Sexual Assault
Kim A. Collins, MD
Detection of spermatozoa can be tedious and difficult. Such detection is scientific proof of sexual contact and extremely important in courts of law. Equally important is not overcalling a microscopic findings resulting in a false positive. Certain entities that have been mistaken for spermatozoa include mucus threads, naked epithelial and inflammatory cell nuclei, and yeast. Depending on the stain used (Papanicolaou, Giemsa, or nuclear fast red-picroindigocarmine), the staining characteristics will aid in differentiating true spermatozoa from artifact.
The spermatozoa are positive proof of sexual contact and their presence is strong evidence for the prosecution of any sexual assault case. Spermatozoa, a free-swimming cell, were first described by Leeuwenhoek in the 17th century. The basic structure is composed of a head, neck, midpiece, and tail. The head is approximately 4-5 um (length) x 2-3 um (width) and has an anterior acrosome which contains enzymes needed to penetrate the ovum during fertilization. The head is packed with DNA which determines its staining characteristics. The head is connected by a neck to a midpiece (7-8 um in length) and a tail (50-55um in length). Spermatogenesis occurs in the testes, spermatozoa are stored in the epididymis, and then carried via the vas deferens to the urethra during ejaculation. Spermatozoa are extremely motile and this motility can be observed if the specimen is collected soon after ejaculation. In reported studies of living victims, after three hours only 50% of the smears will have motile sperm. Generally, motile spermatozoa are not found in cervicovaginal smears after 12 hours. Spermatozoa are sensitive to the environment and begin to degenerate within hours after ejaculation. The first obvious sign of degeneration is loss of the tail. In living victims, this loss occurs after approximately 16 hours. Nonmotile spermatozoa and spermatozoan heads can be detected in cervicovaginal smears days after intercourse; however, with time, the positive recovery rate is low. After 72 hours, only 50% of cervicovaginal smears taken from living victims will have any spermatozoa present. Spermatozoa are rarely found in cervicovaginal smears after 10 days. In the rectum, spermatozoa seem to loose their tails more quickly, after 6 hours. Successful recovery of spermatozoa in the rectum can occur up to 24 hours after ejaculation and has been reported up to 65 hours. However, these positive findings are usually only the spermatozoan heads. Recovery from the oral cavity requires an even shorter time interval of approximately 6 hours. This decreased survival is due to the action of saliva and bacteria in the mouth. Of course, the positive identification of spermatozoa in the vagina/anorectum/mouth may be detrimentally affected by douching, urinating, oral contraceptive pills, defecation, mouth washing, and drinking respectively. Prepubertal young girls also have decreased spermatozoal survival in cervicovaginal specimens due to a decreased cervical mucus.
Spermatozoa apparently survive longer in deceased individuals. At the postmortem examination, motile spermatozoa can be sought by a saline wet mount. In a current study of deceased individuals, spermatozoa were found in the vagina up to 5 days (intact), 7 days (intact and heads), and 2.5 months (heads only) with averages of 23 hours, 38.4 hours, 28 hours respectively. Spermatozoa were detected in the rectum up to 17 hours(intact), 15 hours (intact and heads), and 48 hours (heads only) with averages of 17 hours, 12.5 hours, and 28 hours respectively. A major variable is the environmental temperature, especially in certain geographic locales. Cool temperatures preserve the spermatozoa. In rare cases where the body is frozen, spermatozoa may be found for longer periods. Reported examples include: 16 days in a cold temperature, spermatozoa were found; after 85 hours, spermatozoa recovered; after a 3 month PMI, spermatozoa recovered, conditions not documented. Furthermore, decomposition and embalming do not preclude finding spermatozoa. In fact, the embalming tends to preserve spermatozoa. In one reported case, 30 hours after embalming, spermatozoa were detected.