Semen and Sperm
Semen and sperm (Forensic Science)
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.
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Composition of Semen (Forensic Science)
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 evidence 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...
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Detection of Semen and Sperm (Forensic Science)
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.
One confirmatory test...
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Further Reading (Forensic Science)
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. Discusses semen identification and the use of classical serological techniques for analyzing whether a semen stain originated from a given person.
Gaensslen, R. E. Sourcebook in Forensic Serology, Immunology, and Biochemistry. Washington, D.C.: National Institute of Justice, 1983. Presents a comprehensive review of the fundamentals and practice of forensic serology relevant to semen evidence.
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. Discusses the forensic science techniques used in relation to semen evidence.
Nash, Jay Robert. Forensic Serology. New York: Chelsea House, 2006. Provides background information on the use of serology in the forensic sciences and discusses various techniques used in the handling of semen evidence.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007. Introductory textbook includes a chapter that provides a general introduction to serology and discusses semen analysis.
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Semen and Sperm (World of Forensic Science)
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.