Fertilisation
Fertilisation happens when a sperm enters an egg and their nuclei join. It takes place in the oviduct (egg tube). The egg has half the genes of the mother and the sperm half of the father, so the zygote has a full set.
Only one sperm gets in. Then the egg changes its outer layer so no other sperm can enter.
Early development: cleavage, morula, blastocyst
- Cleavage: the zygote divides 2, 4, 8, 16... times without growing. The cells (blastomeres) become smaller.
- Morula: a solid ball of about 16 cells (like a mulberry).
- Blastocyst (about day 5): a hollow ball. The inner cell mass becomes the baby. The outer layer (trophoblast) becomes the placenta.
- Implantation (about day 6 to 7): the blastocyst sticks into the soft wall of the uterus.
While travelling down the egg tube to the uterus, the embryo is already dividing.
IVF: in vitro fertilisation
In vitro means “in glass”. Steps: the woman takes hormones so that several eggs mature; eggs are collected; sperm are added in a dish (or one sperm is injected: ICSI); embryos grow for 3 to 5 days; the best one or two are moved to the uterus.
It can help when the egg tube is blocked, sperm are few, or the cause is unknown. The first IVF baby was born in 1978. Related methods: ZIFT (zygote or early embryo put into the egg tube) and IUT (older embryo into the uterus).
Embryo transfer and freezing
Embryo transfer means placing an embryo in the female’s womb through a thin tube. Extra embryos can be frozen at −196 °C in liquid nitrogen (cryopreservation) and used later.
In cattle breeding (MOET: multiple ovulation embryo transfer): a champion cow is given hormones to release many eggs; she is mated or inseminated; the 6 to 8 day embryos are washed out; each is put into an ordinary cow, who carries the calf. Result: many calves with the best genes each year.
Embryo splitting and testing
Embryo splitting (embryo cloning) cuts an early embryo into two or more parts with a fine blade. Each part has the same genes and can grow into a separate animal, like identical twins. It works only in very early stages.
Testing: a few cells can be checked for serious inherited diseases before the embryo is moved. This raises ethical questions, so rules and doctors’ advice matter.
Try it
In the 3D, slide the stage from 1 to 16 cells and notice that the ball stays about the same size. Then tap Split at 8 cells. At home, fold a paper in half again and again and count the boxes: 1, 2, 4, 8, 16. That is how fast the cells double.
Key formulas and definitions
- Cells after n divisions = 2ⁿ (1, 2, 4, 8, 16, 32...)
- Zygote → morula (about 16 cells) → blastocyst (about day 5) → implantation (day 6 to 7)
- IVF: egg + sperm in a dish (37 °C) → embryo → transfer
- Embryo splitting: 1 embryo → 2 parts = identical twins
- Freezing embryos: −196 °C in liquid nitrogen
Worked examples
1. How many cells are present after 4 rounds of cleavage?
2⁴ = 16 cells (the morula stage).
2. A champion cow gives 8 embryos in one go. 6 are healthy and 4 are placed into surrogate cows successfully. How many calves can be expected?
About 4 calves, one for each successful pregnancy.
3. An 8-cell embryo is split into two equal parts. How many cells are in each part and do the twins have the same genes?
4 cells each. Yes, both parts come from one fertilised egg, so they have the same genes.
Common mistakes
- Thinking the embryo grows bigger during cleavage. The cells divide but the ball stays the same size.
- Mixing up IVF (fertilisation in a dish) with cloning (no sperm, nucleus transfer).
- Thinking the outer layer of the blastocyst makes the baby. The inner cells make the baby; the outer layer makes the placenta.
- Believing embryo splitting can be done at any age of the embryo. It works only at very early stages.