Library · Cellular process
Human fertilization
Make it clearer
Fertilization is the process in which a sperm and an egg join to form a single new cell, the zygote. It is not an instantaneous collision: it happens in several steps and takes 24 hours to complete.
The egg is not bare. It is wrapped in two layers that the sperm must get through: first the corona radiata, then the zona pellucida. The sperm carries a structure called the acrosome, which contains enzymes, proteins that can break down other substances. Digesting the zona pellucida takes an enzyme called acrosin.
And here is the key point: as soon as one sperm gets through, the egg seals itself off. Cortical granules, tiny secretory sacs, release enzymes that make the zona pellucida impenetrable to any other sperm. It is a door that closes behind the first one. This ensures that the zygote ends up with the right number of chromosomes.
In brief
Fertilization is a multistep process, completed in 24 hours, in which a sperm meets the egg and together they form a zygote.
The sperm, carrying enzymes in its acrosome, must get through two layers: first the corona radiata, then the zona pellucida, which acrosin must digest.
Once one acrosome penetrates the egg, vesicles called cortical granules make the zona pellucida impenetrable to other sperm, which ensures the correct number of chromosomes.
A door that shuts behind the first one in.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingEmbryology, Fertilization, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
Updated