Egg Fertilization

Egg fertilization occurs in a woman’s body in the Fallopian tubes.

This event occurs within the first 24 hours after the eggs have been released from the ovaries and can occur only if there are sperm present around the eggs at this critical time. The penetration of the egg by the sperm AND the resulting formation of the nucleus of the sperm and the nucleus of the egg within the interior of the egg is known as fertilization. In our Austin IVF lab, the eggs and sperm are combined by one of two processes, by insemination or by ICSI.

There are two possible methods in order to fertilize the eggs in the IVF lab following egg retrieval. There is an insemination procedure and there is Intracytoplasmic sperm injection (ICSI).


In order to perform an insemination, several factors must be met. There must be a high concentration of sperm present in the sample that has a high percentage of normal morphology. In addition, the sample must have a high concentration of forward progressing sperm. Once all of these criteria have been met, an insemination can be performed.

This procedure will be performed 6 hours post egg retrieval. With insemination, a concentration of highly-motile and normal sperm will be administered into a culture dish within drops that contain eggs.

Fertilization takes place by the vigorous movement of the sperm which penetrate the shell of the egg, bind to the inner membrane of the egg and then enter the cell material, or cytoplasm, of the egg.

Here, the chromosomes of the sperm and egg combine and produce an embryo.

The sperm enters the egg by dissolving a path through the outer shell, or zona pellucida, with an enzyme that is located on the head of the sperm. This entry also requires vigorous swimming by the sperm until it gets through the shell. At that point, the sperm and egg cell membranes chemically bind and the sperm is pulled into the interior, or cytoplasm, of the egg. When the sperm enters the interior of the egg, its tail breaks off. The significance of this is that the cell membrane that surrounds the head and the tail is broken. The breaking of this membrane allows water to enter the sperm head from the cytoplasm of the egg. Then, both the egg and the sperm form a nucleus within the egg. These two nuclei are called pronulei until they merge to form the nucleus of the new embryo. They each contain half of the chromosomes that the embryo will have. The formation of these two pronuclei is what the embryologist looks for when they perform a fertilization check on the morning after the retrieval.

Intracytoplasmic Sperm Injection (ICSI)

Intra-cytoplasmic sperm injection, or ICSI, refers to the placement of a single sperm directly into the cell material, or cytoplasm, of the egg by an Embryologist at our Austin IVF laboratory. This process becomes necessary when the penetration of the shell and inner membrane of the egg by the sperm is not likely to happen due to a low concentration of optimal sperm.

With ICSI, a single sperm is injected into each egg. This technique eliminates the need for the sperm to dissolve a path through the shell of the egg and to bind to the egg membrane. And these are capabilities that some sperm do not have. However, the placement of a sperm into the interior of the egg by ICSI does not necessarily result in fertilization. This is because the nucleus of the egg and the nucleus of the sperm must form within the interior of the egg as a result of the sperm entering the egg. Just like other events in the process of fertilization, this formation does not always occur. And just as with eggs that have been inseminated, the appearance of these two pronuclei is what the embryologist looks for when they check the eggs for fertilization the morning after retrieval.

Once fertilization has been confirmed, the eggs are put into culture dishes with a new type of media that will support their growth during the initial stages of division and growth. This culture media mimics the tissue fluid that is present in the Fallopian tubes in the woman’s reproductive tract.

The newly-fertilized eggs are known as zygotes.