1. Fertilization
Definition:
Fertilization is the process where a sperm fuses with an ovum to form a zygote, marking the beginning of human development.
Site of Fertilization:
- Occurs in the ampulla of the fallopian tube (widest part).
Steps of Fertilization:
- Sperm Capacitation:
- Sperm undergoes biochemical changes in the female reproductive tract, enhancing its motility & ability to penetrate the ovum.
- Acrosome Reaction:
- The sperm releases enzymes (hyaluronidase & acrosin) to penetrate the zona pellucida.
- Sperm-Oocyte Fusion:
- One sperm enters the ovum, triggering a cortical reaction to prevent polyspermy (entry of multiple sperm).
- Completion of Meiosis II in the Ovum:
- The secondary oocyte completes meiosis II, forming the female pronucleus.
- Zygote Formation:
- The male & female pronuclei fuse, forming a diploid (46 chromosomes) zygote.
2. Early Embryonic Development
|
Day |
Event |
|
Day 1 (Fertilization) |
Zygote formation |
|
Day 2–3 |
Cleavage (Mitotic divisions) → Formation of
Blastomeres |
|
Day 4 |
Morula (16-cell stage) reaches the uterus |
|
Day 5–6 |
Blastocyst formation (Fluid-filled cavity develops) |
|
Day 6–7 |
Blastocyst hatches from zona pellucida |
|
Day 7–10 |
Implantation into the endometrium |
Key Structures in Blastocyst:
- Trophoblast: Outer layer → Forms placenta.
- Inner Cell Mass: Becomes embryo & fetal structures.
- Blastocoel: Fluid-filled cavity.
3. Implantation (Days 6–10)
Definition:
- The blastocyst attaches & invades the endometrium, establishing pregnancy.
Steps of Implantation:
- Apposition: Loose contact between the blastocyst & endometrium.
- Adhesion: Trophoblast cells firmly attach to the endometrium.
- Invasion:
- Trophoblast differentiates into:
- Syncytiotrophoblast (Secretes hCG to maintain corpus luteum).
- Cytotrophoblast (Forms future placenta).
Clinical Importance:
- Ectopic Pregnancy: If implantation occurs outside the uterus (e.g., fallopian tubes).
- Implantation Bleeding: Mild spotting around Day 10, often mistaken for a period.
4. Formation of Germ Layers (Gastrulation – Week 3)
Three Primary Germ Layers & Their Derivatives:
|
Germ Layer |
Derivatives |
|
Ectoderm |
CNS (Brain, Spinal cord), Skin, Hair, Nails, Sensory
organs |
|
Mesoderm |
Muscles, Bones, Heart, Kidneys, Gonads, Blood
vessels |
|
Endoderm |
GI Tract, Liver, Pancreas, Lungs, Thyroid, Bladder |
5. Placental Development (Weeks 2–12)
- Function: Exchange of nutrients, gases & hormones between mother & fetus.
- Hormones Secreted:
- hCG (Human Chorionic Gonadotropin) → Maintains corpus luteum.
- Progesterone & Estrogen → Maintain pregnancy.
Summary Table: Fertilization & Embryo Development
|
Stage |
Event |
|
Fertilization (Day 1) |
Sperm & ovum fuse in ampulla → Zygote formation |
|
Cleavage (Day 2–4) |
Rapid mitotic divisions → Morula formation |
|
Blastocyst (Day 5–6) |
Formation of trophoblast (placenta) & inner cell
mass (embryo) |
|
Implantation (Day 7–10) |
Blastocyst invades the endometrium |
|
Gastrulation (Week 3) |
Formation of three germ layers |
|
Placental Development |
Supports fetal growth & hormone production |
Key Takeaways
- Fertilization occurs in the ampulla of the fallopian tube.
- Blastocyst implants into the endometrium by Day 7–10.
- Three germ layers form all body structures during gastrulation.
- The placenta develops by 12 weeks to sustain pregnancy.
