Fertilization & Early Development

Embryo

From fertilization→ week 8

Fetus

Week 9→ birth

Gestation

40 weeks from last menstrual period (LMP) — the woman is “2 weeks pregnant” at the moment of fertilization

How far along sperm can make it to uterine tube

• Ejaculate: begins with 300 million sperm

• Vagina: Millions lost due to leakage + acidic pH of 3.5-4

• Cervical canal (mucus barrier): small fraction pass

• Uterus: Thousands remain due to phagocytes destroying millions

• Ampulla of uterine tube: a few hundred left

Sperm viability in female tract

up to  5 days (fertilizing capacity declines over time)

Oocyte viability after ovulation

12-24 hours

How does sperm know where to swim to

Sperm follow chemical signals (chemotaxis via olfactory-like receptors) to the oocyte in the uterine tube

Capacitation

• A further maturation process sperm must undergo in the female reproductive tract before they can fertilize an oocyte.

• NOT completed in the epididymis — requires secretions of the female tract.

What happens during capacitation

Removal of cholesterol and proteins from sperm plasma membrane

Results of capacitation

• Result 1: enhanced/hyperactive motility (increased flagellar beat)

• Result 2: plasma membrane becomes more fragile→ facilitates the acrosome reaction

Inhibitor of capacitation

• Seminal fluid contains capacitation-inhibiting factors → capacitation begins as sperm are separated from seminal fluid

IVF relevance to capacitation

Capacitation induced artificially by washing sperm (removes seminal fluid) or Percoll gradient

Why capacitation exists

Prevents premature acrosome reaction; ensures sperm become fully activated only near the oocyte; provides a time delay that coordinates with oocyte availability

3 main barriers around oocyte

• The oocyte in the ampulla is surrounded by three barriers (outermost → innermost):

  • Corona radiata — layer of granulosa cells (from the follicle)

  • Zona pellucida — thick glycoprotein coat (formed at primary follicle stage)

  • Oocyte plasma membrane

5 steps of sperm penetration

1. Approach

   • Sperm weaves through corona radiata using surface enzymes on its head

     to separate granulosa cells

2. Acrosomal approach

   • Sperm head binds to sperm-binding receptors in the zona pellucida →

     Ca²⁺ rise within sperm → triggers release of acrosomal enzymes (hydrolases)

3. Zona digestion

   • Acrosomal enzymes from many sperm digest holes through the zona

     pellucida — this is a collective effort

4. Binding

   • First sperm to reach the oocyte plasma membrane binds to receptors and

     triggers fusion

5. Fusion

   • Sperm and oocyte plasma membranes fuse; sperm contents (nucleus,

     centriole, mitochondria) enter oocyte

Why does acrosome reaction occur only after sperm binds to zone pellucida

• prevents premature enzyme release

• This is why capacitation makes membranes fragile but doesn’t trigger the acrosome reaction prematurely

• Many sperm sacrifice their acrosomal enzymes to clear a path — only one enters

Polyspermy

• Penetration of the oocyte by more than one sperm → produces a cell with too many chromosomes → lethal.

• Only monospermy (one sperm) results in successful development

2 blocks to polyspermy

• Fast block

• Slow block (cortical reaction)

Fast block

• Occurs within seconds

• triggered by sperm-oocyte membrane fusion

• occurs vis oocyte rapidly shedding all remaining sperm-binding receptors from its plasma membrane

• results in no additional sperm being able to bind to/fuse with oocyte membrane

• Analogy→ removes the door handle

Slow block (cortical reaction)

• Occurs within a few minutes

• triggered Ca2+ surge(wave released from oocyte ER/cortical granules)

• Occurs via Ca2+ triggering exocytosis of cortical granules→ released enzymes cause the zona reaction

• Results in zona pellucida sperm receptors destroyed +zona hardened (glycoprotein cross-linking)→ impenetrable

• Analogy→ seals the door shut

Additional efect of Ca2+ surge

The same Ca²⁺ surge that triggers the cortical reaction also triggers completion of meiosis II:

7 steps of meiosis II

1. Secondary oocyte (arrested in metaphase II since ovulation) completes meiosis II → ovum

   + 2nd polar body

2. Ovum nucleus swells→ female pronucleus

3. Sperm nucleus swells→ male pronucleus

4. Both pronuclei replicate their DNA

5. Pronuclei approach each other→ nuclear envelopes dissolve

6. Chromosomes intermingle on a mitotic spindle→ all 46 chromosomes together= true fertilization

7. Results in zygote: first cell of a new organism

Fertilization is a process, not a moment. The zygote doesn’t exist until chromosomes from both pronuclei are combined.

Zygote

The single diploid cell formed by the union of sperm and oocyte pronuclei — the first cell of the new organism. Contains 46 chromosomes (23 maternal + 23 paternal).

Cleavage

• Period of rapid mitotic divisions of the zygote without intervening growth

• cells get smaller with each division.

Location of cleavage

Occurs while zygote travels down the uterine tube toward the uterus

Transport mechanism of cleavage

peristalsis + cilia of the uterine tube

Purpose of cleavage

Produce many smaller cells (blastomeres) rapidly → high surface-to-volume ratio → enhances nutrient/O₂ uptake

1st cleavage

36 hours after fertilization → 2 blastomeres

progression of cleavage

2 → 4 → 8 → 16+ cells

Stages of fertilization→ implantation

• Fertilization→ zygote

  • occurs on Day 0

  • location: in ampulla of uterine tube

  • pronuclei fuses

• 2-cell??

  • occurs 36 hours after fertilization

  • location: in uterine tube

  • first cleavage occurs

• Morula (16+ cells)

  • occurs on day 3 (72 hours)

  • location: uterine tube

  • is a berry-shaped cluster of 16+ cells ; still in zona pellucida

• Blastocyst (100+ cells)

  • occurs on days 4-5

  • location: uterine cavity

  • is a hollow shaped sphere with fluid; hatching from zona

  • formed by morula hollowing out and filling with fluid

• Implanting blastocyst

  • occurs on days 6-7

  • location in uterine wall (endometrium)

  • trophoblast adheres to endometrium

• Implantation complete

  • occurs on day 12

  • location: embedded in endometrium

  • sealed within endometrial tissue

2 types of blastocysts

1. Trophoblast

   • single outer layer of large, flat cells

   • Fate→ Chorion (embryo’s part of the placenta); produces immunosuppressive factors

2. Embryoblast (inner cell mass)

   • Small cluster of 20-30 rounded cells

   • Fate→ Embryo proper + three extraembryonic membranes (amnion,

     yolk sac, allantois)

Outer cells become the interface with the mother; inner cells become the baby

Hatching

• Zona pellucida breaks down → blastocyst “hatches” out

• Significance: Blastocyst is now free to interact with and implant into the endometrium

• IVF note: Assisted hatching (laser thinning of zona) can improve implantation rates

Timing of Implantation

Begins day 6-7 after fertilization (day 20-22 of menstrual cycle — the implantation window of the secretory phase)

Site of implantation

High in the uterus; trophoblast over the embryoblast adheres to endometrial epithelium

Preimplantation

Blastocyst floats 2 days in uterine cavity, nourished by glycoprotein-rich endometrial secretions (“uterine milk”)

Trophoblast differentiation during implantation

• Cytotrophoblast

  • inner layer

  • role= layer of distinct cells

• Syncytiotrophoblast

  • outer layer

  • role= cells fuse into multinuclear mass (syncytium); send protrusions that invade and digest endometrial cells

Blastocyst burrows deeper; endometrial cells proliferate and cover it, sealing it off

By day 12: implantation complete

hCG rescue

• Source: Syncytiotrophoblast

• Hormone: hCG (human chorionic gonadotropin)

• Action: Acts like LH → maintains corpus luteum → continues progesterone + estrogen production → prevents menstruation

• Detection: Detectable in blood  2 days after implantation (day 9 after ovulation) — basis of pregnancy tests

• Peak: 8 weeks of pregnancy

• Handoff: By end of month 3, placenta takes over hormone production → corpus luteum degenerates

Failure rates

• 2/3 of all zygotes fail to implant or spontaneously abort.

• Additional embryos miscarry due to genetic defects or uterine malformations.

Ectopic pregnancy

Implantation outside the uterus, most commonly in the uterine tube — a medical emergency.

After implantation

Embryoblast → two-layered embryonic disc (epiblast + hypoblast)

3 extraembryonic membranes

• Amnion: fluid-filled sac (buoyancy, protection)

• Yolk sac: Earliest blood cells; primordial germ cells originate here

• Chorion: outermost membrane; forms placenta

Gastrulation (week 3)

Transforms 2-layer disc into 3-layer embryo:

• Primitive streak forms → establishes body axes

• Epiblast cells migrate inward → three germ layers

3 germ layers

• Ectoderm

  • position: surface

  • major derivatives: nervous ystem, epidermis

• Mesoderm

  • position: middle

  • derivatives: muscle, bone, blood cardiovascular, urogenital, connective tissues

• Endoderm

  • Position: deepest

  • major derivatives: epithial linings of GI tract, resp. tract; glands (liver, pancreas, thyroid)

By end of week 8: all organ systems recognizable → now called a fetus