Insemination refers to the introduction of semen (sperm and seminal fluid) to an egg, or oocyte. The primary goal of insemination is fertilization, during which time a sperm enters the internal portion of an egg and deposits its DNA into the egg. The result of fertilization is a one-cell embryo, also known as a zygote.
For any insemination to occur, sperm is needed. A sperm sample can either be fresh or frozen (in which case it will need to be thawed). The sperm is always verified and prepared (cleaned and processed) in media prior to insemination.
There are 2 methods of insemination:
ICSI (intracytoplasmic sperm injection): sperm are individually sucked up into a small needle (the best sperm are chosen), and then 1 sperm is injected into each mature egg to force fertilization to occur. This is the most common form of insemination in IVF clinics and has shown an overall higher fertilization rate than conventional insemination. ICSI is indicated when there is a low sperm count, motility, or morphology, or when conventional insemination has failed in the past. Additionally, ICSI is highly recommended if you are inseminating thawed eggs.
Conventional (standard) insemination (also known as IVF): eggs and sperm are placed into a drop of media together overnight in hopes that fertilization will occur. The eggs are not cleaned or graded for conventional insemination, so the embryologist does not know if the eggs are mature or not prior to insemination.
Insemination typically occurs a few hours after an egg retrieval procedure (or egg thaw). After the insemination occurs, the eggs are not checked for ~16-19 hours. Around this time, an embryologist checks to see if fertilization has occurred. You will receive an update regarding how many eggs fertilized the day after the insemination occurred.
PICSI stands for physiological ICSI. PICSI is essentially the same procedure as ICSI, but the sperm are chosen based on their ability to bind to hyaluronic acid. PICSI dishes contain a ring of hyaluronic acid at the bottom of the dish. When a drop of media is added above the ring and sperm are added to the drop, some sperm will bind to the ring, while others will not. The embryologist will only inseminate the eggs with sperm that are bound to the hyaluronic acid ring.
PICSI is also known as HBA, or hyaluronan binding assay. This assay determines the percentage of sperm that bind to hyaluronic acid in a sample.
The theory behind PICSI/HBA is that only mature sperm are able to bind to hyaluronic acid. Hyaluronic acid is the main component of the cumulus cells that surround an egg. Mature sperm must bind to the hyaluronan in order to reach the egg for fertilization.
Doctors may recommend PICSI for poor sperm counts (though sperm with poor motility cannot use PICSI) or when the sperm do have a low HBA percentage.
The morning after insemination, embryologists are looking for 2 key signs of fertilization:
1. Two circles inside the embryo called pronuclei. This indicates that the DNA from the sperm and egg are both present. The pronuclei will combine to form a cell with 2 sets of chromosomes (a diploid cell), and then regular cell division will begin to occur.
2. Two polar bodies. This indicates that the egg has been fertilized and has completed meiosis II (fertilization is required for this to occur).
Normal fertilization. These zygotes (early embryos) have 2 pronuclei and polar bodies. They will remain in culture (inside media in a petri dish) in an incubator for a few days.
No fertilization. These do not have 2 pronuclei and/or polar bodies, so normal fertilization did not occur. These are known as 0PNs since there are no pronuclei present in them. These are typically not cultured, though some clinics may culture them to see if they grow. In rare cases, these embryos do end up growing even though they did not show signs of fertilization.
Abnormal fertilization. Sometimes there are 1 or 3+ pronuclei present, or the embryo prematurely divides into 2 cells, or some other abnormality occurs. These are typically not cultured since they are abnormal.
Degeneration. It's possible that the eggs do not survive the insemination process, or that some abnormality has caused the egg to not survive even if fertilization occurs. These are typically not cultured.
Note: it's normal for some eggs to not properly fertilize. Typically, only ~50-80% of the eggs that are inseminated will fertilize for one reason or another. If your fertilization rate is lower than that, it could indicate an issue with egg or sperm quality or some other underlying issue.
1. ICSI: What Is ICSI + How Is It Different From Conventional IVF Fertilization (cnyfertility.com)
2. What Is PICSI or Physiological ICSI in IVF? (invitra.com)