Lesson 5 of 6 / Reproduction in humans
From fertilisation to implantation
Why are fertilisation and implantation different events in different places?
In this lesson: Trace fertilisation, formation of a ball of cells and implantation in the uterine lining.
About 4 min
The key ideaFertilisation normally occurs in an oviduct. The zygote divides into a ball of cells that travels to the uterus and implants in its lining.
Explore the idea
Where does each event happen?
Fertilisation normally occurs in an oviduct, forming a zygote.
Original sequence model, not to scale or a timed record. The cell-division image represents an early stage; implantation involves a larger developing ball of cells.
Explanation
If a sperm reaches an egg in an oviduct, the male and female nuclei can fuse. Fertilisation forms a zygote. The usual site is the oviduct, not the ovary or the uterine lining.
The zygote divides repeatedly to form a ball of cells. This early development occurs as it moves along the oviduct towards the uterus. An increase in cell number follows fertilisation; it does not mean that the sperm keeps fertilising each new cell.
The ball of cells reaches the uterus and becomes embedded in its lining. This is implantation. The uterine lining has been prepared during the menstrual cycle, linking the organ route with the hormonal changes studied earlier.
Keep the order and location clear: fertilisation in an oviduct, early cell divisions during movement towards the uterus, then implantation in the uterine lining. Later development is supported there; implantation is not a second fusion of gamete nuclei.
Step by step
- 1
Fuse nuclei
A zygote forms after fertilisation in an oviduct.
- 2
Divide and travel
Repeated divisions produce a ball of cells moving towards the uterus.
- 3
Implant
The ball of cells embeds in the uterine lining.
Worked example
Two labels, two places
A diagram labels fertilisation on the uterine lining and implantation in an oviduct. Correct the two labels and describe the intervening change.
One way to explain it
Fertilisation normally occurs in an oviduct. The zygote divides into a ball of cells while moving towards the uterus. Implantation occurs when it embeds in the uterine lining.
Why this answer works
- Identify the usual site of nuclear fusion.
- Describe the increase in cell number.
- Locate embedding in the uterine lining.
Is this true? "Implantation is when a sperm first enters an egg."
Fertilisation involves fusion of gamete nuclei. Implantation happens later when the developing ball of cells embeds in the uterine lining.