At fertilization check, the embryologist often sees some eggs that have abnormal fertilization. These eggs exhibit abnormal fertilization for several reasons including:
Three or more pronuclei instead of the normal two pronuclei. This can occur for one of two reasons.
Multiple sperm fertilize an egg. When an egg is placed with several thousand sperm during the process of insemination, a mature, healthy egg will allow a single sperm to bind to its cell membrane and enter its cytoplasm. Immediately, a chemical process occurs within the egg that will block any other sperm from entering, even though it is surrounded by thousands of sperm. This is known as the cortical granule reaction. Some eggs are defective in this process and the chemical reaction is slow or incomplete. In these cases, a second sperm may enter the egg’s cytoplasm and form a third pronucleus. This third set of chromosomes will add to the plan for building an embryo and completely confuse and shut down the process of embryo growth. These abnormally fertilized eggs may grow for a few divisions and then stop, or worse if transferred, may grow to the point of early implantation, implant on the uterine wall and then result in a miscarriage. These eggs are referred to as polyspermic eggs. Because these abnormally fertilized eggs have no potential for producing a viable pregnancy, they are immediately discarded.
When intracytoplasmic sperm injection (ICSI) is performed. The possibility for multiple sperm entering an egg is eliminated. However, there are eggs that have been injected with a single sperm and not exposed to any other sperm, but still have three pronuclei. The process which results in this outcome involves the division of the female chromosomes. When an egg matures, it releases one half of its chromosomes in a polar body. As a sperm enters an egg, by insemination or by ICSI, its presence is signaled to the nucleus of the egg. This causes the egg chromosomes to align and to form a duplicate set of chromosomes which align next to the first set of chromosomes. The two sets of chromosomes then exchange sections in a process that results in each set of female chromosomes having a different composition than they had before the crossing over occurred. The final step in the normal process is for one half of the chromosomes to be expelled into a second polar body outside of the cytoplasm of the egg. Sometimes this does not occur and the extra set of chromosomes is retained within the egg. The result is a third pronucleus that has the same effect as if the source of these chromosomes was from a second sperm. Too many chromosomes will prevent the egg from developing normally. These eggs are referred to as polygynic or polyploid. Because they have no potential for producing a viable pregnancy, they are immediately discarded. Since this source of too many chromosomes can also occur in eggs that have been inseminated, the source of the third pronucleus is not always known. For this reason, these eggs with more than two pronuclei are referred to as polyploid, or having more than two sets of chromosomes, regardless of the source.
An egg with only one pronucleus.
This usually occurs when a defective sperm enters an egg signaling the egg to form its pronucleus. Meanwhile, the defective sperm is incapable of forming its pronucleus. With only one half of the chromosomes functional, the plan for building an embryo is incomplete and the process shuts down. These eggs are known as 1pn and have no potential for life. They are immediately discarded.
