Anatomy chapters
Fertilisation, cleavage and implantation
The first week of development: where fertilisation happens, how the zygote divides into a morula and blastocyst, and where and when it implants.
- Fertilisation normally happens in the ampulla of the uterine tube.
- The zygote divides (cleavage) into a morula, then a blastocyst, which reaches the uterus.
- Implantation begins about day 6, usually in the posterior wall of the uterine body.
On this page
Overview
The first week of development runs from fertilisation to the start of implantation.
During this week the zygote divides as it moves down the uterine tube, becomes a hollow blastocyst and starts to bury itself in the wall of the uterus.
Fertilisation
Fertilisation is the fusion of a sperm and a secondary oocyte. It usually takes place in the ampulla, the widest part of the uterine tube.
The usual site of fertilisation is the ampulla of the uterine tube.
Getting the sperm ready
- Capacitation: in the female tract, a glycoprotein coat and plasma proteins are removed from the sperm head. It takes several hours. Only capacitated sperm can pass the corona radiata.
- Acrosome reaction: when the sperm binds to the zona pellucida, enzymes are released from the acrosome. They digest a path through the zona.
Entry and the block to polyspermy
When the first sperm touches the oocyte membrane, the oocyte releases enzymes from its cortical granules. These change the zona pellucida so no other sperm can get through. This is the zona reaction.
The cortical and zona reactions prevent polyspermy (entry of more than one sperm).
Results of fertilisation
- The oocyte finishes meiosis II and releases the second polar body.
- The diploid number (46) is restored.
- The sex of the embryo is set: an X-bearing sperm gives a girl, a Y-bearing sperm gives a boy.
- Cleavage begins.
The father's sperm decides the sex of the baby, because only the sperm can carry a Y chromosome.
Cleavage
The zygote divides by mitosis into smaller cells called blastomeres. The total size does not increase; the cells just get smaller.
The zona pellucida stays around the dividing cells and stops them from implanting too early in the tube.
- About day 3: a solid ball of about 16 cells, the morula ("mulberry"). It reaches the uterus at about this time.
- Inner cells of the morula form the inner cell mass. Outer cells form the outer cell mass.
The blastocyst
Fluid enters the morula and forms a cavity, the blastocele. It is now a blastocyst, at about day 4 to 5.
| Part | What it becomes |
|---|---|
| Inner cell mass (embryoblast) | The embryo itself |
| Outer cell mass (trophoblast) | The fetal part of the placenta |
The zona pellucida then disappears so implantation can begin.
The trophoblast forms the placenta, not the embryo. The embryo comes only from the inner cell mass.
Implantation
Implantation begins around day 6. The trophoblast over the inner cell mass attaches to the lining of the uterus (the endometrium, in its secretory phase).
The trophoblast then splits into two layers:
- Cytotrophoblast: the inner layer of separate cells that keep dividing.
- Syncytiotrophoblast: the outer layer, a mass of cytoplasm with many nuclei and no cell borders. It invades the endometrium.
The syncytiotrophoblast makes human chorionic gonadotrophin (hCG), which keeps the corpus luteum alive and is the basis of pregnancy tests.
The normal site of implantation is the posterior wall of the body of the uterus, near the fundus.
After implantation, the endometrium becomes the decidua.
Clinical relevance
Ectopic pregnancy
How it happens: the blastocyst implants outside the body of the uterus.
What you see: missed period, pain in the lower abdomen and vaginal bleeding. Tubal rupture causes internal bleeding and shock.
The commonest site is the uterine tube, mostly the ampulla. Other sites are the ovary, the abdominal cavity (often the rectouterine pouch) and the cervix.
The uterine tube is the commonest site of ectopic pregnancy. Previous tubal infection is a common cause.
Placenta praevia
How it happens: implantation near the internal os, in the lower uterine segment.
What you see: painless bleeding late in pregnancy.
Abnormal zygotes
Many early embryos have chromosome errors and are lost before the mother knows she is pregnant.
Hydatidiform mole
How it happens: abnormal fertilisation, in which trophoblast grows without a normal embryo.
What you see: a large uterus for dates, very high hCG and a "snowstorm" pattern on ultrasound.
Monozygotic twins
These come from splitting of one embryo. Depending on when it happens, they may share a chorion, an amnion or both.
Common questions: the site of fertilisation, the reaction that blocks polyspermy, the part of the blastocyst that forms the embryo, the layer that makes hCG, the day implantation starts, and the commonest ectopic site.
Quick revision
- Fertilisation: ampulla of the uterine tube.
- Capacitation in the female tract; acrosome reaction at the zona.
- Zona reaction blocks polyspermy.
- Morula about day 3; blastocyst day 4 to 5.
- Inner cell mass forms the embryo; trophoblast forms the placenta.
- Implantation starts about day 6, usually in the posterior wall of the uterine body.
- Syncytiotrophoblast makes hCG.
Written by The House Job from standard embryology references (Langman's Medical Embryology).
FAQ
Where does fertilisation take place?
In the ampulla of the uterine tube.
What stops more than one sperm entering the oocyte?
The cortical reaction, which changes the zona pellucida (the zona reaction).
When does implantation begin?
At the end of the first week, about day 6.
Which cells produce hCG?
The syncytiotrophoblast.