Question
Describe the process of fertilisation and implantation in humans. What happens to the zygote between fertilisation and implantation?
Solution — Step by Step
Fertilisation happens in the ampullary region of the fallopian tube (oviduct), not in the uterus. The sperm that survives the journey from the vagina reaches here and penetrates the zona pellucida of the secondary oocyte. This triggers the oocyte to complete meiosis II, and the two nuclei fuse — forming the zygote.
The moment a sperm penetrates the egg, cortical granules release enzymes that harden the zona pellucida — this is called the zona reaction (or cortical reaction). This blocks all other sperm from entering. Without this block, polyspermy would result in an abnormal chromosome number.
The zygote starts dividing mitotically while still travelling through the fallopian tube toward the uterus. These divisions are called cleavage — the cells (blastomeres) divide but don’t increase in size. The overall embryo stays roughly the same size because it’s still enclosed in the zona pellucida.
The stages in order: Zygote → 2-cell → 4-cell → 8-cell → Morula (16-cell solid mass).
As the morula enters the uterus, fluid accumulates between cells, creating a fluid-filled cavity called the blastocoel. The structure is now called a blastocyst. Two distinct cell groups form:
- Trophoblast — outer layer, will form the placenta
- Inner cell mass (embryoblast) — will form the actual embryo
This is the stage that implants.
The blastocyst sheds the zona pellucida (“hatching”) and attaches to the endometrium of the uterus around 6–7 days post-fertilisation. The trophoblast cells invade the endometrial lining and embed the blastocyst. The endometrium thickens and vascularises — this is implantation.
Final answer: Fertilisation (ampulla of fallopian tube) → Zygote → Cleavage → Morula → Blastocyst → Implantation in endometrium (~day 7).
Why This Works
The sequence makes sense when you understand the purpose of each stage. Cleavage happens without growth because the embryo has no blood supply yet — it’s living off cytoplasmic stores from the egg. Keeping the same overall size lets it travel freely through the narrow fallopian tube.
The blastocyst must shed the zona pellucida before implantation because the trophoblast cells need direct contact with the endometrium. Think of the zona as a protective packaging that gets discarded once the embryo reaches its destination.
The trophoblast invades the endometrium aggressively — it actually digests maternal tissue to anchor itself and later form the placenta. This invasive behaviour is what makes the embryo-maternal connection so robust.
Alternative Method (Timeline Approach)
For NEET MCQs, it’s often faster to memorise the day-wise timeline:
| Day | Event |
|---|---|
| Day 0 | Fertilisation in ampullary region |
| Day 2–3 | 2-cell → morula, travelling through fallopian tube |
| Day 4–5 | Morula enters uterus |
| Day 5–6 | Morula → Blastocyst (blastocoel forms) |
| Day 6–7 | Zona shedding + Implantation begins |
NEET frequently asks “on which day does implantation occur” — the answer is 6th to 7th day. This timeline has appeared in NEET 2022 and NEET 2019.
Common Mistake
Students often write that fertilisation occurs in the uterus. It does NOT. Fertilisation happens in the ampullary region of the fallopian tube. The zygote then travels to the uterus over several days. Writing “uterus” for the fertilisation site is a direct 1-mark loss in NEET and board exams.
Remember the zona reaction by linking it to “zona = zone of protection.” Once one sperm is in, the zone hardens to keep others out. Also, “trophoblast = tropho (nourishment) + blast (bud)” — it nourishes the embryo by forming the placenta. Etymology makes these terms stick.