Searching on the internet for "sperm" or "sperm quality" yields a great deal of information on infertility, improving sperm count, and the effect of frequent (or infrequent) sex on fertility. However, it is worthwhile to take a step back and look at sperm and its role in fertility when everything is working correctly.
What does a sperm cell look like?
Sperm cells are unusual in the human body in that they are designed to move quickly under their own power. The cell body is called the head, and it contains the cell's DNA in the nucleus. At the tip of the head, there is a sac of enzymes called the acrosome, which is used to aid in penetrating the egg for fertilization. At the neck, the head joins with the midpiece, which has many mitochondria for energy production and is responsible for the sperm's motility. The last major part of the sperm is the long tail, which generates movement by whipping around.
How are sperm formed?
Sperm cells are continuously produced in the seminiferous tubules in the testicles. The developing sperm cells must be kept approximately 3 C cooler than body temperature, which is the reason the testicles are contained in the scrotal sac. It takes about 8 to 9 weeks for sperm to be fully formed. After they are fully developed, the sperm leave the testicles though the vas deferens, which is where they are stored between production and ejaculation. The end of the vas deferens passes through the seminal vesicle and the prostate, which produce the non-cellular components of semen, and then merges with the urethra, which extends the length of the penis.
How do sperm fertilize eggs?
After ejaculation during intercourse, or during intracervical insemination (ICI, see below), the semen is deposited in the vagina, near the cervix. The sperm swim through the cervix, through the uterus, and into the fallopian tubes, where the egg will be waiting. The sperm uses the enzymes in the acrosome to penetrate the protective barrier surrounding the egg. Once a sperm has penetrated the barrier, the cortical reaction occurs, which hardens the barrier to prevent fertilization by more than one sperm. Once the sperm reaches the egg's cell membrane, the membranes fuse and the genetic material of the sperm and egg combine.
How is sperm quality measured?
There are three main factors used in assessing sperm quality: number, shape, and movement.
Number: Sperm count is the number of sperm per milliliter of semen. Typically, men are more likely to be fertile if they have more than 20 million sperm per milliliter.
Shape: The morphology of sperm, or their shape and structure, are also critical for assessing fertility. Normal sperm have an oval head and a long, straight tail. However, a variety of abnormalities in sperm regularly occur, such as abnormally sized or misshapen heads, crooked tails, and even multiple heads or tails on a single sperm. Abnormalities in sperm morphology are so common that the WHO standard for a fertile sperm sample is one that contains only 4 percent or more normal sperm.
Movement: Motility is the measurement of the forward motion of sperm. Sperm with the highest motility move strongly forward, while less motile sperm move generally forward but may curve or wander, and other sperm do not move forward at all. It is common for even a fertile sperm sample to have less than perfect motility: a sample with approximately 50 to 60 percent of the sperm with good forward movement is considered normal.
There are several other secondary characteristics of semen that may also be measured to assess sperm quality, including total semen volume, fructose content, pH, liquefaction, and white blood cell count, but these are less critical than count, morphology, and motility.
Sperm banks measure sperm quality both before and after freezing to ensure adequate fertility of thawed samples.
How is frozen sperm used in artificial insemination?
All donor sperm from a sperm bank is cryogenically preserved for a minimum of 6 months to allow for testing of the donor for disease. After this mandatory quarantine period, sperm samples are kept frozen until they are ready to be used. At the fertility clinic where the insemination procedure will be performed, the sample is thawed and prepared for use.
The most common methods of artificial insemination are intracervical insemination (ICI) and intrauterine insemination (IUI). However, preserved sperm can also be used for in vitro fertilization (IVF).
Intracervical insemination (ICI): This method of artificial insemination most closely mimics ejaculation during intercourse. A needless syringe is used to deposit unwashed or washed (concentrated) sperm high in the cervix. The woman is usually advised to remain lying down for 15-30 minutes, to allow the sperm to travel through the cervix and uterus and into the fallopian tubes. Women may undergo ICI twice within a single fertile cycle and/or take medication to stimulate ovulation, in order to increase the chances of pregnancy.
Intrauterine insemination (IUI): This is the most common form of artificial insemination. A washed sperm sample, which has had the seminal fluid removed and the live sperm cells concentrated, is injected past the cervix directly into the woman's uterus. Women undergoing IUI frequently also use medication to stimulate ovulation.
In vitro fertilization (IVF): In the IVF procedure, mature eggs are harvested from the woman's ovaries and fertilization takes place outside the body. The resulting embryo is then implanted in the woman's uterus. Because fertilization takes place outside the body, both egg and sperm may be donated.
References
Angier, Natalie, "Sleek, Fast and Focused: The Cells that Make Dad Dad," The New York Times, June 12, 2007, http://www.nytimes.com/2007/06/12/science/12angi.html?_r=1
MayoClinic.com, http://www.mayoclinic.com/health/fertility/MC00023
WebMD.com, http://www.webmd.com/infertility-and-reproduction/insemination-procedures-for-infertility
Wikipedia.org, http://en.wikipedia.org/wiki/Sperm, http://en.wikipedia.org/wiki/Human_fertilisation, http://en.wikipedia.org/wiki/Semen_quality
Seattle Sperm Bank: www.seattlespermbank.com
European Sperm Bank USA, located in Seattle, Washington, was established to provide couples and individuals with the donor choices they need to make dreams of conception, pregnancy and childbirth a reality. European Sperm Bank USA is affiliated with Denmark-based European Sperm Bank, and are leading the way for sperm banks in Europe.
