ANS Exam 3

Division of the Estrous Cycle

• Based on Ovarian Structures:

  • Follicular Phase

    • Proestrus

    • Estrus

  • Luteal Phase

    • Metestrus

    • Diestrus

Follicular Phase

• Growth of Follicles

  • Dominant structure- preovulatory follicle

  • Dominant hormone- Estradiol

• Starts with regression of corpora lutea

• Ends with ovulation

• Relatively short (23-33% of the cycle)

• In a 21-day cycle → 5-7 days

• Proestrus + Estrus

Luteal Phase

• Growth of corpus luteum (corpora lutea)

  • Dominant structure - Corpus luteum

  • Dominant hormone- Progesterone

    • Prepares uterus for pregnancy

    • Supports embryo development

• Starts with ovulation

• Ends with luteal regression

• Longer phase (66-75% of cycle

• In a 21 days cycle → 14-16 days

• If not pregnant:

  • CL undergoes luteolysis (regression)

  • Cycle restarts with new follicular phase

• Metestrus + Diestrus

Proestrus

• Increased estrogen

  • Follicles growing

• 1st stage of follicular phase

• Progesterone decreasing

  • Due to CL regression

Estrus aka “heat”

• Sexual receptivity

• Peak estrogen production

• Lowest Progesterone

  • Maximum follicle size and estrogen

• LH & FSH surges

• Ovulation

  • Mare: ovulates during estrus (estrus lasts 4-9 days)

  • Cow/Sheep: ovulate after estrus, in metestrus (last 8 hours)

• High estrogen stimulates GnRH surge → LH surge → ovulation

• Behavioral sign

  • Affectionate behavior

  • Increases body temp

Metaestrus

• Increasing progesterone production

• Estrogen decreasing (follicle ruptured)

  • Formation of the CL following ovulation

• Ovulation - Cow & Ewe

• LH and Estradiol- low

Diestrus

• Sustained progesterone production

• Maximum progesterone

• Estrogen= lowest

• Fully functional CL

  • CL dominant structure

  • Follicles continue to grow in waves

  • Shift in LH & FS

• Longest period of cycle

• Why estrogen stays low

  • Progesterone blocks the surge center in the hypothalamus

    • → Prevents LH surge

      • Only the tonic center releases small pulses of LH + FSH

        • → Not enough to cause ovulation

Differences between Estrous and Menstrual Cycles

• Cycle based on what we can SEE

  • Menses’ vs. Estrus

• Menstrual 50 follicular: 50 luteal

  • Estrous cycle: 25 follicular: 75 luteal

• Ovulation in the middle (day 14)

• Menstrual cycles occur in primates

• Ovulation is silent (no behavioral signs)

Menstrual cycle sequence

1. Consist of six events:

2. Menstruation: Sloughing of endometrial lining, low estrogen + progesterone

3. Follicular growth: Follicle develop, estrogen rises, Endometrium begins rebuilding

4. Ovulation (day 14): Triggered by LH surge, Marks end of follicular phase

5. Luteinization: CL forms, progesterone rises, Endometrium thickens further

6. Endometrial growth

7. Luteolysis

Estrous vs. Menstrual Cycle: Key Comparison

Estrous Cycle

• Seen in livestock

• Defined by estrus (heat)

• Ovulation often tied to estrus

• Follicular phase = 25%

• Luteal phase = 75%

• No menses

• Sexual receptivity is obvious

Menstrual Cycle

• Seen in primates

• Defined by menses

• Ovulation is silent

• Follicular = 50%

• Luteal = 50%

• Endometrium sloughs if no pregnancy

• No clear behavioral estrus

Progesterone + Estrogen

Progesterone

• Produced by CL

  • At ovulation → low

  • After ovulation → increases CL forms

  • Mid cycle → Peak

  • Before next ovulation → decreases due to PGF2a

• P4 blocks the surge center

Estrogen

• Produced by growing follicles

• just before ovulation → high

• Mid cycle → low

• End of cycle → rises again as follicles regrow

 Add the Gonadotropins (Anterior Pituitary)

Follicle Stimulating Hormone (FSH)

• Stimulates follicle growth

• Follicles → produce estrogen

• FSH rises early in the cycle

• Small “blip” at ovulation

FSH pattern:

Small rise → supports follicle waves → small bump at ovulation

 Luteinizing Hormone (LH)

• Controlled by:

  • Tonic center (low pulses)

  • Surge center (big spike)

• Estrogen stimulates the surge center
  → LH surge → ovulation

LH pattern:

Low pulses → massive spike → back to low pulses

Add the Uterine Hormone

 Prostaglandin F₂α (PGF₂α)

• Produced by: Uterine endometrium

• Function: Kills the corpus luteum (luteolysis)

• Begins pulsing around day 10

• Pulses increase in:

  • Amplitude

  • Frequency

• Causes progesterone to drop

PGF₂α pattern:

Low → pulses begin → pulses intensify → CL dies → drops again

Full hormone sequence

1. Follicular Phase Begins (P4 low)

• CL regresses → progesterone drops

• Low P4 removes block on surge center

• FSH rises → follicles grow → estrogen increases

 

2. Estrus + LH Surge

• Estrogen peaks → positive feedback to surge center

• Surge center releases GnRH

• Anterior pituitary releases LH surge

• Ovulation occurs (if P4 is low)

 

3. Luteal Phase Begins

• Ovulation → CL forms → progesterone rises

• High P4:

  • Blocks surge center

  • Prevents LH surge

  • Prevents ovulation

  • Supports uterine lining

 

4. Diestrus (Longest Phase)

• CL fully functional → max progesterone

• Follicles grow in waves but undergo atresia

• Estrogen stays low

 

5. Luteolysis

• Uterus releases PGF₂α pulses

• Pulses intensify → CL dies

• Progesterone drops sharply

 

6. Cycle Restarts

• Low progesterone → surge center unblocked

• FSH rises → follicles grow → estrogen rises

• New estrus approaches

Estrous vs. Menstrual Cycle (Final Comparison)

Estrous Cycle (Livestock)

• Defined by estrus (heat)

• Follicular = 25%

• Luteal = 75%

• No menses

• Ovulation tied to estrus (species‑dependent)

Menstrual Cycle (Primates)

• Defined by menses

• Follicular = 50%

• Luteal = 50%

• Ovulation ~day 14

• Endometrium sloughs if no pregnancy

Ovulation Occurs when progesterone is LOW

• Progesterone must be low for the LH surge to occur.

• When P4 is low → surge center is unblocked → LH surge → ovulation.

• After ovulation:

  • Follicle ruptures

  • Corpus luteum (CL) forms

  • Progesterone begins to rise

If the female is NOT pregnant

• The uterine endometrium detects no embryo.

• It releases prostaglandin F₂α (PGF₂α).

• PGF₂α:

  • Kills the CL (luteolysis)

  • Causes progesterone to drop

  • Allows the cycle to restart

PGF₂α pattern:

• Begins pulsing around day 10

• Pulses increase in amplitude + frequency

• High pulses → CL death → P4 drops → follicular phase begins again

 

Parts of the Follicle

• Oocyte

  • Oogenesis- development of the oocyte= able to fertilize

• Granulosa cells

  • Inner cell layers

  • Two types:

    • Mural granulosa

    • Cumulus granulosa

  • Functions:

    • Covert testosterone → estrogen

    • Support oocyte development

    • Communicate via follicular fluid

• Thecal cells

  • Outer yellow layer

  • Produces testosterone

  • Testosterone diffuses into granulosa cells → converted to estrogen

    • This is the two-cell, two- gonadotropin model

• Follicular fluid

  • Fills the antrum

  • Allows communication between granulosa cells

  • Stores nutrients, growth factors

Components of oocyte

• Zona Pellucida- assists fertilization and may provide protection

  • Thick, protective glycoprotein shell

    • Functions:

      • Protects the oocyte

      • Sperm bind here during fertilization

• Vitelline membrane- surface layer of the oocyte

  • Immediately beneath the zona pellucida

  • Called the oolemma

• Perivitelline space

  • Space between zona pellucida and vitelline membrane

  • Provides cushioning and structural separation

• Cytoplasm (yolk)- contains many proteins, enzymes, nutrients needed for survival of the zygote

  • Supports embryo until it reaches the uterus

• Germinal vesicle- nucleus containing genetic information

  • The nucleus of the oocyte

  • Contains the genetic material

  • Will undergo meiosis during maturation

Oogenesis

• Oogenesis- formation and development of the ovum (egg) - gametes fixed number of germ cells

  • It begins in fetal life, pauses for years, and only resumes after LH surge during reproductive cycle

• Oogonium- the primordial cell which develops into the oocyte

• Oocyte- the early, not yet fully developed ovum

• Ovum- the egg- the cell that is capable of developing into a new individual

 Primordial Germ Cells → Oogonia → Oocytes

 Primordial Germ Cells

• Originate in the yolk sac

• Migrate to the gonadal ridge

• Undergo mitosis to increase in number

• Fill the sex cords

Oogonia

• Earliest form of female germ cells

• Undergo mitosis in fetal ovary

• After mitosis, they begin meiosis I

Primary Oocyte

• Formed in fetal life

• Frozen in meiosis I (prophase I)

• This is the stage found in:

  • Primordial follicles

  • Primary follicles

  • Secondary follicles

  • Tertiary (antral) follicles

  • Even preovulatory follicles

Key point:

The oocyte stays frozen in meiosis I for YEARS until the LH surge.

Oocyte Formation & Atresia

• Primary oocytes formed from mitotic divisions of oogonia

• Primary oocytes enter MEIOSIS 1

  • Become dormant

• Maximum number formed during mid to late fetal life

• Once max number is attained, atresia or natural degeneration begins and continues for life

• Decrease in number of gamete’s

Atresia (Programmed cell death)

• After peak numbers in mid‑late fetal life, oocyte numbers decline.

• Atresia = apoptosis‑like death of germ cells.

• Continues throughout life.

• Reason for reproductive senescence (menopause):
  → Females eventually run out of oocytes.

Follicle Stages vs. Oocyte Stages

Follicle Stage	Oocyte Stage
Primordial	Primary oocyte (meiosis I arrest)
Primary	Primary oocyte
Secondary	Primary oocyte
Tertiary (antral)	Primary oocyte
Preovulatory (Graafian)	Primary oocyte

Important:

The oocyte does NOT complete meiosis I until the LH surge.

 

What triggers meiosis to resume? What happens? Why polar bodies

What triggers meiosis to resume?

• LH surge (caused by high estrogen)

What happens?

1. Primary oocyte completes meiosis I

1. Produces:

   • Secondary oocyte

   • First polar body (genetic trash can)

Why polar bodies?

• Oocyte must go from diploid → haploid

• Polar body = “genetic dumpster” to discard extra chromosomes

Cumulus granulosa cells:

• Surround the oocyte

• LH surge disrupts their tight communication

• This loss of inhibition allows meiosis to resume

Oocyte Activation & Development

• Meiosis resumes only in preovulatory follicle(s)

  • LH & FSH remove inhibitors

  • Activation occurs at estrus

• Only a select few are stimulated to develop

• Repeats every cycle

Oocyte Activation & Development continued

Fertilization → Completion of Meiosis II

If sperm penetrates the zona pellucida:

1. Secondary oocyte completes meiosis II

2. Produces:

   • Ovum (haploid)

   • Second polar body

Result:

• Haploid ovum + haploid sperm → zygote

Secondary oocyte → Ovulation

• The secondary oocyte is ovulated

• It does not finish meiosis II unless fertilization occurs.

• Ovulated oocyte= secondary oocyte, not an ovum

Oogenesis Timeline

• Fetal life:
  Oogonia → primary oocytes → freeze in meiosis I

• Puberty onward:
  Follicles recruited each cycle; oocyte still frozen

• LH surge:
  Meiosis I completes → secondary oocyte + polar body

• Ovulation:
  Secondary oocyte released

• Fertilization:
  Meiosis II completes → ovum + second polar body

Why this matters:

• Only oocytes that reach the LH surge can be ovulated.

• Most oocytes die via atresia.

• Meiosis is tightly controlled by granulosa cells and LH.

Oocyte numbers over the female’s lifetime

 Oocyte Numbers Over the Female’s Lifetime

Key Concept:

Females are born with all the oocytes they will ever have, and the number only decreases from fetal life onward.

 

Human Oocyte Numbers

Fetal Life

• Peak number of oocytes in mid–late fetal development

• ~6–7 million oogonia at maximum

• Begin meiosis → become primary oocytes

• Many die via atresia

At Birth

• Only 1–2 million oocytes remain

• All are primary oocytes frozen in meiosis I

At Puberty

• Only ~300,000 remain

• Still primary oocytes in primordial/primary/secondary follicles

During Reproductive Life

• Only ~400–500 oocytes are ever ovulated

• The rest die via atresia

At Menopause (45–50 years)

• Oocyte pool is essentially depleted

• Reproductive senescence occurs because no viable gametes remain

 

Cattle Oocyte Numbers

Fetal Life

• Peak: ~2.7 million oocytes

At Birth

• Only ~70,000 remain

At Puberty

• ~24,000 oocytes remain

Lifetime Ovulations

• Cattle ovulate far fewer oocytes than humans

• Only ~500 ovulations in a lifetime (depending on lifespan)

 

Gamete Wastage (Atresia)

Your professor emphasized this:

From fetal peak → ovulated oocytes

• 99.98% of oocytes are lost

• Only 0.02% ever reach ovulation

From puberty → ovulated oocytes

• Only ~2.8% of remaining oocytes are ovulated

• 97% die via atresia

Why so much loss?

• Atresia = programmed cell death

• Occurs continuously from fetal life → menopause

• Ensures only the healthiest follicles reach ovulation

 

Why Females Undergo Reproductive Senescence

• They run out of oocytes

• Once the ovarian reserve is depleted → cycles stop → menopause

Primordial follicles

• Primordial Follicles (fetal life → Puberty)

  • Earliest follicles present in the ovary during fetal life.

  • Found in nests/clusters

  • Structure

    • Primary oocyte inside (arrested in meiosis I).

    • Single layer of squamous epithelial cells.

  • These follicles remain present through childhood and puberty

  • After puberty, some primordial follicles begin to activate and grow.

• Activation of Primordial Follicles

  • Growth is stimulated by ovarian growth factors, especially:

    • Members of the TGF-B family

    • Anti-mullerian hormone

    • Activin and inhibin

Primary follicles

• Primary Follicle

  • Structural Changes

    • Oocyte enlarges

    • Follicular cells change from:

      • Squamous → cuboidal/columnar

    • Oocyte gains the zona pellucida:

      • A hardened glycoprotein layer surrounding the oocyte

    • Oocyte is still a primary oocyte, still arrested in meiosis I.

  • Single layer follicle cells

  • Primary oocyte but starts to expand

  • Follicle cells expand (cubodial)

  • Zona pellucida forms (harden layer outside)

Secondary Follicles

• Secondary Follicle

  • Follicular cells proliferate → multiple layers

  • First appearance of two distinct cell types:

    • Granulosa cells (inner)

    • Theca cells (outer)

  • Still pre antral

  • Oocyte remains a primary oocyte

  • Primary oocyte

  • Multiple layers of granulosa cells

  • Thecal layers forms

  • Pre-antral (no follicular fluid)

  • Start to see division of granulosa & Theca)

Tertiary Follicle

• This is where follicuogensis becomes fully active

• Development of the antrum (fluid-filled cavity)

• Oocyte is still a primary oocyte

• Follicle now has distinct functional regions:

  • Oocyte- Still arrested in meiosis I

  • Germinal vesicle- nucleus of the oocyte

  • Mural granulosa cells- Produce estrogen; more cells= more estrogen

  • Cumulus granulosa cells- Surround oocyte; provide meiotic inhibition

  • Theca cells- produces testosterone (precursor for estrogen

  • Follicular fluid- Reservoir for hormones, nutrients, growth factors; enables communication across granulosa layers

Folliculogenesis

• Classification and regulation of follicle growth after they get to tertiary follicle

• Parts of follicle

  • Granulosa cells

  • Thecal cells

  • Follicular fluid

• Theca= testosterone able to transition to granulosa to produce E2

Two cell- Two gonadotropins Model

Theca Cells

• Have LH receptors.

• LH binding stimulates:

  • Cholesterol → pregnenolone → progesterone → testosterone.

• Testosterone diffuses into granulosa cells.

Granulosa Cells

• Have FSH receptors.

• FSH binding increases aromatase.

• Aromatase converts:

  • Testosterone → Estrogen.

Changing hormone Dependency during Follicle growth

• Small follicles → highly FSH-dependent.

• As follicles grow:

  • They become increasingly LH-dependent.

• This shift is essential for:

  • Sustained estrogen production.

  • Preparing for the LH surge and ovulation.

 

Granulosa & Theca cells

• Granulosa

  • From cortex

  • Responsive to FSH

  • Convert testosterone to estradiol

• Theca

  • From stroma

  • Responsive to LH

  • Convert Cholesterol to Testosterone

Follicular Fluid (Antral Follicles)

• Components

  • Hormones

    • Steroids

    • Gonadotropins

    • Prostaglandins

  • Proteins

  • Enzymes

  • Carbohydrates

• Functions

  • Supports follicle growth

  • Aids in oocyte development and health

  • Mediates granulosa cell functions

What is inside follicular fluid and Function?

• Gonadotropins:

  • Luteinizing hormone (LH)

  • Follicle‑stimulating hormone (FSH)

• Steroid hormones:

  • Estrogen

  • Progesterone

  • Testosterone

• Prostaglandins

• Nutrients:

  • Essential proteins

  • Enzymes

  • Carbohydrates

  • Growth factors

Functions of follicular fluid

• Supports follicle growth and development.

• Maintains communication between granulosa cells across the follicle.

• Keeps the oocyte alive and healthy.

• Provides nutrients in a hypoxic environment (granulosa cells lack direct blood supply).

• If follicular fluid deviates from normal → follicle loss.

Stages of follicular growth

Stage 1: Recruitment

• A large number of small antral follicles are recruited.

• These follicles produce low estrogen.

• Many follicles enter this stage, but most will not continue.

 

🔹 Stage 2: Selection

• A refinement process.

• Some follicles continue growing; many undergo atresia (cell death).

• Selected follicles:

  • Increase in size.

  • Increase in granulosa cell number.

  • Produce more estrogen.

• Atresia reduces the number of follicles but increases the size and estrogen output of survivors.

 

🔹 Stage 3: Dominance

• Only one dominant follicle in nonovulatory species (humans, cattle, mares).

• Litter‑bearing species may have co‑dominance.

• Dominant follicle characteristics:

  • Largest follicle on the ovary.

  • Produces a tremendous amount of estrogen.

  • Reaches maximum size.

  • Has the most granulosa cells and the most follicular fluid.

Early Antral (tertiary)

• Primary oocyte

• Oocyte reaches maximum size

• Granulosa cells expand

• Antrum forms

• Thecal layer more prominent

  • Interna

  • Externa

Follicular pool and timeline

• The follicular pool includes:

  • Primordial follicles

  • Primary follicles

  • Secondary follicles

• Transition from primordial → tertiary follicle takes 45–60 days (species‑dependent).

• Once in folliculogenesis, growth occurs much more rapidly.

 

Regulation of Folliculogenesis

What determines which follicles advance?

• Exact mechanisms are still unknown.

• But we know it depends heavily on gonadotropin support:

  • Which follicles receive FSH?

  • Which receive LH?

• Follicles that fail to receive adequate support undergo atresia.

 

Graafian Follicle (Preovulatory)

• Last stage of tertiary follicle

• Primary oocyte

• Features:

  • Maximum granulosa cell number.

  • Maximum follicular fluid volume.

  • Oocyte is still a primary oocyte, arrested in meiosis I.

  • Contains the cumulus–oocyte complex (COC).

    • This is what is expelled at ovulation.

 

Follicular Development

• How do follicles grow?

  • Waves

  • Number of follicles change throughout estrous cycle

• Stimulated or regulated by?

  • Gonadotropins

  • Growth factors

  • Activin or inhibin

• Fates?

  • Ovulation

  • Atresia

  • Cysts

Follicular Waves During the Estrous Cycle

• Follicles grow in waves.

• During the luteal phase, high progesterone suppresses follicular progression.

• During the follicular phase, progesterone drops → follicle growth increases.

Activin & Inhibin: Regulation of FSH

Produced by granulosa cells

• Activin

  • Produced by small follicles

  • Increases FSH from the anterior pituitary

  • “Activin activates”

• Inhibin

  • Produced by large follicles

  • Decreases FSH

  • “Inhibin inhibits”

Why this matters

• Helps regulate which follicles continue to grow.

• Prevents too many follicles from becoming dominant.

• Coordinates follicle fate with the hormonal environment.

Follicle Fate

Possible outcomes

1. Ovulation

   • Requires low progesterone

   • Must occur during the correct stage of estrus

   • Only the dominant follicle has this opportunity

2. Atresia (death)

   • Most follicles undergo this

   • Occurs at all stages of folliculogenesis

   • Ensures only the best‑supported follicle survives

Most follicles undergo Atresia

• The majority of follicles will not ovulate.

• They undergo atresia (programmed cell death).

• Atretic follicles are removed from the ovary.

• Atresia can occur at any stage of folliculogenesis.

Cystic Follicles (Pathological Fate)

A. Cystic Follicles in Cattle

• Occur when follicles stop responding to normal hormonal signals.

• Common in high‑producing dairy cows due to disrupted hypothalamic–pituitary–ovarian signaling.

• Cystic follicles:

  • Do not ovulate.

  • Do not undergo atresia.

  • Persist on the ovary.

• Hormone profile:

  • Low estrogen

  • Low progesterone

• Defined as >25 mm in diameter.

• A major reproductive management challenge in cattle.

B. Polycystic Ovarian Syndrome (PCOS) in Humans

• Different phenotype from cattle.

• Characterized by:

  • Many small recruited follicles that remain on the ovary.

  • Follicles fail to progress beyond early antral stage.

• Strongly associated with metabolic dysregulation.

• Results in irregular cycles and impaired ovulation.

Folliculogenesis Review (Recruitment → Selection → Dominance)

Follicular Pool

• Includes:

  • Primordial follicles

  • Primary follicles

  • Secondary follicles

Once follicles reach the tertiary (antral) stage, they enter folliculogenesis:

1. Recruitment

   • Many follicles enter.

   • Low estrogen production.

2. Selection

   • Some follicles continue; many undergo atresia.

   • Selected follicles grow larger and produce more estrogen.

3. Dominance

   • In monovulatory species (humans, cattle, mares): one dominant follicle.

   • In litter‑bearing species: multiple co‑dominant follicles.

   • Dominant follicle produces very high estrogen.

 

Dominant Follicle & Ovulation Conditions

• Dominant follicle can ovulate only if progesterone is low.

• If progesterone is high (luteal phase):

  • Dominant follicle undergoes atresia.

  • A new wave of recruitment → selection → dominance begins

Follicle growth

• Many recruited

• Few selected

• One dominant follicle

  • Where do the rest of the follicles go?

  • What about litter bearers?

• Increase progesterone dominant follicle undergo atresia

Litter-Bearing Species

• Species like pigs, dogs, cats:

  • Recruit more follicles.

  • More follicles survive selection.

  • Multiple follicles become co‑dominant → multiple ovulations.

 

 Oocyte Status in Tertiary Follicles

• Even in large tertiary follicles:

  • The oocyte is still a primary oocyte.

  • Still arrested in meiosis I.

• Oocyte reaches maximum size in the tertiary stage.

• Surrounded by the cumulus–oocyte complex (COC).

  • This entire complex is expelled at ovulation.

Waves of follicular growth

• Occur in patterns

• Ovulatory vs. non-ovulatory

• Dominant vs. subordinant

• Signals

  • LH

  • FSH

  • Inhibin

• Follicles grow in waves throughout the cycle.

• Luteal phase:

  • High progesterone suppresses follicular progression.

• Follicular phase:

  • Progesterone drops → follicle growth accelerates.

• Waves repeat until:

  • A dominant follicle ovulates
    OR

  • Progesterone rises again and the dominant follicle undergoes atresia.

 Follicular Waves Occur Only in Tertiary (Antral) Follicles

• At this point, we are focusing only on tertiary follicles.

• These follicles grow in waves throughout the estrous cycle.

• Waves occur in two types of periods:

  • Obligatory period

  • Non‑obligatory period

 

 Obligatory vs. Non‑Obligatory Periods

Non‑Obligatory Period

• Occurs when progesterone is high (luteal phase).

• High progesterone:

  • Blocks the surge center in the hypothalamus.

  • Prevents the LH surge.

• Without an LH surge:

  • Dominant follicles cannot ovulate.

  • They undergo atresia.

Obligatory Period

• Occurs when progesterone is low.

• Low progesterone:

  • Removes inhibition on the surge center.

  • Allows estrogen from the dominant follicle to activate the surge center.

  • Leads to the LH surge → ovulation.

First wave

Second Wave

Follicle Categories Within a Wave

• Recruited follicles

  • Many enter; most die.

• Selected follicles

  • Survive initial refinement; grow larger.

• Dominant follicle

  • One per wave in monovulatory species.

  • Suppresses subordinate follicles.

  • Only ovulates if progesterone is low.

Subordinate follicles = all recruited/selected follicles that are not dominant.

 

 Hormonal Regulation During Waves

Key regulators

• FSH

• LH

• Inhibin (from dominant follicle)

Why no Activin here?

• Dominant follicles produce inhibin, not activin.

• Inhibin suppresses FSH, preventing new follicles from rising to dominance.

 

First Follicular Wave After Ovulation

• Ovulation → corpus luteum forms → progesterone rises.

• High progesterone = non‑obligatory wave.

• Follicles still undergo:

  • Recruitment

  • Selection

  • Dominance

• BUT the dominant follicle cannot ovulate → undergoes atresia.

 

Real Estate Effect

• Dominant follicles take up physical space on the ovary.

• When a dominant follicle undergoes atresia:

  • Space opens up.

  • Allows new recruited follicles to grow.

• This is why waves repeat.

Second Follicular Wave

• Same pattern:

  • Recruitment → selection → dominance.

• Progesterone is still high.

• So this wave is also non‑obligatory.

• Dominant follicle again undergoes atresia.

Signals for follicle growth

• Follicle stimulating Hormone

  • Granulosa cell mitosis

  • Increases LH receptors

  • Steroidogenesis (increase E2)

  • Effects preantral & antral follicles

• Luteinizing Hormone

  • Steroidogenesis

  • Effect antral follicles

  • Surge starts ovulation

  • Resumption of meiosis -oocyte

How FSH and LH Drive Follicle Growth

FSH

• Stimulates mitosis in granulosa cells.

• More granulosa cells → more estrogen.

• FSH also increases LH receptors on granulosa cells.

  • This is crucial for later LH responsiveness.

LH

• Acts on theca cells → produces testosterone.

• Testosterone diffuses into granulosa cells → aromatized to estrogen.

• LH is also required for ovulation once the surge occurs.

Transition to the Ovulatory Wave

• In a three‑wave cow, the third wave occurs when:

  • Progesterone is falling (luteolysis).

• Low progesterone:

  • Removes inhibition on the surge center.

  • Estrogen from the dominant follicle activates the surge center.

  • LH surge occurs.

  • Dominant follicle becomes pre‑ovulatory (Graafian).

  • Ovulation happens.

 How Many Waves Do Cows Have?

• Cows can have 1–6 waves per estrous cycle.

• Most commonly: 2 or 3 waves.

• Three‑wave cows are more fertile than two‑wave cows.

Why?

• In two‑wave cows:

  • The dominant follicle remains dominant longer.

  • It is exposed to high progesterone for a longer time.

  • This prolonged exposure → follicle aging → reduced fertility.

 

 Dominant Follicle Suppression of Subordinates

• Dominant follicle produces:

  • Estrogen

  • Inhibin

• These suppress FSH.

• Low FSH prevents subordinate follicles from:

  • Growing

  • Becoming co‑dominant

  • Competing for dominance

This ensures only one follicle reaches ovulatory size in nonovulatory species.

How the Dominant Follicle Suppresses Subordinate Follicles

The dominant follicle actively prevents other follicles from continuing growth. It does this in three major ways:

A. Physical Suppression

• The dominant follicle is large and takes up significant space on the ovary.

• This physically limits the ability of subordinate follicles to grow.

B. Hormonal Resource Competition

• The dominant follicle has:

  • More granulosa cells

  • More theca cells

  • More receptors for FSH and LH

• Because of this, it absorbs most of the gonadotropins delivered to the ovary.

• Subordinate follicles receive less FSH and LH, so they cannot continue developing.

C. Inhibin Production

• Dominant follicles produce high levels of inhibin.

• Inhibin suppresses FSH at the anterior pituitary.

• FSH is critical for early recruited follicles.

• Low FSH → subordinate follicles undergo atresia.

Follicular Waves in a Three‑Wave Cow

Wave 1 (Yellow Wave)

• Recruitment → selection → dominance.

• Progesterone is high → dominant follicle cannot ovulate.

• Dominant follicle undergoes atresia.

Wave 2

• Same pattern: recruitment → selection → dominance.

• Progesterone still high → dominant follicle again undergoes atresia.

Wave 3 (Pink Wave)

• Recruitment → selection → dominance.

• Progesterone decreases due to luteolysis.

• Dominant follicle now has the opportunity to:

  • Activate the surge center

  • Trigger the LH surge

  • Ovulate

 

Inhibit follicle growth

• Presence of a dominant follicle

• Inadequate production of gonadotropins

• Steroids

  • Estrogen and androgens

• Inhibin

  • Protein produced by follicles which inhibits other follicles from developing by decreasing FSH release

Follicular growth

• growth

• plateau

• regress?

• ovulate?

Endocrine Control of Follicular Waves

A medical‑textbook style diagram summarizes the logic:

Step 1: Hypothalamus

• Releases GnRH.

Step 2: Anterior Pituitary

• Releases FSH and LH in response to GnRH.

Step 3: Ovary

• FSH and LH stimulate follicular growth.

• Granulosa cells proliferate → estrogen increases.

• If progesterone is low, high estrogen activates the surge center → LH surge.

Step 4: Ovulation

• LH surge triggers:

  • Rupture of the dominant follicle

  • Release of the cumulus–oocyte complex (COC)

  • Remaining cells differentiate into the corpus luteum (CL)

Hormonal regulation

Hormonal Dependency Across Follicular Stages

A. Recruited Follicles

• Highly dependent on FSH

• FSH stimulates:

  • Granulosa cell mitosis

  • Aromatase expression

  • Early estrogen production

• LH is less important but still needed for testosterone production.

B. Selected Follicles

• Begin shifting from FSH‑dependent → LH‑dependent

• Why?

  • FSH increases LH receptors on granulosa cells.

• Estrogen production increases.

• Inhibin begins to rise → suppresses FSH.

C. Dominant Follicle

• Strongly LH‑dependent

• Minimal reliance on FSH.

• Produces:

  • Very high estrogen

  • Very high inhibin

• Inhibin suppresses FSH → prevents new follicles from rising.

 Application: Superstimulation / Superovulation

If you want to increase the number of oocytes a prized animal produces:

Use exogenous FSH

• Why?

  • FSH keeps recruited follicles alive.

  • Prevents atresia.

  • Allows more follicles to continue through selection and dominance.

• This is the basis of superstimulation protocols used in:

  • Cattle

  • Sheep

  • Goats

  • Embryo transfer programs

Why not use LH?

• LH targets later‑stage follicles.

• It does not rescue early recruited follicles.

• To increase oocyte number, you must support the recruited pool, not the dominant follicle.

 

 Transition to Ovulation

• The pre‑ovulatory (Graafian) follicle contains:

  • Cumulus–oocyte complex

  • Mural granulosa cells

  • Theca interna + externa

• LH surge triggers:

  • COC expulsion

  • Collapse of remaining follicular cells

  • Luteinization → formation of the corpus luteum

Question

What would you do if you were a producer and you wanted to increase the number of ova produced by your prize cow?

 Ovulation Requires an LH Surge (Across All Mammals)

• All mammalian species—spontaneous ovulators (humans, cows, mares) and induced ovulators (cats, rabbits)—require a luteinizing hormone (LH) surge for ovulation.

• LH surge = universal trigger for follicular rupture.

What about FSH?

• FSH also increases during the surge.

• Its exact role in ovulation is not fully understood.

• It may help prime small follicles in earlier waves.

• But LH is the true driver of ovulation.

Ovulation = Pressure + Wall Weakening

Your professor’s analogy:

Ovulation is like popping a pimple.

You need:

1. Pressure pushing outward

2. Weakening of the follicular wall

The LH surge initiates both processes.

Mechanism 1: Increasing Pressure on the Follicle

A. Prostaglandin E2 (PGE2)

• LH surge → ↑ PGE2 (from the ovary).

• PGE2 is a vasodilator.

• Effects:

  • ↑ blood flow to the ovary

  • ↑ fluid accumulation

  • Edema forms around the follicle

  • Pressure increases on the follicular wall

This swelling helps “push” the follicle toward rupture.

 

B. Prostaglandin F2α (PGF2α)

• LH surge → ↑ ovarian PGF2α (not uterine).

• PGF2α causes smooth muscle contractions in the ovary.

• These contractions:

  • Squeeze the follicle

  • Add mechanical pressure

  • Help force the oocyte outward

PGF2α also has a second role (see below).

 Mechanism 2: Weakening the Follicular Wall

A. PGF2α → Lysosomal Enzyme Release

• PGF2α triggers release of lysosomal enzymes from:

  • Theca cells

  • Granulosa cells

• These enzymes:

  • Digest components of the follicular wall

  • Break down cellular junctions

  • Soften the tissue

This makes the follicle easier to rupture under pressure.

 

B. LH Surge → ↑ Progesterone → ↑ Collagenase

• LH surge stimulates granulosa + theca cells to produce progesterone.

• Progesterone increases collagenase activity.

• Collagenase:

  • Breaks down collagen in the extracellular matrix

  • Weakens the follicular wall

  • Helps create the rupture point

Ovulation Resembles an Inflammatory Response

Ovulation shares features with inflammation:

• Increased blood flow

• Edema

• Tissue breakdown

• Enzyme release

• Structural remodeling

This is why the ovary becomes red, swollen, and structurally altered at the ovulation site.

 

 LH Surge Effects on the Oocyte

The LH surge also acts directly on the cumulus–oocyte complex (COC):

• Disconnects cumulus granulosa cells from the oocyte.

• Removes meiotic inhibition.

• Allows the oocyte to transition:

  • Primary oocyte → Secondary oocyte (completes meiosis I)

• If fertilization occurs:

  • Secondary oocyte completes meiosis II → haploid ovum

This prepares the oocyte for fertilization.

 

The Physical Act of Ovulation

• The follicle ruptures at the weakened spot.

• Follicular fluid + COC are expelled into the peritoneal cavity.

• The fimbriae of the oviduct sweep the COC into the oviduct.

A 2008 surgical observation captured this process in real time:

• The “egg” seen was actually follicular fluid, not the oocyte (too small to see with the naked eye).

 

After Ovulation: Luteinization

• Remaining granulosa + theca cells collapse inward.

• They undergo terminal differentiation.

• This forms the corpus luteum (CL).

• CL produces progesterone, which:

  • Supports pregnancy

  • Suppresses new LH surges

  • Prevents additional ovulations

A preovulatory follicle has several key structural components:

Cell Layers & Their Functions

Structure	Color (from lecture)	Function
Theca cells	Light green	Produce testosterone (precursor for estrogen).
Granulosa cells (mural)	Natural/regular green	Convert testosterone → estrogen.
Cumulus granulosa cells	Red	Surround the oocyte; form the cumulus–oocyte complex (COC).
Oocyte	Center	The egg cell that will be ovulated.

Ovulation

• Stimuli

  • LH surge

  • FSH

• Specific cascade of events

  • Degradation of follicle wall

  • Extrusion of oocyte

    • Resumption of meiosis

  • Collapse of the follicle

LH Surge

 Hormonal Conditions Leading to Ovulation

• High estrogen + decreasing progesterone → positive feedback on the surge center.

• This triggers the LH surge.

• LH surge = the key signal that initiates follicular rupture.

 Events Required for Ovulation

To ovulate, the follicle must:

A. Increase pressure inside the follicle

• Fluid accumulation in the antrum.

• Swelling pushes outward on the follicular wall.

B. Weaken the follicular wall

• Enzymatic digestion.

• Breakdown of connective tissue.

• Lysosomal enzymes from theca + granulosa cells help digest the wall.

Outcome

→ Rupture of the follicle

→ Expulsion of the cumulus–oocyte complex (COC)

 After Ovulation: What Happens to the Follicle?

The remaining cells (theca + granulosa + some cumulus cells) collapse inward.

This process forms the corpus luteum, but it happens in stages:

 

 Stage 1: Corpus Hemorrhagicum (CH)

• Immediately after rupture, blood vessels tear.

• Creates a wound-like structure filled with blood.

• Appears as a red body protruding from the ovary.

• Duration: hours to 1–2 days, depending on species.

Luteal Phase

• Corpora lutea formation

  • Luteinization

• Progesterone production

• Longer phase

  • 66-75% of cycle

• Ends with luteolysis

• Work with uterus to aid with embryo

• Luteolysis= CL dies and decrease in P4

Luteinization

• Process that transforms the granulosa and theca cells in luteal cells. Undergo terminal differtiation → into luteal cells

• Triggered by the surge of LH

• Causes profound changes in the follicles that become corpora lutea: Disruption the tissue of ovary is damaged.

Definition:

The process where theca + granulosa cells undergo terminal differentiation into luteal cells.

Key points:

• Triggered by the LH surge.

• Terminal differentiation = once they become luteal cells, they cannot revert.

• Cells reorganize, blood vessels regrow, and the structure becomes the corpus luteum (CL).

The CL

A fully formed CL is:

A heterogeneous tissue containing:

• Large luteal cells

• Small luteal cells

• Endothelial cells (forming a dense capillary network)

• Fibroblasts

• Mast cells

• Immune cells

All of these contribute to CL function and maintenance.

Pre-ovulatory Follicle- → Ovulatory → CL

🔬 6. Origin of Large vs. Small Luteal Cells

Based on a classic (and rarely replicated) antibody‑labeling study:

Luteal Cell Type	Origin
Small luteal cells	Theca cells
Large luteal cells	Granulosa cells

This remains the accepted dogma, though the original study had limitations and has not been repeated due to difficulty.

 

🌙 7. Function of the Corpus Luteum

The primary function of the CL:

→ Produce progesterone

Progesterone:

• Maintains pregnancy (if it occurs)

• Suppresses LH/FSH

• Prevents new follicular waves from ovulating

• Prepares the uterus

 

📈 8. The Luteal Phase in the Estrous Cycle

The luteal phase begins at metestrus, right after ovulation.

Key features:

• Progesterone rises as the CL forms.

• The process of CL formation = luteinization.

• The longest phase of the estrous cycle:

  • 66–75% of the cycle.

• CL remains until it receives the signal for luteolysis (death of the CL).

• After luteolysis:

  • Progesterone drops sharply.

  • The cycle transitions back toward the follicular phase.