ANS 220 Exam 4

Compare and Contrast male and female gametes

• Female

  • 1 oocyte

  • Largest cell in the human body (120 to 150 microns)

• Male

  • Billions of Sperm

  • Smallest cell in the human body (2.5 to 3.5 microns)

  • Quantity over quality

Male reproductive tract

Overall Pathway of Sperm

• Seminiferous tubules (testis)

  • Sperm are produced here

  • Temperature must be 4-6 degrees cooler than body temp

• Rete testis → Efferent ducts → Epididymis

  • Epididymis= motility + fertility acquisition

  • Also, the storage warehouse for sperm

• Vas deferens → Pelvic urethra

  • Here sperm encounter seminal plasma

• Penile urethra → Glans penis

  • Delivery system for ejaculation

Seminal Plasma

• Acts like bubble wrap for sperm

• Provides:

  • Protection

  • Nutrients

  • Stabilization for travel through female tract

Testicle

Seminiferous tubule

• Sertoli cells

  • “Nurse cells” supporting developing sperm.

  • Shape varies depending on surrounding germ cells

• Leydig cells

  • outside the tubule

  • Produce testosterone

• Basement membrane

  • Protective outer layer of each tubule

• Capillaries

  • Provide nutrients and blood supply

Spermatogenesis overview

Stem cells

• Located along the basement membrane.

• Called spermatogonial stem cells (SSC).

• Two simplified types:

  • Type A = undifferentiated

  • Type B = differentiated

Progression of germ cells

1. Type A SSC → Type B SSC

2. Primary spermatocyte

3. Secondary spermatocyte

4. Spermatid (round)

5. Elongated spermatid → Spermatozoa

Movement

• As cells mature, they migrate toward the lumen of the seminiferous tubule.

Cell division

• Mitosis occurs early (stem cell replication).

• Meiosis occurs during transition from primary → secondary spermatocytes → spermatids.

 

Function of Epididymis Functions

• Gain motility

• Gain fertility

• Storage until ejaculation

Two important cells

• Intersitial cells (LEYDIG)

  • Produce testosterone

  • Location

    • Sit outside the seminiferous tubules, in the interstitial space.

    Function

    • Produce testosterone — absolutely essential for spermatogenesis.

    • Equivalent to theca cells in the female.

    Hormonal Regulation

    • Stimulated by LH from the anterior pituitary.

    • LH binds to receptors on Leydig cells → activates steroidogenesis.

    • Converts cholesterol → testosterone.

    Why testosterone matters

    • Must be very high inside the seminiferous tubule for sperm production.

    • Drives:

      • Spermatogenesis

      • Sertoli cell function

      • Secondary sex characteristics

      • Negative feedback to hypothalamus/pituitary

• Sertoli cells

  • Nurture germ cells through development

  • Produce numerous steroids

  • Location

    • Inside the seminiferous tubules, directly contacting developing germ cells.

    Female Equivalent

    • Granulosa cells (closest to gametes, nurture them).

    Functions

    Sertoli cells produce:

    • Estrogen (via aromatization of testosterone)

    • Inhibin

    • Anti‑Müllerian Hormone (AMH / MIH)

    • Growth factors

    • Androgen Binding Protein (ABP)

    They:

    • Support and nourish germ cells

    • Regulate the microenvironment

    • Control progression from spermatogonia → spermatozoa

    • Form the blood‑testis barrier

    FSH Regulation

    • Sertoli cells express FSH receptors.

    • FSH stimulates:

      • Estrogen production

      • Inhibin production

      • ABP production

      • Growth factor secretion

Endocrine Regulation

• Male doesn’t have surge center only tonic pulses

Key difference from females

• Males do not have a surge center.

• All hormone release is tonic (steady pulses).

Hormone flow

1. Hypothalamus → GnRH (tonic pulses)

2. Anterior pituitary →

   • LH (stimulates Leydig cells → testosterone)

   • FSH (stimulates Sertoli cells → ABP, estrogen, inhibin)

3. Testis →

   • Testosterone (negative feedback)

   • Estrogen (local mitosis)

   • Inhibin (FSH regulation)

Pulsatility

• GnRH pulses → LH pulses → testosterone pulses.

• Testosterone rises ~3 hours after each GnRH pulse.

• Over a full day, testosterone looks “flat,” but actually oscillates.

Androgen Binding Protein

Produced by

• Sertoli cells (in response to FSH).

Function

• Binds testosterone inside the seminiferous tubule.

• Keeps testosterone concentrations extremely high locally.

• Prevents testosterone from diffusing out of the tubule.

Why this matters

• Without ABP → testosterone would escape → spermatogenesis would fail.

• ABP is like a “testosterone magnet” ensuring the tubule stays saturated.

4. Estrogen in the Testis — Not Just a Female Hormone

Produced by

• Sertoli cells (aromatizing testosterone).

Function

• Stimulates mitosis of germ cells.

• Drives:

  • Type A → Type B spermatogonia

  • Type B → primary spermatocyte

Key point

• Estrogen is required for the mitotic expansion phase of spermatogenesis.

Inhibin and Anti-Mullerian Hormone

Inhibin — FSH Regulator

Produced by

• Sertoli cells.

Function

• Negative feedback to anterior pituitary.

• Decreases FSH secretion.

• Helps fine‑tune Sertoli cell activity.

 

Anti‑Müllerian Hormone (AMH / MIH)

Produced by

• Sertoli cells (even in adults).

Function

• In development: regresses Müllerian ducts.

• In adults: helps regulate differentiation of spermatogonial stem cells.

Pre-pubertal vs Post-pubertal Steroid Production

Before puberty

• Primary steroid = androstenedione

• Testis is not fully activated.

After puberty

• Primary steroid = testosterone

• Due to activation of Leydig cells by LH.

Why High Testosterone Is Required Inside the Tubule

• Spermatogenesis cannot occur unless testosterone is:

  • High

  • Local

  • Bound by ABP

• Testosterone drives:

  • Meiosis

  • Spermiogenesis

  • Sertoli cell function

Endocrine Functions; What do the hormones do?

• Leydig cells

  • Respond to LH

  • Product androstenedione (prepubertal)

  • Testosterone required for spermatogenesis

• Sertoli cells

  • Responds to FSH

  • Produces:

    • Estrogen

    • Inhibin

    • AMH

    • Various growth factors

    • Androgen Binding Protein

  • MIH differentiation

  • Inhibin regulates FSH

  • ABP maintains high levels of testosterone

  • Estrogen causes the changes of sperm and simulates Mitosis

Summary of Hormone Functions

Hormone	Produced By	Target	Function
LH	Anterior pituitary	Leydig cells	Testosterone production
FSH	Anterior pituitary	Sertoli cells	ABP, estrogen, inhibin, growth factors
Testosterone	Leydig cells	Sertoli cells + germ cells	Spermatogenesis, negative feedback
Estrogen	Sertoli cells	Germ cells	Stimulates mitosis
Inhibin	Sertoli cells	Anterior pituitary	Decreases FSH
AMH/MIH	Sertoli cells	Germ cells	Differentiation
ABP	Sertoli cells	Seminiferous tubule	Concentrates testosterone

Why the Seminiferous Tubule Produces So Many Sperm

• Tubules are extremely long and tightly coiled.

• Spermatogenesis occurs along the entire length.

• Continuous tonic hormone pulses maintain constant production.

2-cell, 2- Gonadotropin Model

Leydig Cells = Theca Cells

• Stimulated by LH

• Produce testosterone

• Located outside the seminiferous tubules

Sertoli Cells = Granulosa Cells

• Stimulated by FSH

• Directly contact germ cells

• Produce:

  • Estrogen

  • Inhibin

  • AMH/MIH

  • Growth factors

  • Androgen Binding Protein (ABP)

Key point

Both LH and FSH are essential for spermatogenesis.

Without either → sperm production collapses

Spermatogenesis

Human spermatogenesis takes ~70 days

• From spermatogonial stem cell → spermatozoa

• Species differences:

  • Bull: ~60 days

  • Ram: ~48 days

Why this matters

If the testis is injured (heat stress, fever, trauma), you won’t see infertility immediately.

Instead:

• Damage today → infertility 60–70 days later

• Because the injury affects early-stage spermatogonia, not mature sperm already stored in the epididymis.

Clinical example

• A bull spikes a fever for several days

• His sperm output looks normal right now

• But 60 days later, sperm count + quality drop sharply

This is why reproductive managers track:

• Heat stress events

• Illness

• Environmental temperature

• Scrotal insulation

 

3. Puberty and Onset of Spermatogenesis

Before puberty

• No sperm production

• Hypothalamus–pituitary–testis axis is not fully connected

• Primary steroid = androstenedione

After puberty

• GnRH pulses activate LH + FSH

• Leydig cells begin producing testosterone

• Sertoli cells begin supporting germ cells

• Spermatogenesis begins and continues for life

 

4. Spermatogenesis Occurs in Waves

Your professor emphasized this pattern.

Waves

• Along the length of a seminiferous tubule, different sections are at different stages of development.

• This ensures continuous sperm output.

• If you “straightened out” the tubule, you’d see:

  • Region A: releasing sperm

  • Region B: meiosis

  • Region C: spermatogonia dividing

  • Region D: spermiogenesis

Why waves matter

• Prevents “batch production”

• Ensures daily sperm release

• Allows billions of sperm to be produced per day

 

• Continuous production = billions per day

Spermatogenic Cycles

Definition

• A “cycle” = the repeating pattern of cell associations within a segment of the seminiferous tubule.

Why cycles matter

• They help identify:

  • Where damage occurred

  • What stage is disrupted

  • How long recovery will take

 

⭐ 6. The 3 Major Phases of Spermatogenesis

Your professor breaks spermatogenesis into three phases, based on cell type and type of division.

 

Spermatogenesis continued

• Onset- puberty

• Hormonal requirements

  • LH, FSH, testosterone regulation

• Occurs in waves and cycles

• Three phases

  • Spermatogonia phase

    • Proliferation, renewal and differentiation

    • mitosis

  • Spermatocyte phase

    • Meiosis

  • Spermatid phase

    • Final maturation steps

• 1 spermatogonium (stem cell)= 256 spermatozoa

Phases of Spermatogenesis

 Phase 1: Spermatogonial Phase (Mitotic Phase)

Location

• Along the basement membrane

What happens

• Spermatogonial stem cells (SSCs) proliferate

• Type A SSCs:

  • Self‑renew

  • Maintain the stem cell pool

• Type B SSCs:

  • Differentiate

  • Commit to meiosis

Division type

• Mitosis

Purpose

• Expand the pool of germ cells

• Prepare cells for meiosis

 

Phase 2: Spermatocyte Phase (Meiotic Phase)

Location

• Middle region of the seminiferous tubule

What happens

• Primary spermatocytes undergo meiosis I

• Secondary spermatocytes undergo meiosis II

• Haploid spermatids are formed

Division type

• Meiosis

Purpose

• Reduce chromosome number

• Create genetic diversity

 

Phase 3: Spermatid Phase (Spermiogenesis)

Location

• Near the lumen

What happens

• Round spermatids → elongated spermatozoa

• No mitosis or meiosis (already haploid)

Major structural changes

• Nuclear condensation

• Acrosome formation

• Tail formation

• Mitochondrial rearrangement

• Cytoplasm reduction

Purpose

• Final maturation

• Create a functional sperm cell capable of fertilization

Massive Amplification: 1 Stem Cell → 256 Sperm

Your professor highlighted this as “impressive.”

Why so many?

• Each spermatogonium undergoes:

  • Multiple mitotic divisions

  • Meiosis

  • Spermiogenesis

Result

• 1 spermatogonial stem cell → 256 spermatozoa

Multiply this across:

• Millions of SSCs

• Entire length of the seminiferous tubules

• Continuous waves

→ Billions of sperm produced daily

 

Why Mammals Are the Focus

• Spermatogenesis in mammals is highly organized into:

  • Waves

  • Cycles

  • Phases

• Fish, reptiles, and other vertebrates have different patterns

• This course focuses on mammalian reproduction

Spermatogonial Stage

ype A Spermatogonia = TRUE Stem Cells

• Located along the basement membrane.

• Capable of self‑renewal:

  • One Type A divides → two identical Type A daughters.

  • This maintains the stem cell pool for life.

• This is why males do not undergo reproductive senescence like females.

Type A Can Also Differentiate

• Type A → Type B spermatogonium

• This is a terminal differentiation:

  • Once a cell becomes Type B, it cannot revert to Type A.

  • It is now committed to entering meiosis.

Type B Spermatogonia

• Still diploid (2n).

• Only two fates:

  1. Become a primary spermatocyte, OR

  2. Undergo apoptosis (if something goes wrong)

Stem Cell Niche

• Type A spermatogonia survive only when attached to their niche along the basement membrane.

• Removing them from this niche → they lose stem cell properties.

• Research (like Dr. Oakley’s lab) shows:

  • The niche provides signals that maintain “stemness.”

  • Without it, they differentiate or die.

Intercellular Bridges Keeping cells in sync

Unique Feature of Spermatogenesis

• Developing germ cells remain connected by intercellular bridges (called cytoplasmic bridges or syncytia).

• These bridges:

  • Keep all linked cells at the same developmental stage.

  • Ensure synchronized mitosis and meiosis.

  • Allow sharing of nutrients, mRNA, and regulatory molecules.

If a cell loses its bridge

• It becomes “out of sync.”

• The body triggers apoptosis.

• This is why one stem cell can produce 256 sperm — but often produces fewer.

Mitosis → Meiosis

Mitosis (Spermatogonial Stage)

• Type A → Type A (renewal)

• Type A → Type B (differentiation)

• Type B → Primary spermatocyte

Primary Spermatocytes

• Large cells.

• Enter Meiosis I.

⭐ 4. Spermatocyte Stage — Meiosis Begins

Primary Spermatocytes → Secondary Spermatocytes

• Meiosis I occurs.

• Chromosomes recombine and segregate.

• Cells become genetically unique.

Blood–Testis Barrier (BTB) Importance

• Primary spermatocytes must cross the tight junctions between Sertoli cells.

• BTB protects them because:

  • Once meiosis begins, they are no longer genetically identical to the rest of the body.

  • The immune system would otherwise attack them as “foreign.”

• BTB prevents immune cells from entering the adluminal compartment.

Secondary Spermatocytes

• Undergo Meiosis II quickly.

• Meiosis II → Haploid spermatids (1n).

Numbers to Know

• 1 Type A stem cell →

  • 2 Type A daughters →

  • 4 Type B →

  • 8 primary spermatocytes →

  • 16 secondary spermatocytes →

  • 256 spermatids

Spermatid Stage

Key Point

• NO mitosis or meiosis here.

• Spermatids are already haploid.

What happens

Round spermatid → elongated spermatozoon

Changes include:

• Nuclear condensation

• Acrosome formation

• Tail (flagellum) formation

• Mitochondrial migration to midpiece

• Cytoplasm reduction

This stage = Spermiogenesis

 Spermatocytogenesis vs Spermiogenesis

Spermatocytogenesis

• Includes:

  • Mitosis (Type A → Type B → primary spermatocyte)

  • Meiosis (primary → secondary → spermatid)

• Requires:

  • LH → Testosterone

• Blocking LH = blocking testosterone = no spermatocytogenesis
  → This is how chemical castration works.

Spermiogenesis

• Round spermatid → elongated spermatozoa

• Requires:

  • FSH → Sertoli cell activation

• Blocking FSH = no spermiogenesis
  → Round spermatids accumulate but do not mature.

⭐ 7. Hormonal Control Summary

Process	Requires	Why
Spermatocytogenesis (mitosis + meiosis)	LH + Testosterone	Drives proliferation + meiosis
Spermiogenesis (final differentiation)	FSH	Sertoli cell support + remodeling

⭐ 8. Key Cell Stages to Memorize

1. Type A spermatogonia (true stem cells)

2. Type B spermatogonia (committed)

3. Primary spermatocyte (Meiosis I)

4. Secondary spermatocyte (Meiosis II)

5. Spermatid (haploid, round)

6. Spermatozoa (elongated, mature)

Spermiogenesis

• Maturation and metamorphosis of a spermatid into a spermatozoan

  • Golgi phase

  • Cap phase

  • Acrosome phase

  • Maturation phase

• Happens inside the seminiferous tubule, at the very tip of the Sertoli cell, right next to the lumen.

• Sertoli cells act as nurse cells, physically supporting the spermatid as it elongates.

• This is the final step before sperm are released into the lumen (spermiation).

⭐ 2. Hormonal Requirement

Spermiogenesis requires FSH

• FSH stimulates Sertoli cells.

• Sertoli cells provide:

  • Growth factors

  • Structural support

  • Enzymes

  • Nutrients

• Blocking FSH → round spermatids cannot elongate → no functional sperm.

Contrast

• Earlier stages (mitosis + meiosis) require LH + testosterone.

• Final differentiation requires FSH.

⭐ 3. The Four Phases of Spermiogenesis

Your professor emphasized these must occur in order:

G → C → A → M

Golgi → Cap → Acrosomal → Maturation

Mnemonic from your professor: “Going Crazy And Mad.”

Golgi Phase

• Acrosome vesicles forms

• Golgi saccules come together and coalesce

Key Events

• Round spermatid with:

  • Nucleus

  • Mitochondria

  • Organelles

  • Large Golgi apparatus

Main Transformation

• Golgi forms the acrosome.

• Acrosome = vesicle filled with digestive enzymes needed for fertilization.

What to memorize

• Acrosome originates from the Golgi.

• This is the defining event of the Golgi phase.

Cap Phase

Key Events

• The acrosome spreads over the nucleus, forming a “cap.”

• The nucleus begins to polarize (one side becomes the head region).

• Tail (axoneme) begins forming from the centrioles.

What to memorize

• Acrosome spreads over the nucleus → forms the acrosomal cap.

Key Events

• The acrosomal vesicle (formed in the Golgi phase) now:

  • Moves to the top of the nucleus

  • Begins to spread over the nuclear surface

• The round spermatid now looks like a mushroom (cap sitting on a round head).

Other events

• Centrioles and other organelles begin to migrate and align.

• These centrioles will later form:

  • The axoneme (core of the tail)

  • The base of the flagellum

What to memorize

• Cap phase = acrosome sits on nucleus + begins spreading.

• Tail precursors begin organizing.

Acrosome Phase

• Nuclear & Cytoplasmic elongation

• Manchette (centrioles) migrate to opposite end of nucleus

• Acrosome wraps around nucleus

Key Events

• Nucleus elongates and begins to condense.

• Acrosome continues to enlarge and cover more of the nucleus.

• Tail elongates further.

• Spermatid rotates so the acrosome faces the basement membrane and the tail points toward the lumen.

What to memorize

• Nuclear elongation + acrosome expansion + tail growth.

This is where the spermatid stops being round and starts becoming a sperm.

Key Events

1. Cell elongation begins

   • Both the nucleus and cytoplasm elongate.

   • This is the first major shape change.

2. Centrioles migrate

   • Move to the opposite end of the nucleus from the acrosome.

   • Begin forming the tail (flagellum).

3. Mitochondria begin migrating

   • Move toward the neck region where the midpiece will form.

4. Acrosome fully wraps around the nucleus

   • Covers ~⅔ of the nucleus.

   • This is essential for fertilization.

What to memorize

• Acrosomal phase = elongation + tail formation + acrosome wrapping.

Maturation Phase

• Final assembly & elongation

• Acrosome completely wraps around nucleus

• Mitochondria wrap around centriole in helical fashion

Key Events

• Final shaping of the sperm head.

• Excess cytoplasm is removed (forms the residual body).

• Mitochondria migrate and wrap around the proximal tail → forming the midpiece.

• Tail becomes fully functional.

What to memorize

• Cytoplasm removed, mitochondria form midpiece, tail becomes motile.

Key Events

1. Final shaping of the sperm head

   • Nucleus becomes extremely condensed.

   • Acrosome finishes wrapping.

2. Mitochondria wrap around the midpiece

   • Form a tight spiral around the proximal tail.

   • Provide ATP for motility.

3. Tail becomes fully functional

   • Principal piece (flagellum) forms.

   • Sperm becomes hydrodynamic.

4. Excess cytoplasm is removed

   • Forms the cytoplasmic droplet.

What to memorize

• Maturation phase = final head shaping + mitochondrial wrapping + cytoplasm removal.

 Final Sperm Structure

Head

• Contains:

  • Nucleus (genetic material)

  • Acrosome (enzymes for penetrating the oocyte)

Midpiece

• Packed with mitochondria → ATP production

• Powers tail movement

Tail (Flagellum)

• Propels sperm through the female reproductive tract

• “Propeller” of the cell

Professor’s joke

• A colleague calls sperm the “ultimate nuclear weapon.”
  (Nucleus + power source + propeller)

Integration With Sertoli Cells

• Sertoli cells cradle the developing spermatid.

• Provide:

  • Structural support

  • Nutrients

  • Hormonal signals

  • Phagocytosis of excess cytoplasm

• Release the mature sperm into the lumen (spermiation).

Cytoplasmic droplet

The cytoplasmic droplet is like a heavy backpack slowing you down.

What it is

• A blob of leftover cytoplasm + organelles.

• Attached to the sperm as it leaves the seminiferous tubule.

Where it is removed

• NOT removed in the testis

• NOT removed in the rete testis

• NOT removed in the efferent ducts

• NOT removed in the caput epididymis

It is removed in the corpus epididymis.

Why removal matters

• Droplet removal = sperm gain motility.

• Until then, sperm are immotile.

What to memorize

• Cytoplasmic droplet removed in corpus epididymis → motility gained.

FINAL SPERM STRUCTURE

Head

• Nucleus

  • Highly condensed DNA

  • Genetic variation due to crossing over during meiosis

• Acrosome

  • Contains enzymes needed to penetrate the zona pellucida

  • Required for fertilization

Midpiece

• Packed with mitochondria

• Provides ATP for motility

• Formed during maturation phase

Tail (Flagellum)

• Also called:

  • Flagellum

  • Principal piece

• Provides propulsion through the female reproductive tract

Professor’s quote

• Sperm = “the ultimate nuclear weapon”
  (Nucleus + power source + propeller

Sperm Components

• Acrosome

  • Enzymes for penetration of the egg

• Nucleus

  • Genes for fertilization/syngamy

  • Genetic variation

• Mitochondria

  • Energy for motility

• Flagellum- Principal piece

  • Mechanical basis for motility

Characteristics of Spermatogenesis

• The duration of spermatogenesis at regular intervals

  • Every 16 days in man

  • Every 13.5 days in bull

• Stem cells enter spermatogenesis in groups that are connected by intercellular bridges

Fixed and Constant Spermatogenesis

Fixed” = Spermatogenesis always occurs in the same physical place

Inside the seminiferous tubule, spermatogenesis is anchored:

• Type A spermatogonia sit at the basement membrane

• They differentiate upward toward the lumen

• All development happens alongside Sertoli cells

Fixed means:

• Type A → Type B → Primary spermatocyte → Secondary spermatocyte → Round spermatid → Elongated spermatozoon

• All of this ALWAYS occurs in the same spatial order

• Sertoli cells act as the “scaffolding” that holds the entire process in place

 

“Constant” = Spermatogenesis never stops

Once puberty begins:

• Spermatogenesis runs continuously

• New cohorts of cells begin development every few days

• This ensures nonstop release of sperm from puberty → death

Species timing (important!):

• Bull: new cycle every 13.5 days

• Human: every 16 days

This is the number your professor wants you to remember:

➡ Bull = 13.5‑day cycle

Terminology

• Cycle= progression through sequence of all stages

  • Cells changes

  • internal timeline

  • what the cells are doing

• Stage= specific cellular associations

  • what you see at the top of the Sertoli cell at a single moment. Stages describe which cell types are present at the luminal edge at a given time.

Changes in an individual cell during successive cycles

• Cycle 1: Freshman

  • Type A spermatogonia along basement membrane

  • Spermatogonia (mitosis)

• Cycle 2: Sophomores

  • Intermediate and type B spermatogonia, and as a primary spermatocyte begins meiosis I.

  • 1 layer closer to the lumen

  • Primary spermatocytes (meiosis I)

• Cycle 3: Juniors

  • Developments into a secondary spermatocyte

  • About halfway to the lumen

  • Secondary spermatocytes (meiosis II)

• Cycle 4: Seniors

  • Cell undergoes many morphological changes as it develops as a spermatid

  • near the lumen

  • Round spermatids

• Cycle 4.5: Graduation

  • Undergoes final changes

  • Released into the lumen of the tubule as a spermatozoan

  • Spermiogenesis (Golgi → Cap → Acrosomal → Maturation)

• Further maturation occurs as it travels through the male and female reproductive tracts.

Spermatogenic Waves (Stages)

• Waves

  • Refers to sequentil ordering of stages which occur along the length of the seminiferous tubule

 Stages = the “wave” in a football stadium

• When the wave reaches your section, you stand up (release sperm)

• When it passes, you sit back down (return to round spermatids)

• The wave keeps moving around the stadium

• The seminiferous tubule works the same way

Stage numbers:

• Most mammals: Stages 1–8

• Rodents: Stages 1–9 (not important for exam)

What each stage contains:

• Stage 1: Round spermatids at the top

• Stage 4: Spermatids in acrosomal/mid‑transformation

• Stage 8: Fully mature spermatozoa ready for release

Key point:

• Stages = snapshot in time

• Stages = what is visible at the luminal edge

• Stages = external pattern

How cycles + stages work together to produce billions of sperm

Inside the seminiferous tubule:

• Each Sertoli cell supports multiple cohorts of developing sperm

• Each cohort is at a different cycle

• The luminal edge displays different stages

• Every 13.5 days (bull), the stage shifts forward

• Stage 8 releases sperm → resets to Stage 1

• This repeats forever

Why this matters:

• At any moment, every part of the tubule is producing sperm at a different step

• This creates continuous output

• This is why males can produce billions per day

. Putting it all together — the professor’s “stadium wave” analogy

Your professor wants you to visualize it like this:

• The seminiferous tubule = a circular stadium

• Each Sertoli cell = a section of seats

• Each section has people (cells) at different points in the wave

• When the wave reaches a section → sperm are released

• Then that section resets and waits for the next wave

• The wave keeps moving around the stadium forever

This ensures:

• Continuous spermatogenesis

• Continuous sperm release

• No gaps in fertility (unless heat stress, injury, etc.)

 

⭐ 7. Why the 13.5‑day cycle matters clinically

Your professor emphasized this earlier, but it connects here:

• If a bull has heat stress, fever, or testicular injury:

  • The damage affects spermatogonia first

  • You won’t see infertility immediately

  • You will see it ~60 days later (full spermatogenic timeline)

  • The 13.5‑day cycle helps predict when fertility will drop

The diagram shows spermatogenesis as a repeating 13.5‑day cycle that occurs in waves along the seminiferous tubule.
Each vertical column represents one cohort of developing sperm cells moving from the basement membrane → lumen over time.

Multiple cohorts exist at once, so the tubule always contains cells at different stages.

---

🧱 Bottom Layer: Spermatogonia (Stem + Mitotic Phase)

Near the basement membrane, you see:

• A₁, A₄, Intermediate (I), and B spermatogonia
  These are the mitotic divisions that expand the germ cell population.

• Arrows show how each type transitions into the next.

• These divisions occur in synchronized groups, not individually.

---

🔄 Middle Layer: Meiosis

As the cohorts move upward:

• Primary spermatocytes (big cells) enter meiosis I

• They become secondary spermatocytes (brief stage)

• Then quickly transition into round spermatids

This is the meiotic phase, where chromosome number is halved.

---

🎯 Top Layer: Spermiogenesis (Spermatid Remodeling)

Near the lumen, the diagram shows:

• Round spermatids → elongating spermatids → mature spermatozoa

• This is where the acrosome forms, the tail develops, and the nucleus condenses.

At the very top right, you see spermiation — the release of mature sperm into the lumen.

The diagram notes that full development from spermatogonium → spermatozoon takes ~61 days.

---

⏱ Bottom Timeline: The 8 Stages of the Seminiferous Epithelial Cycle

The colored bar at the bottom shows:

• Stages I–VIII

• Each stage lasts a specific number of days
  (e.g., Stage I = 4.2 days, Stage VII = 1.1 days)

• Total cycle length = 13.5 days

A given cohort of cells moves through all 8 stages, then the cycle repeats.

---

🌊 Why the “Waves” Matter

The five vertical “columns” represent five overlapping cycles.
This explains why:

• The tubule always contains all cell types at once

• Sperm production is continuous, not episodic

• Different regions of the tubule are in different stages at any moment

Pathway of Sperm

1. Seminiferous tubules – site of sperm production.

2. Rete testis → efferent ducts – transport.

3. Epididymis

   • Caput (head): immature sperm, cytoplasmic droplet still attached.

   • Corpus (body): droplet removed → sperm gain motility.

   • Cauda (tail): storage warehouse.

4. Vas deferens → pelvic urethra

   • Sperm meet seminal plasma → “bubble wrap” protection + nutrients.

5. Penile urethra → glans penis

   • Delivery system for ejaculation.

Temperature Requirment

Temperature Requirement

• Testis must be 4–6°C cooler than body temperature.

• Heat stress damages spermatogonial stem cells, causing infertility ~60 days later (bull) or ~70 days later (human).

Seminiferous Tubule Structure

• Basement membrane – protective boundary.

• Spermatogonia (stem cells) sit on the basement membrane.

• Sertoli cells – “nurse cells” that support developing sperm.

• Lumen – where mature sperm are released.

Leydig Cells

Leydig Cells (Interstitial Cells)

• Located outside seminiferous tubules.

• Produce testosterone in response to LH.

• Testosterone is essential for:

  • Spermatogenesis

  • Secondary sex characteristics

  • Supporting Sertoli cell function

Male equivalent of female theca cells.

• Equivalent to theca cells in females

• Produce testosterone in response to LH

• Testosterone:

  • Needed in high concentration inside seminiferous tubules

  • Supports mitosis + meiosis

  • Some enters Sertoli cells → converted to estrogen

Sertoli Cells

• Inside seminiferous tubules, surrounding developing sperm.

• Equivalent to female granulosa cells.

• Functions:

  • Support & nourish germ cells

  • Form blood–testis barrier (tight junctions)

  • Convert testosterone → estrogen

  • Produce:

    • Inhibin → ↓ FSH

    • Anti‑Müllerian hormone (AMH)

    • Growth factors

    • Androgen-binding protein (ABP) → traps testosterone in tubule

• Required for spermiogenesis (final differentiation).

• Equivalent to granulosa cells in females

• Functions:

  • Support and nourish developing sperm

  • Form blood–testis barrier (tight junctions)

  • Produce:

    • Estrogen (from testosterone)

    • Inhibin → ↓ FSH

    • Anti‑Müllerian hormone (AMH)

    • Growth factors

    • Androgen Binding Protein (ABP) → traps testosterone in tubules

• Required hormone: FSH

Hormonal Regulation in males

Hypothalamus

• Releases GnRH in tonic pulses.

Anterior Pituitary

• LH → stimulates Leydig cells → testosterone

• FSH → stimulates Sertoli cells → ABP, estrogen, inhibin, growth factors

Feedback Loops

• Testosterone → negative feedback on LH.

• Inhibin → negative feedback on FSH.

Important Distinction

• Males do NOT have a surge center → no LH surge → constant tonic pulses.

Full Process of Spermatogenesis

Total Time

• Human: ~70 days

• Bull: 61 days (4.5 cycles × 13.5 days each)

• Ram: ~48 days

Three Major Phases

 

1. Spermatogonial Phase (Mitotic)

Location: Basement membrane

Cells: Type A → Type B spermatogonia

Key points:

• Type A = true stem cells

  • Can self‑renew (mitosis)

  • Or differentiate into Type B

• Type B = committed; cannot revert

  • Must become primary spermatocytes or undergo apoptosis

• Cells are connected by intercellular bridges (syncytium)

  • Keep divisions synchronized

  • If a cell disconnects → apoptosis

2. Spermatocyte Phase (Meiotic)

Location: Moving toward lumen

Events:

• Primary spermatocyte undergoes Meiosis I → secondary spermatocyte

• Secondary spermatocyte undergoes Meiosis II → round spermatids

• Round spermatids are haploid.

Blood–Testis Barrier

• Tight junctions between Sertoli cells protect meiotic cells from immune attack.

• Primary spermatocytes must cross the barrier before meiosis.

3. Spermiogenesis (Differentiation Phase)

Round spermatid → elongated spermatozoon

Requires: FSH + Sertoli cell support

NO mitosis or meiosis here.

Four Phases (Going Crazy And Mad)

1. Golgi Phase

   • Golgi forms the acrosome vesicle.

2. Cap Phase

   • Acrosome spreads over nucleus like a cap.

   • Centrioles migrate to opposite pole → future tail.

3. Acrosomal Phase

   • Cell elongates.

   • Acrosome fully wraps nucleus.

   • Tail axoneme begins forming.

   • Mitochondria migrate toward midpiece.

4. Maturation Phase

   • Final assembly of head, midpiece, tail.

   • Mitochondria wrap midpiece.

   • Excess cytoplasm forms cytoplasmic droplet.

Cytoplasmic Droplet

• Present in seminiferous tubules, rete testis, efferent ducts, and caput epididymis.

• Removed in corpus epididymis → sperm gain motility.

 

Sperm Structure

• Head

  • Nucleus -tightly packed DNA

  • Acrosome- enzymes for penetrating zona pellucida

• Midpiece

  • Mitochondrial sheath → ATP for motility

• Tail

  • Axoneme (9+2 microtubule structure)

  • Propulsion through female tract

Sperm production Scale

Daily Production

• Bull: 9–13 billion/day

• Boar: 17–22 billion/day

• Human: ~1000 sperm per heartbeat

Why so many?

• Spermatogenesis is:

  • Fixed (always occurs in same spatial pattern)

  • Constant (continuous waves of development)

CYCLES & STAGES OF SPERMATOGENESIS

Cycles = Cellular progression

• 4.5 cycles in all mammals

• Bull cycle length: 13.5 days

• 4.5 × 13.5 = 61 days total

Stages = What’s happening at the Sertoli cell apex

• Stage 1: round spermatids

• Stage 8: elongated spermatozoa ready for release

• After release → stage resets to 1

Wave Analogy

• Like a stadium wave:

  • Each Sertoli cell region “stands up” (releases sperm)

  • Then resets and waits for next cycle

NJURY, HEAT STRESS & INFERTILITY TIMING

Why infertility appears later

• Damage hits spermatogonial stem cells first.

• Mature sperm already in epididymis still function.

• Infertility appears one full spermatogenic cycle later:

  • Bull: ~61 days

  • Ram: ~47–48 days

  • Human: ~70 days

Examples

• Fever

• Scrotal heating

• Laptop on lap

• Obesity (poor thermoregulation)

EPIDIDYMAL MATURATION

Caput

• Immature sperm

• Cytoplasmic droplet present

• No motility

Corpus

• Droplet removed

• Motility gained

Cauda

• Storage

• Fertilization‑competent after ejaculation + capacitation in female tract

Semen Production

• Functions

  • Fluid environment for transport

  • Provides energy source

  • Buffer

  • Maintains osmolality

  • Need to rely on environment

• Composition

  • Fructose, inositol, citric acid, prostaglandins, growth factors, cholesterol, lipids

• Contributions from testes & accessory glands\

• Testes contributions

  • Sperm (immature)

  • Rete testis fluid

• Epididymis contributions

  • Maturation of sperm

    • Loss of cytoplasmic droplet

    • Gain forward motility

  • Concertation

  • Storage- caput, corpus, cauda

• Accessory gland contributions

  • adds the rest of the semen components (highly variable)

Epididymis

• Caput= head

  • Absorption to concentrate sperm

  • Transport

• Corpus= Body

  • Secretions “mature” sperm

  • Remove cytoplasmic droplet

  • Foward/progressive motility

• Cauda= Tail

  • Storage for ejaculation

Sperm Transport

• Seminiferous tubules

  • Passively moved by flow of fluids produced by Sertoli cells & flowing to rete testis

• Efferent ductules

  • Flow of fluids into the ducts aided by the reabsorption of fluids within the ducts

  • Flow through the ducts aided by cilia

• Epididymis

  • Spontaneous peristaltic contractions of smooth muscle lining the wall

• Vas deferens

  • Flow into vas deferens due to steady flow through epididymis

  • Flow through vas deferens at ejaculation due to peristaltic contractions into uretha

  • Emptying of vas deferens leaves room for further flow from the epididymis

• Pelvic & Penile urethra

  • Rhythmic contractions of bulbospongiosus & ischiocavernosus muscles at ejaculation

  • Simultaneous emptying of accessory glands to provide fluid vehicle

  • Help push seminal plasma into urethra to deposit in female

Sperm transport more

1. Seminiferous Tubules

• Passive movement

• Carried by fluid secreted by Sertoli cells

2. Rete Testis

• Fluid movement continues

3. Efferent Ducts

• Cilia + fluid flow pull sperm into epididymis

4. Epididymis

• Caput: concentration

• Corpus: maturation, droplet removed, motility gained

• Cauda: storage

Movement: smooth muscle peristalsis

Hormone: oxytocin enhances contractions

5. Vas Deferens

• Strong smooth muscle contractions move sperm to urethra

6. Pelvic & Penile Urethra

• Skeletal muscles (ischiocavernosus, bulbospongiosus)

• Forceful contractions → ejaculation

Important:

Sperm do NOT swim in the male tract.

Movement is entirely due to fluids + muscle contractions.

Seminal Plasma & Accessory sex glands

Seminal Vesicles

• Fructose

• Energy substrates

• Majority of seminal plasma volume

Prostate

• Buffers

• Enzymes

• Helps activate sperm

Bulbourethral (Cowper’s) Gland

• Mucus

• Lubrication

• Neutralizes urethra

Functions of seminal plasma

• Protect sperm

• Provide nutrients

• Package sperm for transport (“bubble wrap”)

• Facilitate movement through female tract

Semen evaluation & characteristics

• Volume

• Concentration

  • Sperm cells per volume

• Sperm motility

  • percentage of motile sperm

• Sperm morphology

  • percentage of normal sperm

• Volume increases the concentration decreases. Volume decreases the concentration increases.

Volume & pH

sperm cells are extremely sensitive anything can kill sperm

Concentration

• Use hemocytometer

  • Has 1mm X 1 mm grid

  • Calculate number

• Spectrophotometer

  • Decrease density more light lower concentration

  • Increase density less light higher concentration

Morphology

• Collect Smaple

• Fix and stain sperm cells

• Look abnormalites

  • Head- pear shape, slender, double head, micro or macro, cephalic

  • Midpiece- kinked, double, swollen

  • Tail- coiled, cytoplasmic droplet, absent, double

Spermatozoan

• Head

  • plasma membrane

  • apical ridge

  • acrosome

  • nucleus

• Tail

  • Midpiece

    • Proximal centriole

    • Mitochondrial sheath

    • Distal centriole

  • Principal piece

Motility

• Droplet test

  • Place fresh sample on slide

  • Estimate percentage (7/10)

  • Ranking system

    • High

    • Medium

    • Low

• Time lapsed photography

• Narrower down by precent is 100% moving No is 50% moving yes so look for percent in between

Mate selection

• Rely on:

  • odors

  • Visual cues

  • Vocalization

Pre-copulatory

• Searching for females

• Flehmen response → detects pheromones

• Courtship: nudging, vocalizations, mounting attempts

Copulatory

• Erection (hypothalamus-mediated)

• Mounting

• Intromission

• Ejaculation

Post-copulatory

• Dismount

• Refractory period

• Memory-based learning → older males more efficient

0. Mating Behavior & Copulation Sequence

Pre-copulatory

• Searching for females

• Flehmen response → detects pheromones

• Courtship: nudging, vocalizations, mounting attempts

Copulatory

• Erection (hypothalamus-mediated)

• Mounting

• Intromission

• Ejaculation

Post-copulatory

• Dismount

• Refractory period

• Memory-based learning → older males more efficient

Cus function on hypothalamus

• Step 1

  • Erotogenic stimuli cause sensory nerves to fire

• Step 2

  • Sensory nerves activate

  • “Reproductive Behavior Center” in hypothalamus

• Step 3

  • Stimulation of parasympathetic nerves that innervate penile arterioles

• Step 4

  • Parasympathetic nerve terminals release nitric oxide

• Step 5

  • Nitric oxide initiates biochemical cascade that causes erection

 What starts everything?

Sensory cues → Hypothalamus → Erection

The male detects:

• Sight of females in estrus

• Smell (pheromones via Flehmen response → vomeronasal organ)

• Sound / behavior (mounting invitations, restlessness)

These cues are processed by the hypothalamus, specifically the reproductive behavior center, which then activates:

Parasympathetic nerves → Nitric oxide (NO) release

• Parasympathetic fibers innervate the penile arterioles.

• They release nitric oxide (NO).

• NO causes vasodilation + trapping of blood in the corpora cavernosa and corpus spongiosum.

This is the physiological basis of erection in both:

• Vascular penis species (stallion, human)

• Fibroelastic penis species (bull, ram, boar — with sigmoid flexure)

 

2. Erection vs Ejaculation: Two Different Control Systems

Erection = Parasympathetic + Hypothalamus

• Requires sensory input + hypothalamic processing.

• Uses NO to trap blood.

Ejaculation = Spinal Reflex (NO brain involvement)

Once the glans penis receives the correct mechanical stimulus, ejaculation is triggered by a simple spinal reflex:

1. Sensory nerves in glans penis detect pressure/temperature.

1. Signal travels to spinal cord.

2. Motor neurons fire to:

   • Ischiocavernosus muscle

   • Bulbospongiosus muscle

These skeletal muscles contract rhythmically → expel semen.

Species differences in the stimulus:

• Boar: corkscrew penis locks into cervix → pressure triggers ejaculation

• Ram/Bull: vaginal pressure + temperature

• Stallion: must press glans firmly against cervix

Copulation Physiology

A. Sexual Arousal

• Female cues (sight, smell, sound) → hypothalamus

• Hypothalamus → parasympathetic nerves → nitric oxide release

• Nitric oxide traps blood in erectile tissue → erection

B. Intromission

• Glans penis enters female

• Species-specific stimulation:

  • Boar: pressure + corkscrew lock in cervix

  • Stallion: pressure against cervix

  • Ruminants: vaginal pressure + temperature

C. Ejaculation

• Does NOT involve the brain

• Simple spinal reflex:

  • Sensory nerves in glans penis → spinal cord → motor neurons

  • Motor neurons → ischiocavernosus & bulbospongiosus muscles

  • Rhythmic contractions → semen expelled

D. Refractory Period

• Male temporarily unresponsive

• Longer in young males

• Shorter in experienced males

 

🐂 12. Optimizing Ejaculate Output (False Mount Technique)

• Used in AI studs (e.g., Select Sires).

• One false mount → 55% increase in sperm per ejaculate.

• Two false mounts → no additional benefit.

• Works because:

  • Extra stimulation → more oxytocin → stronger epididymal/vas deferens contractions

  • More sperm moved into pelvic urethra before ejaculation

Site of Semen Deposition

Ruminants (bull, ram, buck)

• Anterior vagina

• Goal: deposit as close to cervix as possible

• Cervical mucus becomes less viscous under estrogen → easier passage

Boar

• Intra‑cervical

• Corkscrew penis locks into cervix

• Very large volume ejaculate

• Ends with a gel plug to prevent backflow

Stallion

• Intrauterine + cervical

• High volume

• Cervical folds guide semen into uterus

Sperm transport in the Female

Two phases:

A. Rapid Phase (minutes)

• Triggered by oxytocin release during copulation

• Oxytocin → uterine peristaltic contractions

• Moves sperm quickly toward the uterus/oviduct

B. Slow Phase (hours)

• Combination of:

  • Sperm’s own motility

  • Ongoing uterine contractions

• Takes ~8 hours in ruminants to reach the AIJ (ampulla–isthmus junction)

 

6. Cervical Phase (Ruminants)

The cervix acts as:

• A filter

• A barrier

• A selector for normal sperm

During estrus:

• Estrogen dilates the cervix

• Mucus becomes watery → easier passage

• Abnormal sperm get trapped in cervical folds

Two types of Penes:

• Vascular

  • No Sigmond flexure

  • Penis fills with blood

  • Increase blood pressure

  • Humans and Stallions

• Fibroelastic

  • Rigid in non-erect state

  • “S” shape due to Sigmond flexure

  • Increase blood pressure= straightens

  • Bulls, Rams, Boars

Phases of Ejaculation

What the image is about

It’s a diagram of the ejaculation reflex — a spinal reflex that coordinates sensory input from the penis with rapid, rhythmic muscle contractions that push semen out of the urethra.

Think of it as:
Stimulus → Spinal cord reflex → Powerful pelvic muscle contractions → Semen expulsion

---

🔍 Step‑by‑step explanation1. Intromission

This just means penetration.
Once the penis is inside the reproductive tract, stimulation begins.

---

2. Sensory stimulation of the glans penis

The glans (tip of the penis) has dense sensory receptors for:

• Pressure

• Temperature

• Touch

These sensory signals travel through afferent neurons to the lumbosacral spinal cord.

This is important:
Ejaculation is controlled by a spinal reflex center — not the brain.

---

3. Spinal reflex activates pelvic muscles

The spinal cord sends motor signals back to specific muscles:

• Urethralis muscle
  Squeezes the urethra like a pump.

• Bulbospongiosus muscle
  Provides the strongest contractions that actually propel semen.

• Ischiocavernosus muscle
  Stabilizes the penis and increases pressure inside erectile tissue.

These muscles contract suddenly and powerfully, in rhythmic bursts.

---

4. Expulsion of semen

Those coordinated contractions force semen through the urethra and out of the body.

This is the ejaculatory phase, following emission (movement of semen into the urethra).

---

🧩 Why this matters physiologically

• Ejaculation is not voluntary once the reflex is triggered.

• It depends on sensory input, spinal integration, and striated muscle contractions.

• The muscles involved are the same ones used in pelvic floor function.

Erection & Ejaculation

• Sensory stimuli and Psychic stimuli cause Reflex, sympathetic & parasympathetic nerves

• Reactions:

  • Increased vascular supply causes erection

  • Smooth muscle contractions of accessory glands cause emission

  • Contraction of muscles causes ejaculation

Post Copulatory Phase

• Characterized by period of Refractory

  • Time in which additional stimuli will not stimulate male to copulate again

  • Results in satiation & unwillingness

• Differs from exhaustion

  • Occurs when over copulated

  • Unable to copulate even with appropriate stimuli

Refractory Period

• A temporary period where the male cannot be stimulated to mate again.

• Length depends on:

  • Age

  • Libido

  • Temperature

  • Number of females

• Young bulls: rule of thumb = 1 female per month of age
  (12‑month bull → ~12 females)

Exhaustion (different from refractory)

Occurs when:

• Many females come into estrus at once (e.g., synchronization)

• Male attempts to breed too many in a short window

Results in:

• Depletion of sperm reserves

• Inability to copulate even after refractory period ends

Sperm Transport

• Rapid Phase

  • Occurs within minutes

  • Peristaltic contractions induced by copulation

  • Oxytocin

• Slow phase

  • Fertilizing sperm to AI junction

    • 8 hrs.

  • Motility + Contractions

Sperm Transport: Cervical Phase

• Billions Phase

• Roles of Cervix

  • Receptive at estrus (mucus)

  • Reservior cervical crypts (minor)

  • Protection from vagina (phagocytosis)

  • Energy (from mucus)

  • Filtration of dead + defective sperm

• Influenced by estrogen

• Motility required to traverse mucus

Where it happens: Cervix

When: Immediately after semen is deposited in the anterior vagina (ruminants)

What estrogen does during estrus:

• Opens (dilates) the cervix

• Thins the cervical mucus → easier for sperm to swim through

What the cervix does:

• Acts as a filter

• Removes:

  • Dead sperm

  • Dying sperm

  • Abnormal sperm

• These get trapped in cervical crypts (little pockets in the cervix)

Immune system role:

• Estrogen activates immune cells

• Neutrophils perform phagocytosis of sperm that don’t make it through

After ovulation:

• Progesterone rises → cervix closes

• Mucus becomes thick → forms a cervical plug (pregnancy protection)

Key takeaway:

Billions enter → only thousands make it past the cervix.

Sperm Transport: Uterine Phase

• Thousands survive

• Movement

  • Primarily contractions

• Contraction stimulators

  • PGF in Semen

  • Oxytocin form Posterior Pituitary

• Sperm Capacitation

  • Hypermotility

  • Acrosome Reaction

Where: Uterine body → uterine horns

How long: ~8 hours in cattle

What moves sperm forward:

• Oxytocin pulses (from posterior pituitary)

• Prostaglandin F2α (from seminal plasma)

• Both stimulate smooth muscle contractions → peristalsis

What sperm must do here:

• Continue swimming

• Survive immune attack

• Begin capacitation

 

Capacitation — Final Maturation Step

Where: Uterus

When: During the slow phase of transport

Why: Sperm MUST complete capacitation to fertilize the oocyte

What capacitation actually is:

Removal of seminal plasma proteins (specifically glycocalyx glycoproteins / glycosaminoglycans) from the sperm head.

What capacitation accomplishes:

1. Hyper‑motility

   • Tail beats become faster + more forceful

   • Needed to penetrate the zona pellucida

2. Exposes the acrosome

   • Acrosomal membrane becomes accessible

   • Prepares sperm for the acrosome reaction

Timing:

• Sheep: ~1.5 hours

• Pigs: 3–6 hours

• Cattle: ~7 hours

Cool fact:

Capacitation is reversible.

If you put capacitated sperm back into seminal plasma → proteins re‑coat the head → motility decreases.

The cervix removes:

• Dead sperm

• Abnormal sperm

• Low-motility sperm

Why?

• Cervical crypts trap defective sperm.

• Estrogen (estrus):

  • Dilates cervix

  • Thins mucus → easier passage

  • Activates immune cells → phagocytosis of dead sperm

After ovulation:

• Progesterone rises

• Cervix closes

• Mucus becomes thick → forms a cervical plug

Only thousands of sperm make it past the cervix.

 

4⃣ Uterine Phase — Longest Part of the Journey

Sperm move by:

• Uterine contractions (oxytocin + PGF2α)

• Their own motility (now beginning to matter)

Key event: Capacitation

Occurs in the uterus.

Capacitation =

Removal of seminal plasma proteins (glycocalyx/glycoconjugates)

→ Exposes the acrosome

→ Enables hyperactivated motility

→ Prepares sperm for acrosome reaction

⏱ Species timing:

• Sheep: ~1.5 hr

• Pig: 3–6 hr

• Cattle: ~7 hr

💡 Capacitation is reversible

If you put capacitated sperm back into seminal plasma, the coating reforms

What is Capacitation?

• Biochemical change

• Sperm outer plasma membrane

• Alters glycosaminoglycans

• Requires 1-7 hours

  • Sheep 1.5

  • Pigs 3-6

• Allows acrosome reaction to occur

• Reversible process

Deposition → Rapid Phase transport

Happens within minutes of copulation.

What drives it?

• Oxytocin release during mating

• Prostaglandin F2α from seminal plasma
  → Both stimulate uterine peristaltic contractions

Purpose:

Push a portion of sperm rapidly toward the uterus and oviducts.

⚠ These sperm are NOT the ones that fertilize.

They arrive too early and are not yet capacitated.

 

2⃣ Retrograde Loss

A huge portion of sperm is immediately lost:

• Flowing out of the vulva

• Trapped in vaginal mucus

• Destroyed by immune cells

This is why billions must be deposited

Sperm Transport: Utero-tubal Junction

• Limits number of sperm reaching oviduct

• Acts as a 2nd filter

• Sperm reservoir

• Affected by estrogen: Progesterone

Analogy: I‑40 going from 4 lanes → 2 lanes.

Functions:

• Limits sperm entry into the oviduct

• Ensures only high‑quality, capacitated sperm pass

• Prevents polyspermy by controlling sperm numbers

Special case:

Some species (e.g., bats) use the UTJ as a sperm reservoir for months.

Sperm Transport: Oviduct Phase

• Only hundreds to thousands survive

• Transport

  • By contractions

  • By fluid “currents” caused by cilia

• Sperm pool (reservoir) in isthmus

• Fertilization occurs at Ampulla-Isthmus junction (AIJ)

Only hundreds to a few thousand sperm reach this point.

Movement aided by:

• Ciliary currents

• Smooth muscle contractions

• Sperm hyperactivation

Hormonal control:

• High estrogen → flow toward AI junction (helps sperm reach oocyte)

• High progesterone → flow toward uterus (helps embryo move back)

Sperm transport

Occurs at the ampulla–isthmus junction.

Steps:

1. Hyperactivated sperm reach the oocyte

2. Acrosome reaction occurs

1. Sperm penetrates zona pellucida

2. Sperm enters oocyte cytoplasm

3. Male + female pronuclei fuse → zygote

Fertilization: What Happens When Sperm Reach the Oocyte

The oocyte has several layers the sperm must get through:

1. Cumulus oophorus

   • Cloud of granulosa cells surrounding the oocyte

   • Held together by hyaluronic acid

   • Sperm use hyaluronidase (from the acrosome) to wiggle through

2. Zona pellucida (ZP)

   • Thick glycoprotein shell

   • Contains ZP1, ZP2, ZP3

   • ZP3 is the key sperm-binding molecule

   • Binding to ZP3 triggers the acrosome reaction

3. Vitelline membrane / oolemma

   • Actual plasma membrane of the oocyte

   • Only one sperm is allowed to fuse

Acrosome Reaction

Triggered when a capacitated sperm binds ZP3.

What happens:

• The acrosomal membrane fuses with the sperm plasma membrane

• Enzymes (acrosin, hyaluronidase) are released

• These digest a path through the zona pellucida

• The sperm becomes “drilled down” to its inner acrosomal membrane

• This exposes the equatorial segment → the part that actually fuses with the oocyte

Fusion and Blocks to Polyspermy

Fusion

Once a sperm reaches the oolemma:

• The sperm’s equatorial segment binds the oocyte membrane

• Membranes fuse

• The sperm nucleus enters the cytoplasm

• The oocyte completes meiosis II

• Second polar body is expelled

• Male and female pronuclei form

• Pronuclei merge → syngamy

• A diploid zygote is created

 

Blocks to Polyspermy

To prevent multiple sperm from entering:

1. Fast block (electrical, in some species—not strong in mammals)

2. Cortical reaction (main mammalian block)

   • Cortical granules release enzymes

   • Zona pellucida hardens

   • ZP3 is modified so no more sperm can bind

   • This is called the zona block

Endogenous Factors affecting Sperm Motility

• Age (sperm and donor)

  • Time between and after ejaculation

• Sperm Maturation

  • Morphology

• Energy Stores (ATP)

  • Flagellar movement

• Cell surface

  • Membrane integrity

Exogenous Factors affecting Sperm Motility

• Biophysical & Physiological Factors

  • pH, temperature, viscosity

• Suspending fluids

  • Male & Female tracts

• Stimulation/Inhibition

  • Hormones

  • Environmental pollutants

  • Inorganic ions

  • Caffeine

Luminal Fluids in contact with Spermatozoa

Fate of Unsuccessful Sperm in the Cow reproductive tract

• 1 billion sperm inseminated

• 73% recovered

  • Mucus discharge- 61%

  • Urine - 1%

  • Vagina & Cervix- 4%

  • Uterus 7 Oviducts (retained)- 6.5%

    • Undergo phagocytosis

  • Abdominal cavity- 0.5%

Ovulation & Egg Transport

• Oocyte and cumulus cells captured by infundibulum

• Transport of Oocyte (ovum)

  • Mechanism

    • Cilia- infundibulum and ampulla

    • Fluid currents

    • Rhythmic segmented peristaltic contractions

• Gamete longevity (hours)

Fertilization

Layers of the Oocyte

From outside → inside:

• Cumulus granulosa cells

  • Sticky cloud of follicular cells surrounding the oocyte.

  • Sperm must push through these first.

• Zona pellucida (ZP)

  • Glycoprotein shell around the oocyte.

  • Contains ZP3 and ZP2 (critical for sperm binding + acrosome reaction).

• Perivitelline space

  • Space between zona pellucida and oocyte membrane.

• Vitelline membrane (oocyte plasma membrane)

  • The membrane the sperm must fuse with.

 

2⃣ Four Major Steps of Fertilization

Fertilization = Acrosome Reaction → Zona Reaction → Vitelline Block → Syngamy

Steps in Fertilization

1. Acrosome Reaction

2. Zona reaction

3. Vitelline Block

4. Pronuclear development & syngamy

tep 1: Acrosome Reaction

Triggered when sperm contacts the zona pellucida.

Key proteins:

• ZP3 = docking protein

○ First contact.

○ Binds the sperm at the apical region (between acrosome + nucleus).

○ Think: “ZP3 = Velcro → holds sperm in place.”

• ZP2 = detonator protein

○ Triggers the acrosome reaction.

○ Causes the sperm to release enzymes.

What happens during the acrosome reaction?

• Outer acrosomal membrane fuses with sperm plasma membrane.

• Releases enzymes:

○ Acrosin (main one) → digests zona pellucida.

○ Hyaluronidase → helps break apart cumulus cells.

• Hyperactivated motility pushes sperm through the hole it creates.

4⃣ Step 2: Zona Reaction (First Block to Polyspermy)

Triggered when the FIRST sperm enters the perivitelline space.

Cortical granules (just under the vitelline membrane):

• Release calcium into the perivitelline space.

Calcium causes:

1. Hardening of the zona pellucida

○ Zona becomes impenetrable.

2. Down‑regulation of ZP3

○ No more sperm can bind.

Purpose:

✔ Prevents additional sperm from binding to the zona pellucida.

5⃣ Step 3: Vitelline Block (Second Block to Polyspermy)

Triggered when the sperm nucleus actually touches the vitelline membrane.

What happens:

• The vitelline membrane engulfs the sperm head.

• A membrane‑level signal spreads across the oocyte.

• Prevents any additional sperm nuclei from fusing with the oocyte cytoplasm.

Purpose:

✔ Prevents multiple sperm nuclei from entering the oocyte.

6⃣ Step 4: Syngamy

Final step = formation of the zygote.

What happens:

• Sperm nucleus detaches from the midpiece.

• Sperm DNA decondenses → forms the male pronucleus.

• Female pronucleus + male pronucleus migrate toward each other.

• They fuse → zygote (2N).

7⃣ Why Polyspermy Is Fatal

• Humans: 23 chromosomes from sperm + 23 from oocyte = 46 total.

• If multiple sperm enter → too many chromosomes → embryo dies.

8⃣ Factors That Reduce Block Efficiency

• Aged oocyte (older egg = weaker blocks)

• Aged female

• Heat stress / high temperature

These increase the risk of polyspermy.

Acrosome Reaction

• Penetration of cumulus granulosa cells and corona radiata

  • Hyaluronidase, corona penetrating enzyme

• Fusion of plasma membrane and outer acrosomal membrane

  • Multiple vesicles appear on cell surface

• Exposure of inner acrosomal membrane

• Penetration of zona pellucida

  • Acrosin

  • Acrosome swells & is lost

  • ZP3: sperm receptor

  • ZP2: initiates acrosome reaction

Step 1 — Acrosome Reaction

Triggered when sperm binds zona pellucida.

Zona proteins:

• ZP3 = docking protein

• ZP2 = triggers acrosome reaction

Acrosome releases enzymes (acrosin) → digests zona → sperm pushes through.

 

Step 2 — Zona Reaction (Block #1 to polyspermy)

• First sperm enters perivitelline space

• Cortical granules release calcium

• Calcium:

  • Hardens zona

  • Down‑regulates ZP3

  • Prevents more sperm from binding

 

Step 3 — Vitelline Block (Block #2)

• Oocyte membrane engulfs sperm nucleus

• Membrane changes prevent additional nuclei from entering

 

Step 4 — Syngamy

• Sperm nucleus decondenses

• Male + female pronuclei fuse

• Zygote formed