What’s in the Shape: Sperm Morphology

The term morphology refers to the form and structure of an object or thing.  Sperm morphology is the term used to describe the appearance (size and shape) of sperm.  Morphologically normal human sperm will have a smooth, oval shaped head with a long tail attached at the distal aspect of the head.  Sperm is described as normal if it has an oval-shaped head that is 5 to 6 micrometers long and about 3 micrometers wide with a well-defined acrosome (cap) covering 40 to 70 percent of the head. There should be no visible vacuoles in the head, midpiece or tail, and no cytoplasmic droplets larger than half the size of the sperm head.

There are many abnormalities a sperm may exhibit, such as head and tail defects including crooked or double tail. The whip like motion of the tail is what propels the sperm forward to swim through the passages of the female reproductive system towards the egg. If abnormal sperm has a defective tail it will have a very difficult time reaching the egg and fertilizing it. The egg is enclosed in a glycoprotein coat called the zona pellucida (ZP). The ZP can be viewed as a selector or filter for morphologically normal sperm because it has been shown that sperm with abnormal head morphology attach to the ZP in lower frequencies than sperm that have normal head morphology. 

Sperm morphology evaluation is a component of a complete semen analysis. Sperm cells are stained to make them visible, and several hundred are viewed under high magnification and scored individually as normal or abnormal based on their shape and size. The percentages of normal and abnormal shaped cells are calculated.

In a typical sample of a man considered to have a normal, healthy semen sample, we would expect to see more abnormal sperm cells than normal. There are several criteria currently used to evaluate sperm morphology; two of the more common criteria are the World Health Organization (WHO) and Kruger Strict.

WHO (Edition 4) criteria classify a sperm with borderline morphology (slightly abnormal) as normal. Based on these criteria men with 30% normal sperm are considered to have normal sperm morphology. WHO (Edition 5) states that men with 4% normal sperm are considered to have normal sperm morphology.  This is a topic that elicits much discussion among the experts, and is rather controversial.

For Kruger Strict criteria, which is the method used by the Cryobank, sperm classified as borderline normal sperm are scored abnormal. Based on Kruger analysis men with 14% morphologically normal sperm are considered normal.

The shape of the sperm is a reflection of proper sperm development (spermatogenesis) in the testicles. Men with defective spermatogenesis may produce a high percentage of abnormal sperm. There are many factors that can lead to formation of abnormal sperm. The common ones are genetic trait (inherent), exposure to toxic chemicals, increased testicular temperature and infection. Nothing can be done if it is a genetic trait; however, the other causes may be reversible. Spermatogenesis takes place at a lower temperature than the overall body temperature. Anything that increases the testicular temperature e.g. wearing tight clothes that hold the testes close to the body will cause the production of abnormal sperm.

Fairfax Cryobank donors are evaluated for sperm morphology using Kruger Strict morphology.  Any donor who does not pass the morphology analysis during the screening will not be admitted to the donor program.