Hair Shaft Cortex
Definition
The main structural layer of the hair shaft, composed of tightly packed keratin filaments. It contains the melanin granules responsible for hair colour, the mtDNA-bearing residual cortical cells, and the keratin matrix into which drugs and toxins are incorporated during growth.
- Location
- Main structural layer of the hair shaft
- Composition
- Tightly packed keratin filaments
- Pigment carrier
- Melanin granules determining hair colour
- DNA type
- mtDNA from residual cortical cells
- Toxicology role
- Incorporates drugs and toxins during growth
Common questions
Why does hair typically yield mitochondrial DNA rather than nuclear DNA?+
The cortex retains only residual cortical cells with degraded nuclear material by the time a hair is shed or pulled, while mitochondrial DNA has a far higher copy number per cell and survives better, so routine hair typing defaults to mtDNA unless a fresh root with follicular tissue is available.
How does the cortex record drug exposure over time?+
Drugs circulating in the blood are incorporated into the keratin matrix as the hair shaft forms, so segments cut along the shaft's length correspond to sequential growth periods and can be tested to build an approximate exposure timeline.
Can cortex analysis distinguish hair colour changes from natural pigmentation?+
Melanin granule distribution and density in the cortex differ between naturally pigmented and artificially dyed hair, so microscopic or chemical examination of the cortex can flag cosmetic treatment.
Related terms
- Anagen Phase
- The actively growing phase of the hair follicle, during which the keratinocyte is in contact with the dermal papilla capillary and incorporates...
- External Contamination
- The presence of a drug or substance on the outer surface of the hair shaft from environmental exposure rather than systemic ingestion....
- Mitochondrial DNA (mtDNA)
- DNA located in mitochondria rather than the cell nucleus. Present in hundreds to thousands of copies per cell, making it recoverable from...
- Segmental Hair Analysis
- The practice of cutting a hair strand into consecutive segments (typically 1 cm each) and analysing each segment separately. Because hair grows...
- Short Tandem Repeat (STR)
- Short repetitive DNA sequences scattered throughout the genome, with the number of repeats varying between individuals at each locus. DNA profiling compares...