Hair Anatomy and the Growth Cycle

The cuticle forms the outer covering of overlapping keratinised scale cells. Its edges generally point toward the tip. Human hair commonly shows an imbricate pattern, but patterns overlap with other mammals. A smooth scale pattern alone is not a conclusive human-species test.

The cortex forms most of the shaft and contains keratin-rich structures and pigment. Eumelanin and pheomelanin contribute to colour, alongside pigment distribution, hair structure and treatment. Cortical fusi and other internal features can be described but are not established person-specific identifiers.

The medulla may be continuous, interrupted, fragmented or absent and can vary along a hair. Medullary width relative to shaft width can help some assessments, but a single cutoff such as one-third or one-half does not reliably classify every human or animal hair. Use multiple features and suitable references.

Physical survival is not the same as molecular preservation. An apparently intact shaft can have fragmented DNA, while visible weathering does not guarantee failure of every method. Document relevant condition before consuming or modifying the exhibit.

The follicle is a skin structure that produces and supports hair. In anagen, proliferating matrix cells near the dermal papilla give rise to the shaft and inner root sheath. The papilla supplies signals and has a vascular supply; the emerging shaft itself does not contain blood vessels.

The inner and outer root sheaths, connective tissue, sebaceous gland and associated structures have different roles. Matrix cells differentiate and keratinise as the shaft forms. The upper follicle includes the infundibulum opening to the skin surface; the bulge region contains important stem-cell populations.

A recovered hair root is not the entire follicle. Tissue adhering to a root, often described as a follicular tag, can contain useful nucleated cells. Its amount and preservation can influence conventional nuclear-DNA yield, but its presence does not guarantee a complete STR profile.

Root tissue and morphology may be consistent with particular growth stages or removal mechanisms. They do not independently establish the force, intention, pain or timing of removal. Normal grooming, collection and other handling histories must be considered where relevant. Avoid stating that every attached sheath proves anagen or violent pulling.

Anagen is active growth. Scalp follicles often remain in this phase for years, while other body sites have different cycles. Matrix activity produces shaft material, and the lower follicle is developed. Scalp growth is often approximated at about one centimetre per month, but varies between people, follicles and circumstances.

Catagen is regression, generally lasting weeks in scalp hair. Proliferation stops and the lower follicle changes through a coordinated process that includes formation of a club root. It is not simply a miniature anagen phase with identical cellular targets.

Telogen is rest, commonly lasting months for scalp follicles. The club hair can remain retained. Exogen describes its release and can overlap a new growth cycle; shedding is not simply the instant when every telogen follicle restarts anagen. Growth-cycle proportions and durations are approximate biological descriptions, not forensic clocks.

A club-shaped root can support a telogen-type assessment, while a developed pigmented root with tissue may support an anagen-type assessment. Absence of a root on a fragment prevents that direct assessment. A broken end must not be described in one paragraph as rootless and in the next as an observed club root.

Nucleated adhering tissue can provide relatively intact nuclear targets for conventional STR analysis. The actual quantity, quality, contamination history and assay capability determine the result. A single root with tissue can be useful, but there is no guaranteed low-copy protocol that makes every such hair individually identifiable.

Keratinisation destroys or alters nuclei and organelles as the shaft forms. Nevertheless, fragmented nuclear DNA can remain in rootless hair. Published research demonstrates this, and suitable sequencing methods may recover information when conventional STR typing is uninformative. Growth phase therefore does not create a binary nuclear-versus-mitochondrial rule.

Mitochondrial methods have long been useful for hair shafts. The assay may examine defined control-region intervals or the whole mitochondrial genome. Report the compared range, quality and interpretation. A compatible sequence may be shared by maternal relatives and unrelated people; it does not prove membership of a particular maternal family.

Heteroplasmy and mutation can produce differences among tissues or relatives, so “all maternal relatives have identical mtDNA” is too absolute. Conversely, a matching sequence does not identify the donor uniquely. Statistical assessment uses appropriate data and the actual method, not a fabricated frequency attached to a hypothetical hair.

A tag-bearing hair is not automatically stronger evidence of an alleged activity than a rootless fragment. Analytical information and relevance to the event are separate. Transfer route, background and persistence still matter even when a donor comparison is highly informative.

Microscopy can document morphology, assess whether material is hair-like, compare questioned and reference material and help select samples for further examination. Different methods reveal different features: transmitted-light examination, scale impressions, cross-sections and surface imaging are not interchangeable views.

Reference material should represent relevant within-person or within-species variation. One reference hair cannot establish the full range from a person or animal. Body site, treatments, age of reference collection and damage can affect comparisons.

The US Department of Justice and FBI review of historical microscopic hair work documented statements exceeding scientific limits. Similarity must not be presented as unique individualisation, assigned an unsupported numerical rarity or strengthened by an examiner’s personal case count as though it were a population study.

Human ancestry or racial classification from hair appearance is limited and potentially misleading. Describe the observed features rather than assigning an unsupported geographic origin. Molecular testing can add different evidence, but must retain its own limits and must not be treated as confirmation of an overconfident morphological identification.

Record where a hair was recovered and whether it was loose, attached to an item or obtained by a collection method such as tape lifting. Appropriate clean forceps or other validated methods can minimise loss. Preserve exhibits from different locations separately, with documented identifiers and handling.

A paper fold inside a suitable sealed outer container can retain small hairs. The package must prevent loss and cross-transfer and suit the specimen’s condition. Wet material requires an appropriate plan; a blanket paper-versus-plastic slogan does not replace a full packaging procedure.

Agree the examination sequence before mounting, washing, cutting or consuming the hair. Microscopic preparation and decontamination procedures can affect later DNA or chemical work. Washing can remove external material that is itself relevant, so it is not an automatic step applied identically to every exhibit.

Root and shaft testing may be planned separately when sufficient material and the case question justify it. Preserve orientation, length and the portions tested. The method should be selected from actual capability and specimen condition, rather than automatically sending every root to STRs and every shaft only to mitochondrial testing.

The record should distinguish observations, inferred growth stage, genetic results and any activity assessment. Collection authority and reporting duties vary by jurisdiction. Preserve useful teaching about anatomy without adding unsupported country-specific admissibility cases.