Semen and sperm as evidence

DEFINITIONS: Semen is a viscous fluid that results from a combination of secretions from the seminiferous tubules and glands such as the prostate, seminal vesicles, and Cowper’s glands. Spermatozoa (sperm) are reproductive cells carrying male genetic material; they are uniquely found in the fluid of semen.

SIGNIFICANCE: Semen is one of the most commonly analyzed types of biological evidence collected during criminal investigations, particularly cases involving sexual assault. DNA analysis of semen samples can link suspects to crimes or eliminate innocent persons from suspicion.

In 1677, Dutch scientist Antoni van Leeuwenhoek became the first person to report the observation of spermatozoa under a microscope. Sperm, the reproductive cells that carry male genetic material, are morphologically distinctive in that they are characteristically elongated cells divided into regions called head, midpiece, and tail. The sizes of spermatozoa and shapes of the heads vary among species; in human spermatozoa, the heads are oval in shape.

Composition of Semen

General ranges are known for how many spermatozoa are found in a given volume of semen and the amount of semen typically produced. This information can be applicable in the strategies used for testing biological and for reconstructing events at a crime scene. It can also influence the approach taken to extract and analyze DNA (deoxyribonucleic acid) from a semen stain.

Other cells in semen are epithelial cells from the linings of the male reproductive tract. Although these make up less than 1 percent of the volume of semen, they can be sufficient to make DNA typing possible for semen lacking spermatozoa due to congenital conditions or vasectomy. The spermatozoa are usually the main source of DNA in semen that is tested forensically to provide evidence for the source of the semen. It is of forensic importance that spermatozoa DNA can be extracted under conditions different from those used for other types of cells, because this means that a semen component of DNA can be separated from female DNA, thus making it easier to identify the origin of the semen.

The liquid portion of semen is called seminal plasma; it contains certain organic substances in relatively high concentrations, including ascorbic acid, citric acid, fructose, prostaglandins, phosphoryl choline, spermine, and spermidine. Zinc is an element present in particularly high concentrations in semen relative to other body fluids. Certain proteins are found in higher concentrations in semen than in other sources; the most useful of these in terms of forensic testing are prostatic acid phosphatase, prostate-specific antigen (PSA), and semenogelin.

Detection of Semen and Sperm

As a biological stain, semen survives as evidence detectable by biochemical and DNA typing techniques for several years. The survival of DNA in semen stains has allowed for the analysis of evidence in items more than ten years old. Under a forensic lamp that emits blue light, semen stains and some other body fluids fluoresce, which is helpful in their detection at crime scenes or on evidence such as clothing. Additional tests may then be performed to determine whether given stains are semen.

A commonly used presumptive, or screening, test for semen looks for activity of the enzyme acid phosphatase in stains. Acid phosphatase is found at levels four hundred times higher in semen than in other body fluids. In the presence of acid phosphatase, an acid solution of fast blue and alpha-naphthyl phosphate will turn blue-purple. An investigator may sample a stain by rubbing it with a swab or moist piece of filter paper and then adding a drop of the acid solution to the swab or paper; a positive result produces a blue or purplish color within thirty seconds. Where no stains are visually apparent, an area containing a suspected semen stain can be divided into sectors for systematic searching using the acid phosphatase detection technique. This method is not a conclusive indication of the presence of semen, however; other approaches must be used to identify semen definitively for a court of law.

According to a 2021 article in the journal Analytical and Bioanalytical Chemistry, British researchers discovered a method of adding a fluorescent genetic tag to sperm cells that can make them “light up” under a microscope. This method gave scientists a new tool in identifying sperm cells in forensic research.

One confirmatory test for semen relies on the special staining and microscopic observation of spermatozoa. The sample can be taken from a crime victim or from a stain on material such as cloth that has been extracted in water after gentle stirring. A drop of the extract is placed on a microscope slide, processed to cause adherence of any cells to the slide, and stained with picroindigocarmine and fast red. This procedure characteristically stains spermatozoa heads red and midpieces a green to yellow color, leading to the name “Christmas tree stain.”

Spermatozoa can remain alive and moving for four to six hours in the living female body. Intact nonmotile spermatozoa may still be found in the female body up to six days after deposition.

After certain periods of time, nonmicroscopic techniques are not useful in the identification of stains as semen. Another limitation of microscopic techniques is that they do not detect semen that does not contain sperm, such as the semen produced by vasectomized males. In cases where semen stains are aged or suspected to be from vasectomized males, immunological techniques may be used that detect substances that are highly abundant or relatively unique to semen and present in semen from both vasectomized and nonvasectomized males. The most widely used confirmatory immunological test for semen uses antibodies that are specific for PSA. The test format is a type of cassette called a lateral flow immunochromatographic assay. PSA is found in highest abundance in semen, so that at levels detectable in semen, other body fluids are apparently negative for PSA content.

Another immunological test that works by the same principles detects the presence of a protein called semenogelin, which has not been found in any other biological fluids and tissues tested. Semenogelin also appears to be unique to humans and perhaps other higher primates and therefore is potentially the most specific immunological test for semen available.

Bibliography

Baechtel, F. S. “The Identification and Individualization of Semen Stains.” In Forensic Science Handbook, edited by Richard Saferstein. Vol. 2. Englewood Cliffs, N.J.: Prentice Hall, 1988.

Gaensslen, R. E. Sourcebook in Forensic Serology, Immunology, and Biochemistry. Washington, D.C.: National Institute of Justice, 1983.

Jones, Edward L. “The Identification of Semen and Other Body Fluids.” In Forensic Science Handbook, edited by Richard Saferstein. 2d ed. Vol. 2. Upper Saddle River, N.J.: Prentice Hall, 2005.

Nash, Jay Robert. Forensic Serology. New York: Chelsea House, 2006.

"New Method to Detect and Visualise Sperm Cells Recovered From Forensic Evidence." King’s College London, 19 Aug. 2021, www.kcl.ac.uk/news/new-method-to-detect-and-visualise-sperm-cells-recovered-from-forensic-evidence. Accessed 16 Aug. 2024.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.