Image caption : Sperm and Oocyte During Fertilization: Computer Generated Image from Micro-MRI, actual size of zygote = 0.1 mm - This image captures the moment of fertilization, when a sperm breaks through the oocyte's outer shell and breaches the inner membrane. Once fertilized, the egg becomes a zygote, which then begins its 3-day descent to the uterus via the fallopian tubes.


The fusion of a spermatozoon (SPERMATOZOA) with an OVUM thus resulting in the formation of a ZYGOTE.

National Center for Biotechnology Information, U.S. National Library of Medicine


Fertilization, pictured in Figure a is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in Figure b. The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. The nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

Part A is a micrograph that shows a sperm whose head is touching the surface of an egg. The egg is much larger than the sperm. Part B is an illustration that shows the surface of the egg, which is coated with a zona pellucida. The sperm penetrates the zona pellucida and releases its DNA into the egg. At this point, changes in proteins just inside the egg’s cell membrane occur, preventing entry of other sperm.

(a) Fertilization is the process in which sperm and egg fuse to form a zygote. (b) Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus. (credit: (b) modification of work by Mariana Ruiz Villareal; scale-bar data from Matt Russell)

To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. If this mechanism fails, multiple sperm can fuse with the egg, resulting inpolyspermy. The resulting embryo is not genetically viable and dies within a few days.

Sperm and the Process of Fertilization

This figure shows the process of sperm fertilizing an egg. There are many sperm trying to attach to the egg.

Before fertilization, hundreds of capacitated sperm must break through the surrounding corona radiata and zona pellucida so that one can contact and fuse with the oocyte plasma membrane.

Download for free at http://cnx.org/contents/c8105eac-93c4-43e8-83f1-54995429e8ca@3

Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, conception, fecundation, syngamy and impregnation, is the fusion of gametes to initiate the development of a new individual organism. The cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation.

The material on this site is for informational purposes only and is not intended as medical advice. It should not be used to diagnose or treat any medical condition. Consult a licensed medical professional for the diagnosis and treatment of all medical conditions and before starting a new diet or exercise program. If you have a medical emergency, call 911 immediately.