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Microscopy of Hair Part 1: A Practical Guide and Manual for Human Hairs
Published in Forensic Science Communications, Research and Technology, January 2004 – Volume 6 – Number 1
Douglas W. Deedrick , Supervisory Special Agent, Scientific Analysis Section
Sandra L. Koch, Physical Scientist, Forensic Examiner, Trace Evidence Unit, Federal Bureau of Investigation, Quantico, Virginia
http://www2.fbi.gov/hq/lab/fsc/backissu/jan2004/research/2004_01_research01b.htm
Figure 78.
Photomicrograph
of Pili Annulati
Figure 79.
Photomicrograph
of Lice Egg Case
A double medulla (Figure 80) is encountered on occasion (usually in beard hairs, but also in red Caucasian head hairs). However, unless it is a regularly occurring feature in a sample, it is of little value for individualization.
Figure 80.
Photomicrograph
of Double Medulla
The presence of a dark band (Figure 81) at the root end may indicate it was shed postmortem.
Figure 81.
Photomicrograph
of Postmortem
Root Band
Conclusions
There are three basic conclusions that can be reached from a microscopic examination and comparison of hairs.
The hairs from the questioned (Q) source exhibit the same microscopic characteristics as the hairs in a known (K) hair sample and can be associated to the source of the known hairs.
The hairs from the questioned source are microscopically dissimilar to the hairs in a known hair sample and cannot be associated with the source of the known hairs.
The questioned hairs exhibit both similarities and slight differences to hairs found in a known hair sample, and no conclusion can be reached whether they could have originated from the known source. It may be that, in the opinion of the examiner, the differences are not sufficient to eliminate the source of the known hairs as being a possible source of the questioned hairs. At the same time, the presence of these differences precludes an association being made between the questioned and known hairs.
In the first conclusion, it is stated that the questioned hairs can be associated with the source of the known hairs. Hairs are biological specimens and subject to variation. During the analysis of hair, the examiner must establish the range of variation in the sample, and then determine whether the questioned hair fits in that range. It has been found that when two hair samples are randomly selected from different individuals and compared microscopically, it is very unusual that they cannot be distinguished. However, the possibility cannot be dismissed that there may be two hair samples whose ranges of variation overlap and distinguishing between the samples is not possible.
Report
The information contained in the report should be limited to a factual statement of findings concerning the examinations conducted. An interpretation of the evidence is saved for court testimony that includes an explanation about the basis for the examinations. The report should be clear, concise, and easily understood. Technical terminology that is foreign to a layperson or contributor does not serve a useful purpose. The report should contain information pertinent to the requests made by the contributor of the evidence and to the examinations performed. A listing of the items of evidence and their origin, either a description or the contributor's reference number, should be included. Results of examinations should be set out clearly, followed by a statement of the examiner's conclusions. A statement may follow to clearly state the limiting factors of hair examinations.
Example:
Item 1 = item from crime scene
Q1 – knit cap
Item 2 = item from suspect
K1 – head hair sample from John Doe
Results of examinations: Caucasian head hairs that exhibit the same microscopic characteristics as hairs in specimen K1 were found on the Q1 knit cap.
Accordingly, these hairs can be associated to John Doe, the identified source of the K1 hairs.
Hair comparisons are not a means of absolute personal identification. The statement of results sets forth, fairly completely, those determinations that can be made (i.e., that the hairs came from the head, that they exhibit Caucasian characteristics, and that the Q hairs are consistent with the K hairs in microscopic appearance). The resulting conclusion is that given these results, the Q hairs can be associated with John Doe, the source of the K hairs. The last paragraph is optional and is given so that a reader who is not generally familiar with hair examinations can better understand the limits of hair identification.
The introduction of mtDNA sequencing can result in an addendum to the report:
The comparison of the microscopic characteristics in hairs does not constitute a basis for absolute personal identification. The probative value of hair comparisons may be affected by the results of mtDNA analysis.
It must be understood that microscopy and mtDNA sequencing are two different forensic tools—each providing separate and distinct pieces of information. Two hairs can exhibit the same microscopic characteristics and be shown to be different in mtDNA sequence. Conversely, two hairs that have the same mtDNA sequence can have very different microscopic characteristics, as in the case of two individuals with the same mother.
Testimony
Expert witness testimony should include an education component on hair evidence for the jury and judge and a statement of the results as reported. The witness should be prepared to discuss the procedures used in reaching the conclusion(s) and to defend opinions. An expert witness should endeavor to promote a better understanding of the methods of examination, the theory of the transfer of trace materials, and the strengths and limitations of the science.
Significance and Value
The forensic analysis of hair has been accepted in courts of law for many years, but this does not necessarily validate the science. The reliability of hair examinations must be weighed with the education and training of the examiner, as well as with the procedures used in the analysis. The examinations must be objective and impartial, and the weight placed on the results must be in accordance with the experience and training of the examiner.
Human hair identifications are subjective interpretations of objective criteria. The variability and distribution of the microscopic characteristics are useful in determining whether or not a questioned hair could have originated from a particular individual.
It is recognized that hair comparisons do not constitute a basis for absolute personal identification. Whereas hairs cannot be positively identified as originating from a particular individual, it is unusual to find different people having the same hair characteristics. This is based on evidentiary samples received in casework and on proficiency tests prepared in the laboratory.
Studies (Bisbing and Wolner 1984; Gaudette and Keeping 1973) have been conducted to determine the significance of hair associations. Some of these studies attempted to establish a mathematical probability of a match. The FBI Laboratory does not use the mathematical calculations of other researchers nor does it support the feasibility of establishing a numerical probability of a hair match.
The ability to analyze and interpret hair characteristics is a skill gained by training and testing. New examiners must study hair samples from different racial groups and body areas and take hair-matching tests to demonstrate the ability to correctly associate hairs with a particular source.
Hair associations in casework should be subject to a confirmation process when another qualified examiner reviews the match before a report is issued. The significance of a hair match is dependent on the distinct qualities of the hairs and the experience of the examiner. The greater the number of associations found in casework and the greater the number of cross matches, the greater the chance of association. Mitochondrial DNA sequencing testing on matched hairs has shown a high degree of conformity.
Glossary
Achromat: An objective lens system constructed of glass with limited quality of correction for color and spherical aberration.
Anagen: Actively growing root stage.
Apochromat: A complex objective lens system that produces brilliant images and is corrected to the highest degree for color and spherical aberration.
Catagen: A transitional root stage between the actively growing anagen stage and the resting telogen stage.
Comparison microscope: Two microscopes joined by an optical bridge with a split screen to see both fields of views at the same time.
Cortex: Middle portion of hair extending from the cuticle to the medulla and containing the pigment granules, cortical fusi, and ovoid bodies.
Cortical fusi: Air spaces located in the cortex of hairs.
Cuticle: Translucent outer layer of the hair shaft consisting of overlapping scales.
Fluorite system: An objective system constructed of fluorspar that provides improved color and spherical correction over achromatic lens systems.
Fur hairs: Fine hairs that make up the undercoat of mammals and provide warmth.
Guard hairs: Coarse hairs that provide protection and are usually longer than fur hairs.
Keratin: A fibrous protein forming the chemical basis for hair, nails, and feathers.
Medulla: The central portion of hair, the core area.
Melanin: A pigment that gives hair its color.
Miscible: Able to be mixed together.
Numerical aperture: A calculation that shows the ability of an objective lens to make fine structural detail in the specimen distinct (N.A. = n • sinα).
Oil immersion: A system that uses oils of a known refractive index to test the refractive index of other materials and provide greater resolution than air-mounted specimens.
Ovoid bodies: Dark bodies of unknown origin that are a useful discriminatory characteristic in their pattern of appearance.
Papilla: Connective tissue from which hair is generated from the follicle.
Parfocal: A method of setting up the microscope in order to have similar focal distances for objectives of different magnifications.
Pigment granules: Melanin granules whose size, shape, density, and distribution vary.
Refractive index: The ratio of the velocity of light in air to the velocity of light in a medium.
Resolving power: The ability of the microscope to make visible the individual parts of an image [R = λ/(2 N.A.)].
Scales: Outermost portion of the cuticle, flattened and imbricate in humans, pointing toward the distal end of the hair shaft.
Tactile hairs (Vibrissae): Whiskers only found on animals.
Telogen: Resting stage when the root takes on club shape and is ready to be naturally shed.
Vellus: Fine body hair.
References
Bisbing, R. E. and Wolner, M. F. Microscopical discrimination of twins' head hair, Journal of Forensic Sciences (1984) 29:780-786.
Crocker, E. J. A new technique for the rapid simultaneous examination of medullae and cuticular patterns of hairs, Microscope (1998) 46(3):169-173.
Gaudette, B. D. and Keeping, E. S. An attempt at determining probabilities in human scalp hair comparison, Journal of Forensic Sciences (1973) 19:599-606.
Hicks, J. W. Microscopy of Hairs: A Practical Guide and Manual, Federal Bureau of Investigation, U.S. Government Printing Office, Washington, DC, 1977.
Locard, E. The analysis of dust traces, American Journal of Police Science (1930 )1:276-291.
Ogle, R. R. and Mitosinka, G. A. A rapid technique for preparing hair cuticular scale casts, Journal of Forensic Sciences (1973) 18(1):82-83.
Scientific Working Group on Materials Analysis, Trace evidence recovery guidelines, Forensic Science Communications [Online].
Available: www.fbi.gov/hq/lab/fsc/backissu/oct1999/trace.htm.
____________________________________________
Current link to online article: http://www2.fbi.gov/hq/lab/fsc/backissu/jan2004/research/2004_01_research01b.htm
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