A guide to sperm morphology

Donors

If you’re thinking about becoming a sperm donor, you may come across the term sperm morphology and wonder what it actually means. While it might sound like something out of a science fiction film, it’s simply the medical term for the appearance of sperm cells.

Understanding sperm morphology is important for anyone considering donation. It gives valuable insight into your fertility health, and it’s one of the key factors we assess in your sperm analysis results when determining donor eligibility as part of the overall sperm donor process.

In this article, we take a closer look at sperm morphology, defining the term in more detail, explaining what normal sperm look like and exploring the different abnormalities that can occur.

What is sperm morphology?

Sperm morphology refers to the size, shape and structure of sperm. Each sperm cell has three key parts: the head, the midpiece, and the tail. Ideally, these parts are well-formed and proportionate, allowing the sperm to move efficiently and fertilise an egg.

However, it’s perfectly normal for some sperm to look a little unusual. In fact, the majority of sperm in any given sample are considered abnormal, and this doesn’t necessarily mean there’s a problem with your fertility.

At the London Sperm Bank, our andrology team evaluates sperm samples in our laboratory using a staining technique and microscope. This detailed analysis helps us understand your sperm health and highlights any irregularities that could affect your suitability as a donor.

Normal sperm morphology

Sperm morphology is normally classified according to the three segments of the sperm cell: the head, midpiece and tail. The ideal or typical appearance of sperm according to these areas is as follows:

Head: The head of a sperm cell should be smooth and oval-shaped with a well-defined acrosome (a cap-like structure) covering 40-70% of the head.
Midpiece: The midpiece should be even in width and about the same length as the head of the sperm. It should also align with the sperm cell head, attaching straight to the base.
Tail: The tail of a sperm cell is typically long – around ten times the length of the head, straight and uncoiled, although it may bend gently.

Common sperm abnormalities

Sperm abnormalities are more common than you might think, and they can affect any part of the sperm. Even healthy men can have a high proportion of misshapen sperm, which is known as teratospermia.

When discussing abnormal sperm morphology, we can break this down into the defects commonly seen in different parts of the sperm cell. Here are some of the variations our lab team may see:

Sperm head defects

Macrocephaly (giant head sperm)

These sperm have unusually large heads and often carry too much genetic material, which can reduce the chance of successful fertilisation. Sperm with macrocephaly often also have multiple tails.

It is thought that macrocephalic sperm may be caused by the mutation of a specific gene, called the aurora kinase C gene. This means that this type of sperm head defect may be genetic and potentially be able to be passed to male offspring.

Microcephaly (small head sperm)

In contrast, microcephalic sperm have very small heads. They can sometimes have a defective acrosome (which is where the enzymes are stored which help a sperm enter an egg) or have no DNA at all, meaning they will be unable to fertilise an egg.

Pinhead sperm

Pinhead sperm are a subtype of small-headed sperm. In these sperm cells, the head appears as a pin and includes minimal or no genetic material at all.

Tapered head sperm

These sperm cells have heads that are cigar-shaped rather than oval. It is thought that their abnormal shape is the result of a varicocele, which is an enlargement of the veins within the scrotum, or the testicles being constantly exposed to high temperatures from tight clothing, use of saunas or using electronic devices on the lap.

Thin-headed sperm

Thin-headed sperm are a variation of tapered-head sperm, albeit a more extreme one. The heads of these sperm if abnormally thin and long. They are a rare type of morphological defect and are thought to have several underlying causes, including DNA damage, varicocele and disruptions during sperm development.

Globozoospermia (round-headed sperm)

Globozoospermia is rare, occurring in less than 0.1% of cases. It is characterised by sperm cells with round heads that have no acrosome. This causes male infertility as the sperm are unable to break down the outer membrane of an egg cell to then enter and fertilise it.

Acephalic (headless sperm)

These sperm have no head, so they contain no genetic material or chromosomes and are non-functional. This is also referred to as decapitated sperm syndrome. Sometimes, they can be interpreted as pinhead sperm, but on further inspection, it is established that there is no head at all. These sperm are described as looking like a strand of string.

Two-headed sperm

Sperm with two heads are also known as duplicate sperm. They have impaired movement and limited ability to fertilise an egg.

The occurrence of two-headed sperm is linked to exposure to toxic chemicals such as heavy metals and smoke.

One study found that men who have the presence of double-headed sperm in their semen are more likely to also have sperm with other morphological abnormalities.

Sperm midpiece defects

The midpiece of the sperm cell houses mitochondria, which supply the energy that the tail needs to move. Defects affecting the midpiece of the sperm can impact fertility by reducing sperm motility and a sperm’s ability to fertilise an egg.

Sperm midpiece defects can result in:

Irregular shapes
Thickening or swelling
Thinness
Asymmetry
Sharp bends in the midpiece
Small, fluid-filled pockets in the midpiece
Any combination of the above

Abnormalities of the sperm midpiece can be the result of genetic, lifestyle and/or environmental factors.

Sperm tail defects

Tailless sperm

Also known as acaudate sperm, these cells are characterised by the complete absence of a tail. This means that they cannot move to reach an egg for fertilisation. They are typically caused by damage to the cells that produce the sperm.

Multiple tails

Although it might seem like extra propulsion would help, sperm with more than one tail often occur in sperm that also have large heads and abnormal chromosomes, which can affect fertility.

For instance, in one case report, a patient had 100% macrocephalic sperm. Of these, 72% had multiple tails.

Coiled-tail sperm

Coiled-tailed sperm have heavily damaged tails, rendering them unable to swim properly. There are several reported causes of coiled-tailed sperm including:

Imbalanced seminal fluid conditions
The presence of bacteria
Smoking
Varicocele

Stump-tailed sperm

These are sperm with very short, stubby tails, and they often have little to no motility. They can also be referred to as Dysplasia of Fibrous Sheath (DFS) sperm. In many instances, stump-tail sperm are the result of a genetic mutation, meaning that it may be a condition that can be passed on to male offspring.

What is considered a normal sperm morphology percentage?

It may be surprising, but only 4% of sperm in a typical ejaculate need to have normal morphology for the sample to be considered to meet semen analysis parameters. This means that up to 96% of sperm may have at least one structural variation, and that’s completely normal.

Given that the average total sperm number per ejaculate is 39 million or more, this still equates to more than a million normally shaped sperm in a typical semen sample.

When you donate sperm, you’re not just helping someone achieve their dream of parenthood; you’re also learning more about your own reproductive health. Morphology is just one part of the story, and at the London Sperm Bank, we also look at count and motility to get a more comprehensive picture of your sperm health and quality.

If you’re considering donation and would like to understand more about sperm health or the screening process, we’re here to help. Read more about how to become a sperm donor.

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