Question
Describe the process of fertilisation and implantation in humans. Where does fertilisation occur? What happens to the zygote before implantation?
Solution — Step by Step
After ovulation, the secondary oocyte is released from the Graafian follicle and swept into the fallopian tube (oviduct) by fimbriae. Simultaneously, after sexual intercourse, millions of spermatozoa travel through the cervix and uterus into the fallopian tubes.
Most sperm die along the way. Only a few hundred reach the egg, and only one will fertilise it.
Fertilisation occurs in the ampullary-isthmic junction of the fallopian tube (the wider upper part). This is a crucial fact for board exams — not in the uterus, not in the ovary.
Steps of fertilisation:
- Sperm capacitation: Sperm undergo physiological changes in the female tract that enable them to fertilise.
- Acrosomal reaction: The acrosome at the sperm head releases hydrolytic enzymes (acrosin, hyaluronidase) that dissolve the zona pellucida surrounding the egg.
- Plasma membrane fusion: One sperm fuses with the egg’s plasma membrane.
- Cortical reaction: The zona pellucida changes to prevent polyspermy (multiple sperm entry).
- The entry of sperm triggers the secondary oocyte to complete meiosis II, forming a mature ovum and a second polar body.
- The nuclei of sperm and egg fuse → zygote (2n = 46 chromosomes).
The zygote begins mitotic divisions called cleavage as it moves down the fallopian tube. These divisions produce blastomeres — cells that get smaller with each division because there is no growth between divisions (only cell number increases, not cell size).
The stages are:
- 2-cell → 4-cell → 8-cell → morula (16–32 cells, solid ball) → blastocyst (hollow ball with fluid-filled cavity)
The outer cells form the trophoblast (which will become the placenta); the inner cell mass (ICM) becomes the embryo proper.
This entire journey takes about 5–7 days after fertilisation.
The blastocyst reaches the uterus and burrows into the endometrium (uterine lining) — this is implantation. It typically occurs 6–7 days after fertilisation, in the posterior wall of the uterus.
How it happens:
- The trophoblast cells secrete enzymes that digest the endometrial cells.
- The blastocyst sinks into the endometrium.
- The endometrium responds by proliferating — the “decidual response.”
- The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast, which form the early placenta.
Once implanted, the trophoblast secretes hCG (human chorionic gonadotropin) — the hormone detected by pregnancy tests.
| Day | Event |
|---|---|
| Day 0 | Ovulation; fertilisation in fallopian tube |
| Day 1–3 | Cleavage (2→4→8→16 cell stages) |
| Day 4–5 | Morula enters uterus |
| Day 5–6 | Blastocyst formation |
| Day 6–7 | Implantation begins |
| Day 8–9 | Implantation complete; hCG secretion starts |
Why This Works
The sequence makes biological sense. The zygote cannot implant immediately because the uterine lining must be prepared (by progesterone from the corpus luteum) and the embryo must reach the blastocyst stage (when trophoblast cells are ready to invade the endometrium). The 6–7 day journey through the fallopian tube allows both preparations to happen simultaneously.
If the blastocyst implants in the fallopian tube instead of the uterus, it results in an ectopic pregnancy — a medical emergency.
Alternative Method
For NEET, memorise the sequence as: Fertilisation → Zygote → Cleavage → Morula → Blastocyst → Implantation. Each term has a distinguishing feature: morula = solid ball, blastocyst = hollow with ICM, implantation = attachment to endometrium.
Common Mistake
Many students write that fertilisation occurs “in the uterus.” This is incorrect. Fertilisation always occurs in the fallopian tube (specifically the ampullary-isthmic junction). The uterus is the site of implantation, not fertilisation. Confusing the two sites is a 1-mark error in board exams.