SECTION 05: FERTILIZATION, PREGNANCY, PARTURITION, AND LACTATION

By the end of Section 05, you should be able to:

• Describe the processes by which male and female gametes unite.

• Compare and contrast the roles of the corpus luteum and the placenta in secreting hormones to support the fetus.

• Describe the process of parturition with an emphasis on the positive feedback loop.

• Describe the process of lactation with an emphasis on its hormonal control

🌟 SIMPLE OVERVIEW:

Fertilization is when sperm and egg meet and form a zygote. This usually happens in the ampulla of the fallopian tube. After fertilization, the zygote becomes a morula, then a blastocyst, and eventually implants in the uterus.

– STAGES OF FERTILIZATION & IMPLANTATION - 📍 Step 1: Ovum Transport

Ovum (egg) is released from the ovary at ovulation

Fimbriae sweep the egg into the fallopian tube

Peristalsis (smooth muscle waves) moves it to the ampulla

Flashcard:

🥚 Ovum released → Fimbriae → Ampulla via peristalsis

STAGES OF FERTILIZATION & IMPLANTATION 📍 Step 2: Sperm Transport

Sperm deposited in vagina

Must pass cervical mucus (thin only around ovulation due to high estrogen)

Reach uterus → myometrial contractions push them toward oviducts

Reach ampulla ~30 minutes after ejaculation

Flashcard:

🧬 Sperm swim + uterus helps push → Reach egg in ampulla

STAGES OF FERTILIZATION & IMPLANTATION

📍 Step 3: Fertilization

Sperm passes corona radiata (outer follicle layer) using enzymes

Binds to ZP3 on zona pellucida

Triggers acrosome reaction → enzymes released to digest through

First sperm fuses with egg, enters, triggers Ca²⁺ release

Flashcard:

🧠 Key proteins:

Fertilin (sperm) binds to ZP3 (egg)

Acrosome enzymes digest the zona

Ca²⁺ release = blocks other sperm + finishes meiosis

📍 Result of Ca²⁺ Release in Egg

📍 Step 4: Implantation

Zygote divides → becomes morula (solid ball)

Morula → blastocyst (hollow ball with inner cell mass)

Moves to uterus (helped by progesterone)

Blastocyst floats for a few days, nourished by endometrial secretions

Then implants into uterine lining → fully buries itself

Flashcard:

🍼 Morula → Blastocyst → Uterus → Implants in endometrium

✅ Summary Flow (One-Liner Memory Aid):

Ovary → Ampulla → Fertilization → Zygote → Morula → Blastocyst → Uterus → Implantation

how sperm meets the egg and fertilization occurs 🧬 SIMPLE OVERVIEW:

Fertilization is the epic journey where sperm travels through the female reproductive tract to reach and fuse with the egg in the ampulla of the fallopian tube. From 300 million sperm, usually only one wins — forming a zygote, the very first cell of a new human life.

HOW SPERM MEETS THE EGG (9 steps) - 📍 Step 1: Sperm Enters the Vagina

~300 million sperm are ejaculated

Many die in the acidic vaginal environment

Seminal fluid helps protect some sperm

Flashcard:

✅ 300 million sperm enter → many die in acid → few survive

📍 Step 2: Cervix Opens (During Ovulation)

Cervical mucus becomes thin from high estrogen

Sperm swim through; some get stuck, others go on

Flashcard:

✅ Cervical mucus thins at ovulation → sperm can pass

📍 Step 3: Journey Through the Uterus

Uterine contractions help push sperm toward the fallopian tubes

Immune cells kill many sperm

Flashcard:

✅ Uterus helps move sperm → immune system attacks

📍 Step 4: Fallopian Tube & Cilia

Half of sperm go to the wrong side

The rest go toward the egg

Cilia push egg toward uterus, sperm swim against it

Sperm become hyperactive due to chemical changes

Flashcard:

✅ Sperm swim upstream → hyperactive from chemical triggers

📍 Step 5: Reaching the Egg

Only a few dozen sperm make it to the egg

Egg is surrounded by:

• Corona Radiata (outer cell layer)

• Zona Pellucida (protective glycoprotein shell)

Flashcard:

✅ Few sperm reach egg → must get through 2 layers

📍 Step 6: Penetration & Fusion

Sperm bind to ZP3 receptors on zona pellucida

Triggers acrosome reaction → releases enzymes

Sperm burrow into the egg

Flashcard:

✅ Sperm binds to ZP3 → acrosome enzymes digest entry path

📍 Step 7: Zinc Spark + Egg Reaction

First sperm to enter triggers:

• Zinc spark (new discovery!)

• Ca²⁺ release

• Zona hardens to block other sperm

• Sperm tail left outside

• Egg completes meiosis II

Flashcard:

✅ 1 sperm fuses → sparks, hardening, meiosis II finishes

📍 Step 8: Fertilization & Zygote Formation

Male and female DNA form pronuclei (each with 23 chromosomes)

Microtubules pull them together

Chromosomes merge → zygote formed

Unique genetic code is created (eye color, sex, etc.)

Flashcard:

✅ Pronuclei merge → new life starts = zygote

📍 Step 9: Zygote Moves to Uterus

Cilia sweep the zygote to the uterus

It will implant into the endometrial lining

Will grow for the next 9 months if conditions are right

Flashcard:

✅ Zygote → uterus → implants → pregnancy begins

✨ Bonus: Zinc Sparks (from video)

Zinc released by egg during fertilization

Visible as "sparks" using special photography

May help doctors choose healthy embryos during IVF

Flashcard:

✅ Zinc spark = sign of healthy egg activation 🧬⚡

Activity

🌟 SIMPLE EXPLANATION:

• Identical twins = 1 egg + 1 sperm → splits into 2 babies

• Fraternal twins = 2 eggs + 2 sperm → 2 babies at the same time

📍 What are Identical (Monozygotic) Twins?

1 egg is fertilized by 1 sperm

The single zygote splits into two embryos

They share 100% identical DNA

Usually share a single placenta

Flashcard:

👯‍♀ Monozygotic = One egg splits → identical twins (same DNA + usually shared placenta)

📍 What are Fraternal (Dizygotic) Twins?

2 eggs released and fertilized by 2 separate sperm

Develop as two separate zygotes

Genetically like siblings (share ~50% DNA)

Always have separate placentas

Flashcard:

👫 Dizygotic = Two eggs, two sperm → fraternal twins (different DNA + separate placentas)

📍 Key Differences at a Glance:

🧠 Quick Memory Tip:

“Mono” = One egg → Mirrored twins
“Di” = Two eggs → Different twins

🌟 SIMPLE OVERVIEW:

• 

• In a normal pregnancy, the fertilized egg implants in the uterine lining.

• In an ectopic pregnancy, the egg implants outside the uterus, most commonly in the fallopian tube.

• This can be dangerous and requires urgent treatment to avoid serious health risks.

📍 What is a normal pregnancy?

Fertilized egg implants in the uterus
➤ Safe space for embryo growth

Flashcard:

✅ Normal = Uterus implantation → safe for development

📍 What is an ectopic pregnancy?

Fertilized egg implants outside uterus
Most commonly in the fallopian tube (tubal)
Can also implant in the ovary, cervix, or abdomen

Flashcard:

❌ Ectopic = Wrong place (fallopian tube, ovary, cervix)

📍 Why is a tubal pregnancy dangerous?

The fallopian tube is too narrow and thin-walled

Pregnancy only needs to grow to jellybean size to rupture the tube

This can cause severe internal bleeding

Flashcard:

⚠ Tubal pregnancy can burst tube → major bleeding

📍 How common is ectopic pregnancy?

~1 in 60 pregnancies
Rare, but serious

Flashcard:

📊 Ectopic = ~1 in 60 pregnancies

📍 Risk Factors? - ectopic pregnancies

📍 Treatment?

Must be treated medically or surgically

Prevents life-threatening bleeding

Flashcard:

🚑 Treat fast! Ectopic = medical emergency

placenta development and function 🌟 SIMPLE OVERVIEW:

The placenta is an organ that forms during pregnancy to support the growing baby. It connects the baby to the uterus via the umbilical cord and acts like the baby's lungs, kidneys, and digestive system — all in one!

PLACENTA DEVELOPMENT TIMELINE - 📍 4 Weeks

By day 12, the embryo is fully embedded in the uterine lining

The surrounding endometrial tissue begins forming the placenta

Flashcard:

🪴 Week 4 = Implantation complete → placenta starts forming

PLACENTA DEVELOPMENT TIMELINE 📍 8 Weeks

Placenta is not fully formed yet, but is already working

Begins exchanging nutrients and waste

Flashcard:

🔄 Week 8 = Placenta is functional, even if not fully built

PLACENTA DEVELOPMENT TIMELINE 📍 12 Weeks

Maternal blood supply to placenta is now complete

Placenta has all structures needed to support fetus until birth

Flashcard:

✅ Week 12 = Placenta fully developed & fully connected to mom’s blood

PLACENTA DEVELOPMENT TIMELINE 📍 40 Weeks

Placenta continues to grow and function throughout pregnancy

Acts as:

• Lungs (gas exchange)

• Digestive system (nutrients)

• Kidneys (waste removal)

Umbilical cord connects baby to placenta

Flashcard:

🧠 Week 40 = Placenta = baby’s life support (lungs + kidneys + food source)

📍 EXTRA: Placenta Functions

📍 Progesterone

• Also rises steadily during pregnancy

• Maintains endometrium

• Suppresses contractions (prevents early labour)

Flashcard:

🛡 Progesterone = Stabilizes uterus lining + stops early contractions

hCG (Human Chorionic Gonadotropin) 📍 Source

First hormone secreted by the developing placenta
Starts early in pregnancy

Flashcard:

🧬 hCG = First hormone from placenta

hCG (Human Chorionic Gonadotropin) 📍 Function

Mimics LH → binds to LH receptors
Maintains corpus luteum → keeps producing estrogen + progesterone
Corpus luteum becomes “corpus luteum of pregnancy”

Flashcard:

🛡 hCG = Supports corpus luteum → maintains hormone levels

hCG (Human Chorionic Gonadotropin) 📍 Timeline

Peaks early (first 10 weeks)

Drops to low, steady level once placenta takes over hormone production

Corpus luteum stays until birth

Flashcard:

📈 Peaks early → 📉 lowers after week 10

hCG- 📍 Other Roles

Stimulates Leydig cells → testosterone in male fetus

Needed for male reproductive development

May be linked to morning sickness

Flashcard:

👶 hCG = Triggers testosterone in male fetus
🤢 Linked to morning sickness (unclear role)

hCG - 📍 Clinical Use

Detected in urine before embryo seen

Basis of pregnancy tests

Flashcard:

✔ hCG = Detected in pregnancy tests

📍 Can the Placenta Make Estrogen from Cholesterol?

❌ No. The placenta can’t make estrogen from cholesterol directly
It needs DHEA (a hormone made by the fetal adrenal cortex)

Flashcard:

🚫 Placenta can’t make estrogen from cholesterol → needs fetal DHEA

📍 Where Does Estrogen Come From?

1. Cholesterol from maternal + fetal blood

2. Cholesterol → converted into DHEA by fetal adrenal cortex

3. DHEA enters fetal blood → then goes to placenta

4. Placenta converts DHEA → estrogen (estriol) → releases into maternal blood

Flashcard:

🛤 Cholesterol → DHEA (fetus) → Estriol (placenta) → Mom’s blood

📍 Main Type of Estrogen from Placenta?

Estriol
(vs. estradiol from ovaries)

Flashcard:

🧬 Placenta = estriol, Ovaries = estradiol

📍 Why Is Estriol Important?

Used as a marker for fetal health

Indicates proper DHEA production (fetal adrenal health)

Flashcard:

🩺 Estriol = Fetal viability check marker

📍 Functions of Estrogen (Estriol) in Pregnancy

Increases myometrium growth (uterus muscle mass)

Prepares uterus for labour

Promotes mammary duct development

Flashcard:

💪 Builds uterus + 🍼 Preps breast ducts for milk + 🚼 Preps for labour

📍 Source of Progesterone Early in Pregnancy

Corpus luteum produces it first

Placenta takes over by ~10 weeks

Flashcard:

🟡 Week 1–10 = Corpus luteum
🔵 Week 10+ = Placenta makes enough progesterone

📍 How is Placental Progesterone Made?

Cholesterol from maternal blood enters placenta

Placenta converts cholesterol into progesterone

Progesterone is secreted into maternal blood

Flashcard:

🛤 Cholesterol → Progesterone (in placenta) → into mom’s blood

📍 Does Placenta Need Help Making Progesterone?

❌ No. Placenta can make progesterone on its own
Unlike estrogen, it doesn’t need fetal precursors

Flashcard:

✅ Placenta = independent for progesterone
❌ Needs fetal DHEA for estrogen

📍 Functions of Progesterone in Pregnancy

Maintains endometrium

Keeps cervical mucus plug in place

Prepares milk glands

Inhibits uterine contractions (prevents early labour)

Flashcard:

🛡 Progesterone = Protects pregnancy (lining, mucus, milk, stops contractions)

Estrogen & Uterine Contractions (Parturition) 📍 What triggers labor prep at the end of pregnancy?

A surge in estrogen near the end of gestation

Flashcard:

📈 End of pregnancy = ↑ Estrogen surge

estrogen’s role in parturition (childbirth) (2 roles) - 📍 Role 1: Gap Junction Formation

Estrogen increases gap junctions between myometrial cells

This allows the uterus to contract as one unit

Flashcard:

🔗 Estrogen = ↑ gap junctions → uterus contracts together

📍 Role 2: Oxytocin Receptor Increase

Estrogen increases oxytocin receptors in the myometrium

Makes the uterus more sensitive to oxytocin, even in low amounts

Flashcard:

🎯 Estrogen = ↑ oxytocin receptors → uterus responds better to oxytocin

estrogen’s role in parturition (childbirth) - final result

Coordinated uterine contractions that respond strongly to oxytocin
→ Prepares uterus for labour

Flashcard:

🤰 Estrogen = primes uterus for strong, coordinated labour contractions

Positive Feedback Loop in Labour (Parturition) - 📍 What starts the loop?

Estrogen ↑ → uterus becomes more sensitive to oxytocin
Oxytocin (from posterior pituitary) triggers uterine contractions

Flashcard:

🟢 Estrogen ↑ → uterus sensitive to oxytocin → 🔁 contractions start

Positive Feedback Loop in Labour (Parturition) 📍 Step-by-Step Cycle:

1. ↑ Uterine Contractions

2. Fetus pushed against cervix

3. Neuroendocrine reflex triggered

4. ↑ Oxytocin secretion

5. ↑ Prostaglandin production

6. More prostaglandins = stronger uterine contractions

Flashcard:

👶 Pressure on cervix → reflex → ↑ oxytocin → ↑ prostaglandins → ↑ contractions

📍 What kind of feedback loop?

Positive feedback loop
Keeps going until the baby is born and pressure is gone

Flashcard:

🔁 Positive feedback = keeps increasing until birth

Positive Feedback Loop in Labour (Parturition) 📍 When does it stop?

After delivery, cervical pressure is gone → oxytocin returns to normal

Flashcard:

🛑 Birth ends the loop → oxytocin levels ↓

📍 What is the role of positive feedback in parturition?

It ensures a continuous and increasing cycle of contractions by raising oxytocin levels.

Flashcard:

🔁 Positive feedback = ↑ oxytocin → ↑ contractions → ↑ cervix pressure → ↑ oxytocin

Why Positive Feedback Is Necessary for Birth

Strong uterine contractions are needed to push the baby out

Cervical pressure triggers even more oxytocin → stronger contractions

This loop continues until the baby is delivered

Flashcard:

🤰 No feedback = weak contractions → cervix won’t open → baby can’t be delivered

📍 What happens if positive feedback didn’t happen?

Oxytocin would not increase

Contractions would stay weak or stop

Cervix would not dilate

Labour would stall → no birth

Flashcard:

❌ No feedback = no progress → labour fails

📍 What ends the feedback loop?

Once the baby is born, the cervix is no longer stretched, so oxytocin levels fall

Flashcard:

🛑 Birth = cervix pressure gone → oxytocin drops → feedback ends

📍 What are mammary glands made of?

Ducts: carry milk to the nipple

Lobules: clusters of alveoli that produce milk

Alveoli: milk-secreting units lined by epithelial cells

Myoepithelial cells: surround alveoli and contract to eject milk

📍 Where is milk made?

In alveolar epithelial cells
→ milk is secreted into the lumen of alveoli
→ moves through ducts to the nipple

📍 What hormone builds milk ducts?

Estrogen
→ Develops the milk duct system

📍 What hormone forms alveoli?

Progesterone
→ Promotes growth of alveoli (milk sacs)

📍 What stimulates milk production enzymes?

Prolactin

hCG
→ Together, they prep the breast to make milk

📍 Why is milk not secreted before birth?

High estrogen and progesterone block prolactin
→ After parturition, these drop
→ Prolactin is now free to trigger milk production

📍 BONUS – Milk ejection (let-down reflex)

Caused by oxytocin triggering myoepithelial cell contraction

📍 What triggers milk production and ejection?

Suckling → stimulates mechanoreceptors in nipple → signals sent to hypothalamus

STIMULATION OF LACTATION 📍 What two hormones are involved?

Oxytocin (from posterior pituitary)

Prolactin (from anterior pituitary)

📍 Oxytocin – What does it do?

Released from posterior pituitary

Causes contraction of myoepithelial cells

➝ Ejects milk from alveoli

🔄 Can also be triggered by baby crying

📍 Prolactin – What does it do?

Released from anterior pituitary

Stimulates milk production in alveolar epithelial cells

📍 What regulates prolactin?

↓ PIH (Prolactin Inhibiting Hormone = dopamine)

↑ PRH (Prolactin Releasing Hormone = possibly oxytocin)

📍 Key concept from the diagram:

Milk ejection = oxytocin

Milk production = prolactin

Milk must be ejected actively – baby can’t suck it out from alveoli alone

📍 What regulates prolactin? (further explanation)

🔽 PIH = Prolactin Inhibiting Hormone

• Real name: Dopamine

• Job: Tells the anterior pituitary to NOT release prolactin

• Think of it like a brake pedal. When dopamine is high → no prolactin is made.

🔼 PRH = Prolactin Releasing Hormone

• This one encourages prolactin release

• It's less clearly defined, but is suspected to be oxytocin acting within the hypothalamus (⚠ not the same oxytocin that causes milk ejection — just similar).

• Think of it like the gas pedal. When PRH goes up → prolactin gets released.

🧠 How this works during breastfeeding:

1. Baby suckles → sends signal to hypothalamus

2. Hypothalamus ↓ dopamine (PIH) and ↑ PRH

3. This tells anterior pituitary to release prolactin

4. Prolactin → makes more milk 🍼

Module 2 summary and learning outcomes