Microscopical Analysis of Hair

Vocabulary flashcards covering the morphology, ultrastructure, growth cycles, somatic origins, and forensic identification features of human and animal hairs.

Proximal end

The end of the hair shaft situated closest to the root and hair follicle bulb.

Distal end

The end of the hair shaft pointing toward the tip, farthest from the root.

Cuticle

The outermost layer of the hair shaft composed of superimposed, flattened cells with scale edges oriented toward the distal tip, providing chemical and structural protection.

18-methyleicosanoic acid

The major fatty acid constituent forming a monolayer of approximately $3\,\text{nm}$ on the hair cuticle surface, covalently attached via thioester bonds to underlying proteins.

Exocuticle

The outer subcomponent of a cuticle cell, consisting of an amorphous region rich in keratin and keratin-associated proteins (KAP), alongside a cysteine-rich a-layer.

Endocuticle

The amorphous inner subcomponent of a cuticle cell derived from the remnants of the cell nucleus and cytoplasmic organelles.

Cortex

The main body of the hair shaft composed of spindle-shaped cortical cells ($80\text{ to }115\,\mu\text{m}$ in length) aligned along the longitudinal axis, containing pigment granules.

Unit Length Filaments (ULF)

Intermediates in intermediate filament assembly formed by $32\text{-mers}$ ($8\times$ tetramers) that elongate into keratin intermediate filaments (KIF).

Orthocortical cells

Cortical cells possessing the highest fibril-to-matrix ratio; they are longer ($\sim 89\,\mu\text{m}$) and position along the outside of the curvature in curved hair fibres.

Paracortical cells

Cortical cells with a lower fibril-to-matrix ratio; they are shorter ($\sim 73\,\mu\text{m}$) and position along the inside of the curvature in curved hair fibres.

Eumelanin

A melanin pigment responsible for black and dark brown coloration in hair.

Pheomelanin

A melanin pigment responsible for yellow to reddish coloration in hair.

Trichohyalin

The primary protein component of the hair medulla that undergoes citrullination to convert arginine residues into citrulline.

Medulla

The central canal running through the core of thick hair shafts, composed of spongy keratin and air-filled vacuoles.

Inner root sheath (IRS)

The follicular sheath structure surrounding the developing hair shaft, composed of three concentric layers: the IRS cuticle, Huxley layer, and Henle layer.

Anagen phase

The active growth phase of hair lasting up to $6\text{ years}$, encompassing $80\text{ to }95\%$ of human hairs, with growth rates around $400\,\mu\text{m/day}$.

Catagen phase

The regression phase of the hair growth cycle characterized by a reduced growth rate and follicle breakdown, lasting approximately $1\text{ to }2\text{ weeks}$.

Telogen phase

The resting phase of the hair growth cycle lasting $2\text{ to }3\text{ months}$, during which the hair is fully retained but does not grow, comprising $5\text{ to }15\%$ of total human hair.

Exogen phase

The phase of the hair cycle corresponding to the shedding or fall of hair, typically amounting to $50\text{ to }150\text{ hairs/day}$ in humans.

Kenogen phase

A physiological period of follicle emptiness between telogen hair loss and the initiation of a new anagen cycle, associated with balding.

Lanugo hair

The fine, unpigmented first hair produced by fetal hair follicles from $16\text{ to }20\text{ weeks}$ through $8\text{ months}$ of gestation.

Vellus hair

Short, fine, thin hair that develops across the body during childhood.

Terminal hair

Coarse, pigmented hair that emerges during puberty under hormonal control, typically found on the scalp, beard, axillae, and pubic region.

Scale casting

A microscopic preparation method in which a hair is impressed into a setting medium (such as clear nail polish) to produce an imprint of its cuticle scale pattern.

Imbricate scales

Flattened, overlapping cuticle scales with smooth or serrated margins characteristic of human hairs and certain animals such as raccoons and bears.

Coronal scales

Crown-shaped cuticle scales that completely encircle the hair shaft, characteristic of small mammals like bats.

Spinous scales

Petal-shaped, triangular cuticle scales that protrude distinctly from the hair shaft, commonly found in cat hair and mink hair.

Medullary index (MI)

The ratio of the diameter of the medulla to the overall shaft diameter, typically measuring $< \frac{1}{3}$ in humans and $> \frac{1}{2}$ in animals.

Buckling

Abrupt twisting and diameter variation along the hair shaft, serving as a diagnostic somatic characteristic of pubic hair.

Postmortem root band

An opaque, dark band formed near the root of hairs shed from a decomposing body after death.

Follicular tag

A translucent piece of sheath tissue attached to the root of a hair that has been forcibly removed in the anagen phase, suitable for nuclear DNA analysis.

Ovoid bodies

Large, oval-shaped pigment structures situated within the hair cortex that are distinctly larger than normal melanin granules.

Cortical fusi

Small, elongated air spaces or intercellular cavities located in the hair cortex, predominantly found near the proximal root end.

Guard hair

The coarse, protective outer hair of an animal's coat, useful in forensic examination due to its diagnostic cross-sectional and medullary features.

Wineglass-shaped root

The diagnostic root morphology characteristic of hairs belonging to members of the deer family (Cervidae) and antelope.

Spade-shaped root

The distinctive root morphology observed in hairs from domestic dogs.