Lesson 11 of 12 / Reproduction
Fertilisation, implantation and the placenta
How does a fertilised egg become connected to a supply of nutrients and oxygen?
I can explain early cell division, implantation, amniotic protection and placental exchange.
I can explain fertilisation, early division and implantation.Syllabus
Syllabus K325, Topic 13(r). Explain fertilisation, early division and implantation.
I can explain amniotic protection.Syllabus
Syllabus K325, Topic 13(s). Explain amniotic protection.
I can explain placental and umbilical exchange.Syllabus
Syllabus K325, Topic 13(t). Explain placental and umbilical exchange.
About 6 min
Make a guess. You are not marked.
Does the mother's blood flow directly into the fetus?
Show the answer
The two bloods stay separate in the placenta.
Substances cross a thin barrier in the placenta. The mother's and fetus's blood do not mix.
The key idea
The zygote divides and implants in the uterine lining; the placenta exchanges materials while the amniotic sac and fluid protect development.
Simplified front section. The ovaries lie near the open oviduct ends, not inside sealed tubes. The highlighted early embryo is enlarged.
Explanation
A sperm nucleus fuses with the egg nucleus, usually in an oviduct, to form a diploid zygote. Repeated mitotic divisions produce a ball of cells as it moves towards the uterus. It then embeds in the uterine lining: implantation.
As development continues, the amniotic sac encloses the fetus and amniotic fluid. The fluid cushions against mechanical shocks, allows movement and helps provide a stable surrounding environment. The sac contains the fluid; it is not the organ supplying oxygen from maternal blood.
The placenta forms an exchange interface between maternal and fetal supplies. Oxygen and dissolved nutrients, including glucose and amino acids, move towards fetal blood. Carbon dioxide and urea move from fetal blood towards maternal blood for removal by the mother's organs.
Maternal and fetal blood normally remain in separate circulations. Substances cross an exchange barrier; the two supplies do not normally flow together as one shared bloodstream.
The umbilical cord connects the fetus to the placenta and contains fetal blood vessels. It carries blood towards and away from the placenta, enabling delivery of nutrients and oxygen and removal of wastes. The fetus receives oxygen through this route rather than breathing air into its lungs before birth.
Step by step
- 1
Order the early events
Nuclear fusion comes before mitotic divisions and implantation.
- 2
Choose protection or exchange
Use the amniotic sac/fluid for physical protection and the placenta for exchange.
- 3
Trace each substance
State its direction and remember that the cord contains fetal vessels.
Worked example
Removing a fetal waste
Trace how urea produced by the fetus can be removed using the mother's kidneys.
One way to explain it
Fetal blood carries urea through umbilical vessels to the placenta. Urea crosses the placental exchange barrier into maternal blood, which carries it to the mother's kidneys for excretion.
Why this answer works
- The exchange occurs at the placenta.
- Substances cross; the two whole blood supplies do not normally mix.
Watch out for this
A student says: "The umbilical cord connects the fetus directly to the mother's stomach." What is wrong with this?
Show the answer
It connects the fetus to the placenta. Dissolved nutrients reach the placenta through maternal blood after digestion and absorption.
Check your understanding
Which correctly describes support before birth?
Show the answer
Amniotic fluid cushions the fetus; the placenta exchanges substances between separate blood supplies.
Protection and exchange are distinct functions.
Quick recall
How does a fertilised egg become connected to a supply of nutrients and oxygen?