Semen and Sperm

The presence of sperm and semen can be important to a crime investigation. The visual detection of semen can provide evidence in the case of a suspected rape. As well, the genetic material present in the sperm can be analyzed and used as a genetic fingerprint to identify a suspect.

In the male, semen is the fluid expelled during ejaculation. In addition to plasma, the semen ejaculate contains secretions from the seminal vesicles and other glands to support and nourish the living sperm cells (spermatozoa) contained within the semen. Sperm cells are haploid sex cells of the male. Unlike eggs (oocytes and the mature ovum) that are large, non-motile, and generally ovulated one at a time, sperm are tiny, motile, and produced in the millions. While a human sperm contains a relatively long tail (flagella), the volume of an entire sperm, tail and all, is only 1/85,000 of the mature ovum.

Motility of the sperm is due to the long tail, which is a modified flagellum. Cilia and flagella, from protozoa through humans, all have a similar structure that has been intensively investigated since first described in early electron microscope studies. Microtubules that run the length of the sperm tail are arranged in a ring of nine pairs surrounding a pair in the center. Ciliary dynein is associated with each of the nine microtubule pairs. It is the interaction of the dynein with the microtubules which causes flagellar bending and thus propulsion.

It is estimated that a quarter of a billion sperm are released in a single ejaculate of semen in a healthy male human. In addition to a nutrient function, the semen plays an important role in thermal and hydration regulation that promotes viable sperm cells. The semen also provides initial protection against the acidic gradient of the vagina and cervical region.

In a forensic examination, semen can be detected by the presence of the enzyme acid phosphatase. Because this enzyme is present elsewhere in the body, however, the test is not absolute proof of the presence of semen on clothing or in material recovered in a case of suspected sexual assault. But, detection of acid phosphatase is powerful circumstantial evidence , and indicates that further efforts should be made to investigate the possibility that semen is present.

The microscopic detection of sperm is much more conclusive. The chance of recovering intact sperm is less when a sample is older, due to decomposition of the biological material. However, samples that are analyzed soon after collection can be positive for sperm.

Semen can be visualized on clothing and other surfaces using an ultraviolet light. The semen fluoresces under ultraviolet illumination. This test has the advantage of being non-destructive to the scene of the investigation.

see also Crime scene investigation; DNA; DNA profiling; Luminol; Rape kit.