4.7.10Additional lessons
Fertilisation and Implantation — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 64 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5From Zygote to Embryo
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.
Section 2 of 5Parts of the Blastocyst
Inner cell mass
- Inner cell mass The group of cells that will form the embryo itself, and contains cells able to become any type.
Trophoblast
- Trophoblast The outer layer that attaches to the uterine wall and will contribute to the placenta.
Cavity
- Cavity The fluid-filled space inside the blastocyst.
Zona pellucida
- Zona pellucida The protective coat that is shed before the blastocyst attaches.
Section 3 of 5Implantation
Successful implantation depends on a receptive uterine lining and on signals from the embryo.
Definition
- Definition Implantation is the embedding of the blastocyst in the thickened endometrium of the uterus.
Enzymes
- Enzymes The trophoblast secretes enzymes that digest endometrial cells so that the embryo can sink into the lining.
Uterine preparation
- Uterine preparation Progesterone has prepared the endometrium, with a rich blood supply and stored nutrients.
hCG
- hCG Trophoblast cells secrete human chorionic gonadotrophin, which signals the presence of the embryo.
| Stage | Approximate timing after fertilisation | Location |
|---|---|---|
| Zygote | Day 0 | Oviduct |
| Morula | Day 3 to 4 | Oviduct |
| Blastocyst implants | Day 6 to 7 | Uterus |
Section 4 of 5Failure and Variation
Ectopic pregnancy
- Ectopic pregnancy If an embryo implants outside the uterus, usually in the oviduct, it cannot develop normally and is dangerous.
Twins
- Twins Splitting of an early embryo gives identical twins, whereas two fertilised eggs give non-identical twins.
IVF link
- IVF link In vitro fertilisation, early embryos are grown in the laboratory then placed in the uterus to implant.
Section 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.
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