13.4: Semen and Other Bodily Fluids

Although bodily fluids like semen, vaginal secretion, sweat, and saliva are most common identifiers of suspects and victims in criminal investigations involving violent sexual assaults, the importance of this evidence is not confined to investigations that are sexual in nature. Your author once identified a take-over robbery suspect by a small sweat stain left on a shelf in the bank vault. The sweat had likely trickled down to the inner wrist from a sweaty palm that had spent too much time in a latex glove. Therefore, we will start with sweat.

Sweat, or perspiration, is a saline liquid produced in the glands of humans and other animals. The purpose of sweat is to cool the body through evaporation. Although it is approximately 99% water, it has trace amounts of salt, fat, and other organic compounds such as lactic acids and urea. Although it is important to understand that human sweat glands do not secrete human cells, DNA can transfer to sweat through contact with the microbiota of the skin or as free-floating nucleic acid fragments. Microbial DNA is derived from the bacteria, archaea, and viruses that are found on the skin and are transferred to sweat as it passes through. Sweat is the main contributor of touch-DNA. Although the DNA in sweat is very low in quantity, laboratory technicians can replicate the DNA through Polymerase Chain Replication techniques.

Semen, or ejaculum, is a complex biological mixture of spermatozoa, containing reproductive cells called gametes, and seminal fluid, a liquid substance that is secreted by seminal vesicles, prostate gland, and bulbourethral glands and provides the cells with nutrients and safely transports the semen to successfully fertilize the egg. Seminal fluid itself does not contain DNA. DNA is found in the head of the sperm. It is densely condensed and rich in genetic material.

Vaginal fluids are a mixture of secretions from the cervix. They are composed of transudate, through mucous membranes, and skin cells that have been shed. The shed skin cells contain cellular DNA, as well as bacterial DNA. Vaginal secretions can also be a source of DNA from menstrual blood. Commonly in sexual assault investigations, vaginal fluid will be mixed with seminal fluids and therefore contain DNA from booth or multiple contributors.

Saliva is an even more complex biological fluid produced by salivatory glands in the head and mouth. These glands are the parotid, located just in front of the ears, the sublingual, located under the tongue, and the submandibular, which are below the jaw. Saliva, or spit, is 99% water along with over 1,000 proteins including electrolytes, mucus, white blood cells, epithelial cells, enzymes, peptides, and antibodies. Saliva also contains organic components like urea, glucose, cortisol, sex hormones, and the occasional gnat that managed to get in when you were yawning. Epithelial and white blood cells provide the primary sources of DNA in saliva. Mucus is the slippery fluid that coats the mucus membrane. It is primarily composed of water and mucin glycoproteins, but it also contains human cells, including white blood cells, that are a good source of DNA.

Probably the least said about urine and fecal matter the better, other than to say they are excretion and egestion functions of the human body. The question your author is always asked is feces a good source of DNA. The answer is yes, abundantly so. Urine is also a good source of DNA owing to the presence of white blood cells, epithelial cells, and other small fragments of DNA passed through the bloodstream. However, it must be said that DNA from urine and feces will degrade very quickly and therefore analysis must be done either immediately or it is essential to freeze the sample before typing.