
4. Estrogens
   • Formed from testosterone by Sertoli cells when stimulated by FSH.
   • Probably also essential for spermiogenesis.
5. Growth hormone (GH)
   • Maintains the basic metabolic functions of the testes.
   • Specifically promotes the early division of spermatogonia.
   • Severe GH deficiency can cause poor or absent spermatogenesis → infertility.

Maturation of Sperm in the Epididymis

• After formation in the seminiferous tubules, sperm take several days to travel through the approximately 6-meter-long epididymis.
• Fresh sperm from the seminiferous tubules and early epididymis are:
  • Nonmotile.
  • Unable to fertilize an ovum.
• After about 18–24 hours in the epididymis, sperm develop the ability to become motile.
• However, inhibitory proteins in epididymal fluid keep their final motility suppressed until after ejaculation.

Storage of Sperm in the Testes

• The two adult testes can produce up to 120 million sperm/day.
• Most sperm are stored in the epididymis, with a smaller amount stored in the vas deferens.
• Sperm can remain stored and fertile for at least 1 month.
• During storage, substances in duct secretions keep sperm in a strongly suppressed, inactive state.
• With frequent sexual activity and ejaculation, sperm may remain stored for only a few days.
• After ejaculation, sperm become motile and capable of fertilizing the ovum; this functional development is called maturation.
• Sertoli cells + epididymal epithelium produce a nutrient fluid that is ejaculated with sperm.
• This fluid contains:
  • Testosterone and estrogens.
  • Enzymes.
  • Special nutrients needed for sperm maturation.

KEY CONCEPT

LH → Leydig cells → Testosterone → Germ-cell growth and division

FSH → Sertoli cells → Spermiogenesis + Estrogen formation

GH → Early spermatogonial division

Seminiferous tubules → Immature, nonmotile sperm → Epididymis → Maturation potential → Ejaculation → Motile, fertilizing sperm

Conceptual Examples

• Low LH → less Leydig-cell stimulation → less testosterone → impaired spermatogenesis.
• Low FSH → poor Sertoli-cell stimulation → spermatids fail to become mature sperm.
• Freshly formed sperm cannot immediately fertilize an ovum; they first need epididymal maturation.
• Epididymis = main storage and maturation site for sperm.

Physiology of the Mature Sperm

• Normal mature sperm are motile and fertile and move by their flagella at about 1–4 mm/min.
• Sperm activity is greatest in a neutral or slightly alkaline medium, such as ejaculated semen.
• A mildly acidic medium reduces sperm activity, while a strongly acidic medium can rapidly kill sperm.
• Increasing temperature:
  • Increases sperm activity.
  • Also increases metabolism, so sperm survive for a shorter time.
• Sperm can remain alive for many weeks while inactive in the male genital ducts.
• After ejaculation into the female genital tract, sperm usually survive only about 1–2 days.

FUNCTION OF THE SEMINAL VESICLES

• Each seminal vesicle is a coiled, sac-like secretory tube.
• Its secretion contains:
  • Fructose
  • Citric acid
  • Other nutrients
  • Prostaglandins
  • Fibrinogen
• During emission and ejaculation, seminal vesicle fluid enters the ejaculatory duct shortly after sperm from the vas deferens.
• Seminal vesicle secretion forms a large part of semen.
• Fructose and other nutrients provide energy and nourishment to ejaculated sperm until fertilization occurs.
• Prostaglandins may help fertilization in two ways:
  1. Make cervical mucus more favorable for sperm movement.
  2. May produce reverse contractions of the uterus and fallopian tubes, helping move sperm toward the ovaries.
• Because of this transport, a few sperm can reach the upper fallopian tubes within about 5 minutes.

KEY CONCEPT

Neutral/slightly alkaline medium → ↑ sperm activity
Acidic medium → ↓ sperm activity / sperm death

Higher temperature → ↑ activity + ↑ metabolism → shorter sperm life

Seminal vesicles → Fructose + nutrients + prostaglandins → nourish sperm and help sperm movement through the female tract

Conceptual Examples

• Alkaline semen → provides a better environment for sperm movement.
• Fructose → acts as an energy source for sperm.
• Prostaglandins → help sperm move through the cervix, uterus, and fallopian tubes.
• After ejaculation → sperm generally remain viable for only 1–2 days.

FUNCTION OF THE PROSTATE GLAND

• The prostate gland secretes a thin, milky fluid containing:
  • Calcium
  • Citrate ions
  • Phosphate ions
  • Clotting enzyme
  • Profibrinolysin
• During emission, the prostate capsule contracts at the same time as the vas deferens.
• This pushes prostatic fluid into the reproductive tract and adds to the volume of semen.
• Prostatic fluid is slightly alkaline, which is important for sperm function.
• The fluid in the vas deferens is relatively acidic because of:
  • Citric acid
  • Metabolic end products of sperm
• This acidity tends to reduce sperm fertility.
• Female vaginal secretions are also acidic, with a pH of about 3.5–4.0.
• Sperm become optimally motile when the surrounding pH rises to about 6.0–6.5.
• Therefore, the slightly alkaline prostatic fluid helps neutralize acidity during ejaculation.
• This improves sperm motility and fertility.

KEY CONCEPT

Prostate → Slightly alkaline fluid → Neutralizes acidic seminal/vaginal environment → Better sperm motility → Better fertility

Conceptual Examples

• Acidic environment → sperm movement decreases.
• Prostatic alkaline fluid → raises pH toward 6.0–6.5.
• Better pH → sperm become more motile and more capable of fertilization.

SEMEN

• Semen is the fluid ejaculated during the male sexual act and contains sperm + secretions from several glands.
• Main components of semen:
  • Vas deferens fluid + sperm → ~10%
  • Seminal vesicle fluid → almost 60%
  • Prostatic fluid → ~30%
  • Small amount → mucous glands, especially the bulbourethral glands.
• Therefore, the seminal vesicles contribute the largest portion of semen.
• Seminal vesicle fluid is ejaculated last and helps wash sperm through the ejaculatory duct and urethra.
• The average pH of semen is about 7.5.
• The alkaline prostatic fluid more than neutralizes the mild acidity of the other seminal fluids.
• Prostatic fluid gives semen its milky appearance.
• Seminal vesicle + mucous gland secretions give semen its mucoid consistency.
• After ejaculation:
  • A prostatic clotting enzyme acts on fibrinogen from seminal vesicle fluid.
  • This forms a weak fibrin coagulum (clot).
  • The clot helps keep semen in the deeper vagina near the cervix.
• After about 15–30 minutes, the coagulum dissolves.
  • Prostatic profibrinolysin → fibrinolysin
  • Fibrinolysin breaks down the coagulum.
• During the first few minutes, sperm are relatively immobile, possibly because the coagulum is viscous.
• As the coagulum dissolves, sperm become highly motile.
• Sperm can survive for many weeks inside the male genital ducts, but after ejaculation their maximum survival at body temperature is only about 24–48 hours.
• At lower temperatures, semen can be stored for several weeks.
• When frozen below −100°C, sperm can be preserved for years.

KEY CONCEPT

Semen composition:
Seminal vesicles ~60% + Prostate ~30% + Vas deferens/sperm ~10% + small mucous gland secretion

After ejaculation:
Semen coagulates → holds semen near cervix → 15–30 min → fibrinolysin dissolves clot → sperm become highly motile

Average semen pH ≈ 7.5
Ejaculated sperm survival at body temperature ≈ 24–48 hours

Conceptual Examples

• Largest contributor to semen? → Seminal vesicles (~60%)
• Milky appearance? → Prostatic fluid
• Mucoid consistency? → Seminal vesicle + mucous gland fluids
• Why semen initially clots? → To help keep sperm-containing semen near the cervix.
• Why sperm later move freely? → Fibrinolysin dissolves the coagulum after 15–30 minutes.

“Capacitation” of Spermatozoa Is Required for Fertilization of the Ovum

• Sperm are considered mature when they leave the epididymis, but their activity is still suppressed by inhibitory factors from the male genital ducts.
• Therefore, freshly ejaculated sperm are not yet able to fertilize the ovum.
• After entering the female genital tract, sperm undergo several activating changes called capacitation.
• Capacitation normally takes about 1–10 hours.

1. Removal of inhibitory factors
   • Fluids of the uterus and fallopian tubes wash away substances that were suppressing sperm activity.
2. Loss of excess cholesterol
   • In the male genital ducts, sperm are exposed to cholesterol-rich vesicles.
   • Cholesterol is added to the membrane covering the acrosome, making it stronger and preventing enzyme release.
   • After ejaculation, sperm move away from these cholesterol-rich vesicles and gradually lose excess cholesterol.
   • Therefore, the acrosomal membrane becomes weaker and easier to open.
3. Increased calcium entry
   • The sperm membrane becomes more permeable to Ca²⁺.
   • Increased Ca²⁺ changes flagellar movement from a weak undulating movement → powerful whiplash movement.
   • Ca²⁺ also changes the membrane over the acrosome so that acrosomal enzymes can be released rapidly.
   • These enzymes help sperm pass through the granulosa cell mass and especially the zona pellucida surrounding the ovum.

• Without capacitation, sperm cannot penetrate into the ovum and fertilization cannot occur.

KEY CONCEPT

Ejaculated sperm → Female genital tract → Capacitation (1–10 h) → Fertilization ability

Capacitation =
Remove inhibitory factors + Lose excess cholesterol + ↑ Ca²⁺ entry

↑ Ca²⁺ → Powerful flagellar movement + Easier acrosomal enzyme release → Ovum penetration

Conceptual Examples

• Before capacitation: sperm is mature but not ready to fertilize.
• Loss of cholesterol: makes the acrosomal membrane easier to break/open.
• More Ca²⁺: gives the sperm a strong whiplash movement.
• Acrosomal enzyme release: helps sperm penetrate the coverings of the ovum.

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