Hair Anatomy and the Growth Cycle

The hair shaft is composed entirely of dead, keratinised cells arranged in three distinct layers that can be visualised by transmitted light microscopy. Each layer has a different structural role and provides different information for forensic comparison.

The cuticle is the outermost layer, one cell thick, formed from flat scale cells that overlap like roof tiles with their free edges pointing toward the tip of the hair. In human hair the scales are generally smooth-edged and tightly overlapping, a feature that distinguishes human from most animal hair where the scales are more prominent and widely separated. The cuticle protects the cortex from mechanical and chemical damage. Damage patterns in the cuticle, caused by heat, bleaching, or physical abrasion, can indicate the treatment history of a hair and assist in distinguishing hairs from different sources.

The cortex forms the bulk of the shaft. It consists of spindle-shaped cortical cells packed with keratin fibrils and, in pigmented hair, melanin granules. The two types of melanin are eumelanin (brown to black) and phaeomelanin (yellow to red), and their ratio determines hair colour. Forensic examiners assess pigment granule distribution (peripheral, central, or even), density (sparse, moderate, dense), and morphology (fine, coarse, clumped) as comparison characteristics. The cortex also contains ovoid bodies and cortical fusi, small air-filled spaces whose pattern is individual-specific enough to be useful in microscopic comparison.

The medulla is the innermost structure, a column of loosely packed or hollow cells running along the central axis. Its pattern is classified as continuous (an unbroken column), interrupted (broken into segments), fragmented (short disconnected sections), or absent. The medullary index, the ratio of medulla diameter to shaft diameter, is routinely above 0.5 in animal hair and typically below 0.3 in human hair. Medullary pattern combined with scale morphology is the primary basis for distinguishing human from non-human hair at the microscopic level, a task also supported by the wildlife forensics discipline.

The follicle is a tubular invagination of the epidermis that extends into the dermis and, in some regions, into the subcutaneous fat. Its lowest portion, the hair bulb, is the site of active cell division and shaft production. Understanding follicle anatomy is essential for interpreting what is present at a hair root and whether nuclear DNA can be recovered.

The dermal papilla is a small connective tissue structure at the base of the bulb, indented into it like a finger pushing into a balloon. It is richly vascularised and contains fibroblasts that produce the signalling molecules, including Wnt ligands and fibroblast growth factors, that control follicle cycling. The papilla does not divide but directs the matrix cells above it.

The matrix is the proliferating cell population surrounding the papilla at the base of the bulb. Matrix cells divide rapidly, differentiate upward, and gradually keratinise as they move toward the skin surface to form the shaft and root sheaths. These are the nucleated cells that provide nuclear DNA when a hair is in anagen. The inner root sheath forms a sleeve around the growing shaft and is the source of the follicular tag tissue when a hair is forcibly removed.

Above the bulb, the follicle has a middle segment containing the sebaceous gland duct and, in body hair, the arrector pili muscle attachment. The infundibulum, the uppermost segment, is the portion of the follicular canal that opens at the skin surface. Sebum from the sebaceous gland coats the emerging shaft and contributes to the chemical profile detectable on shed hairs.

Hair follicles cycle through three phases independently of one another. At any given time on a healthy human scalp, approximately 85 to 90 percent of follicles are in anagen, 1 to 2 percent in catagen, and 10 to 15 percent in telogen. This asynchronous cycling means that normally shed hairs from the scalp are predominantly telogen hairs.

Anagen lasts two to six years for scalp hair, which is why scalp hair can grow to a substantial length before shedding. The duration varies by body site: eyebrow anagen lasts only three to five months, which is why eyebrow hairs stay short. During anagen the matrix cells at the bulb divide at a rate of about 1 mm of shaft per three days. The root in anagen is deeply anchored in the dermis, surrounded by the inner and outer root sheaths, and strongly attached to the follicle, making forcible removal painful and leaving cellular material behind.

Catagen is a transitional phase of two to three weeks. Cell division ceases, the lower follicle involutes, and the bulb shrinks as the dermal papilla retracts upward. The keratogenous zone migrates up to form the club root. Catagen hairs are rare in forensic samples because the phase is brief and hairs in this phase are transitional between the deeply anchored anagen state and the loosely attached telogen state.

Telogen lasts two to four months. The follicle is quiescent, and the club root, a rounded, non-pigmented, non-sheathed structure, sits loosely in the upper follicle. Telogen hairs shed passively through normal grooming, brushing, or contact with surfaces. Because they detach without force, they leave no sheath material at the root. The telogen hair found at a crime scene is the most common hair type encountered in casework.

The forensic significance of growth phase reduces to a single question: does the hair root carry nucleated cells? If yes, nuclear DNA can be extracted and short tandem repeat (STR) profiling can individualise the contributor with high statistical discrimination. If no, only mitochondrial DNA is available, and that DNA can include or exclude a maternal lineage but cannot identify a specific person.

An anagen hair pulled from the scalp carries matrix cells and inner root sheath cells in the root zone. These cells are nucleated, and each nucleus contains the full diploid nuclear genome. A single anagen root with intact sheath can yield sufficient nuclear DNA for full STR profiling using current low-copy number techniques. The presence of a follicular tag, translucent sheath tissue visible to the naked eye as a gelatinous coating on the proximal few millimetres of the root, substantially increases the nuclear DNA yield because it adds a larger population of nucleated epithelial cells.

A telogen hair that has shed naturally carries no living nucleated cells at its root. The shaft cells are fully keratinised and their nuclei have degraded. However, shaft cells do contain mitochondria, and mitochondria contain multiple copies of a circular genome that is far more abundant per cell than nuclear DNA. Mitochondrial DNA extracted from the shaft or root of a telogen hair can be sequenced at two hypervariable regions (HV1 and HV2, and increasingly HV3) of the control region, generating a haplotype that can be compared to a reference sample. Because all maternally related individuals share the same mtDNA haplotype, an mtDNA match cannot individualise: it supports the hypothesis that the hair shares a maternal lineage with the reference contributor but cannot exclude maternal relatives.

The distinction also matters for touch DNA and trace deposits covered in touch DNA and trace biological material: a hair with a follicular tag found on a victim's clothing is stronger evidence than a shaft fragment with no root tissue, even if both items are the same species and hair colour.

Before DNA analysis became routine, microscopic hair comparison was the primary tool for associating a questioned hair with a potential source. An examiner using a comparison microscope would assess 20 to 30 morphological characteristics including shaft diameter and its variation, pigment granule distribution and density, medullary pattern and index, cortical fusi distribution, scale pattern, and root morphology, then render an opinion that the questioned hair was consistent with the reference sample, dissimilar, or inconclusive.

Large-scale review projects in the United States and United Kingdom have demonstrated that microscopic hair comparison evidence was overstated in many historical cases. The Federal Bureau of Investigation (FBI) review initiated in 2012 found that FBI examiners had provided testimony that exceeded the scientific limits of the technique in a significant proportion of cases reviewed. Many convictions based substantially on hair comparison evidence are being re-examined. This does not mean microscopic comparison has no value: it remains a valid screening tool for excluding hairs from dissimilar sources and for directing limited DNA resources. It means that microscopic comparison cannot individualise a hair to a specific person, and testimony claiming it can is unsupported.

Modern practice in laboratories governed by accreditation standards such as ISO 17025 treats microscopic comparison as a preliminary step that informs the decision to submit a hair for DNA analysis. The National Commission on Forensic Science (NCFS) in the United States and the Forensic Science Regulator in England and Wales have both issued guidance requiring that microscopic hair comparison reports acknowledge the absence of a validated statistical framework for the technique. Similar guidance has been adopted by the European Network of Forensic Science Institutes (ENFSI).

Hair is among the most physically durable biological evidence types. The keratinised shaft resists enzymatic degradation and can survive decades in dry conditions, which is why hair has been recovered from mummies, archaeological burials, and cold cases. However, DNA within hair, both nuclear DNA in roots and mitochondrial DNA in shafts, degrades with time, humidity, UV exposure, and microbial activity. Collection and packaging decisions directly affect the quality of DNA that can be recovered.

At a scene, individual hairs should be collected with clean forceps, placed in paper folds (not sealed plastic bags), and stored cool and dry. Plastic packaging traps moisture and accelerates fungal growth and hydrolytic DNA degradation. Each hair should be packaged separately to prevent cross-contamination between items from different locations. Hairs collected from a victim by combing, or taken as reference samples from a known source, should be documented with the collection site, number of hairs, and phase if determinable.

In casework, a hair is first examined visually and then by low-power microscopy to determine species, body region, and root phase. This examination is non-destructive and completed before any DNA extraction. If DNA analysis is planned, the examiner will wash the shaft to remove surface contamination, cut the root portion from the shaft, and submit them to separate DNA workflows. The shaft goes to mtDNA sequencing; the root, if nucleated, goes to nuclear STR profiling. This parallel approach maximises information recovery from a single item.

Environmental degradation affects nuclear DNA faster than mitochondrial DNA because the nuclear genome has a single copy per haploid set while mitochondria contain hundreds to thousands of genome copies per cell. This copy-number advantage means that mtDNA sequencing succeeds from samples where nuclear STR profiling has failed. The relative success rates inform triage decisions in cold case laboratories and are covered in depth in the broader forensic biology curriculum under the forensic biotechnology subject area.