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How does human fertilisation work?

Human fertilisation occurs when the spermatozoa meets and successfully penetrates an egg, releasing its activation factor and the male half of the chromosome content.
Human fertilisation

How are eggs and sperm produced?

In humans, the road to fertilisation is long.  Sperm must swim through the vagina, uterus and Fallopian tubes. The egg must pass down the Fallopian tubes towards the oncoming sperm.  Once the two gametes meet, the sperm must still force its way through the cumulus (‘helper’) cells surrounding the egg, break througth the zona pellucida (‘shell’) and, if that wasn’t already enough, fuse with the egg membrane and fertilise it.
Only one sperm must fertilise the egg. Occasionally, when two sperm get in, an abnormal pregnancy will result and always ends in miscarriage.  The egg has a shield against further sperm penetration once fertilised, called the ‘block to polyspermy’. Female human embryo as demonstrated by FISH

Female human embryo as demonstrated by FISH

Calcium wave in fertilised egg. This is one of the first signals the egg receives.
 Calcium wave in fertilised egg
Eggs receive the signal for fertilisation from the sperm, in the form of a wave of calcium throughout the cytoplasm.  This tells the egg that the sperm has arrived and the egg can start to divide to form an embryo.
Embryogenesis is composed of two parts in the human.  These are preimplantation embryogenesis (i.e. before implantation) and postimplantation development. Preimplantation embryogenesis involves the devision of the egg into diverse blastomeres, followed by compaction of the embryo (‘morula’) and formation of the blastocyst.
Human unfertilised eggHuman fertilised egg (zygote).Human embryo at first mitotic cleavage.Human embryo at second mitotic cleavage.Human embryo at third mitotic cleavage.Human embryo at morula stage.Human blastocyst.
Human preimplantation embryogenesis.
Fertilisation, embryo development and pregnancy in the uterus. Naturally, fertilisation and embryo development occur during the journey of the egg through the Fallopian tube to the uterus. Implantation of the fertilised egg and hence pregnancy occur within the uterus itself.
Meiotic apparatus in an unfertilised human egg. Chromosomes in red, tubulin in green.
Completion of anaphase just after fertilisation in a human egg. Chromosomes in red, tubulin in green.
Chromosome segregation in a fertilsed egg
Several factors are important for preimplantation embryogenesis.  These are chromosome duplication, cell division and genome activation.  To successfully complete these processes, the embryo must have sufficient energy-generating mechanisms and protein stores. It follows that embryo implantation is highly dependent on the availability of energy (ATP in the cell).  In humans, maternal age plays a large part in this process, as less energy is produced through mitochondrial metabolism.
What chance do I have of getting pregnant?  

Wife’s age



6 Responses to How does human fertilisation work?

  1. Arminator says:

    So is it possible now to turn a somatic cell into germ cell which may eventually be fertilized plus become another human?

  2. Rishi says:

    For a normal human karyotype, what will be the chromosome number plus the diploid chromosome number? (are they the same?)
    For an abnormal human karytype (Down syndrome disorder), what will be the chromosome number plus the diploid chromosome number?

  3. Mark says:

    Almost every website states that fertilization happens inside the fallopian tube without providing the reason. So are there any supporting details?

  4. Kobe says:

    I am trying to find out about the formation of the human embryo for my science homework and everything i look at is too complicated so i’m wondering if any of you guys know what it is and put it in a simplest way you can put it!
    its about how its turning from fertilization to a human baby kinda thing in the simplest words

  5. Andrew S says:

    I know the answer is probably obvious but I’m missing something. What’s the point of having synapsis and crossing over when you’re just making gametes? It wouldn’t matter if the gametes were genetically unique because they’re going to combine with the opposite sex’s gametes later on and that will make the final daughter cell genetically unique. Also, once the egg and sperm combine during fertilization, why does mitosis occur? Isn’t this the part where you want swapping of genes from meiosis?

    So many questions; sorry.

  6. Jonathan says:

    What is the difference between self-fertilization and cross-fertilization?

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