Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.7.10Additional lessons

Fertilisation and Implantation — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 64 of 101

📚 IB Biology HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 5From Zygote to Embryo

4.7.10 Fertilisation and Implantation
1
After fertilisation in the oviduct, the zygote divides by mitosis in a process called cleavage, without growing in size.
2
A solid ball of cells called a morula forms after about three to four days.
3
Fluid fills a cavity inside, producing a blastocyst with an inner cell mass and an outer layer, the trophoblast.
4
Cilia and muscular contractions move the embryo along the oviduct into the uterus.
5
The blastocyst reaches the uterus and attaches to the endometrium about a week after fertilisation.

Part 2 of 5Parts of the Blastocyst

Inner cell mass
  • The group of cells that will form the embryo itself, and contains cells able to become any type.
Trophoblast
  • The outer layer that attaches to the uterine wall and will contribute to the placenta.
Cavity
  • The fluid-filled space inside the blastocyst.
Zona pellucida
  • The protective coat that is shed before the blastocyst attaches.

Part 3 of 5Implantation

Successful implantation depends on a receptive uterine lining and on signals from the embryo.

Definition
  • Implantation is the embedding of the blastocyst in the thickened endometrium of the uterus.
Enzymes
  • The trophoblast secretes enzymes that digest endometrial cells so that the embryo can sink into the lining.
Uterine preparation
  • Progesterone has prepared the endometrium, with a rich blood supply and stored nutrients.
hCG
  • Trophoblast cells secrete human chorionic gonadotrophin, which signals the presence of the embryo.
StageApproximate timing after fertilisationLocation
ZygoteDay 0Oviduct
MorulaDay 3 to 4Oviduct
Blastocyst implantsDay 6 to 7Uterus

Part 4 of 5Failure and Variation

Ectopic pregnancy
  • If an embryo implants outside the uterus, usually in the oviduct, it cannot develop normally and is dangerous.
Twins
  • Splitting of an early embryo gives identical twins, whereas two fertilised eggs give non-identical twins.
IVF link
  • In vitro fertilisation, early embryos are grown in the laboratory then placed in the uterus to implant.

Part 5 of 5Exam Checklist & Tips

4.7.10 – Key Points for the Exam

  • Cleavage Rapid mitotic divisions of the zygote without growth.
  • Blastocyst A hollow ball of cells with an inner cell mass and trophoblast.
  • Implantation Embedding of the blastocyst in the endometrium.
  • hCG A hormone secreted by the embryo that maintains the corpus luteum.

Common Mistakes

  • Saying the zygote grows larger during cleavage; the total size stays about the same.
  • Placing implantation in the oviduct.
  • Stating that the trophoblast forms the baby; the inner cell mass does.
  • Confusing fertilisation with implantation.

Last updated:

\n