Fundamental to life, keratin abundant tissue has provided clothing, shelter, adornments, tools and utensils over millennia. Keratin is the key component of our skin, hair and nails. Its protein building blocks have amino acid chains, coiled, cross-linked and classified as either hard or soft. It separates us from our environment by forming thin and flexible skin, strong hair and rigid nails. Even embryonic teeth have a keratin layer!
Internal linkages provide keratin with its stability, enzyme resistance and insolubility (although it can absorb water). Skin, hair and nails are multi-layered, maturing from a nail matrix, hair bulb or epidermal basal layer. Keratinocytes leave their germinal base, change shape, and begin producing and accumulating keratin. They then harden and become flexible, flat and water resistant corneocytes. This cornification differs between each structure, but general processes and purposes are shared. Finally, keratin producing cells die. Keratinised tissue is avascular.
The primary characteristic of keratin tissue is its appearance of overlapping scales. However, other characteristics include:
Stratum corneum cells (SC): These are packed, brick-wall like, within a lipid matrix. Enzymes degrade corneocyte linkages enabling shedding – also known as desquamation.
Cuticle (hair) cells: These cement in protein, face away from the scalp and are protected by sebum. Below these are the cortex, which has strong, elongated, tightly-packed keratinised cells.
Nails with long narrow fibres embedded in protein are highly flexible. Nails get their strength from three linked keratin layers, where oils and moisture cement and lubricate the nail plate. Functionality results from the nails shape, which mirrors that of the underlying bone.
Hair and nails retaining tight inter-corneocyte connections don’t desquamate. However, hair sheds through life cycle programming and nail corneocytes are buffed off with friction and activity.
Multiple genes regulate keratin production, primarily in accordance with tissue type and cell maturity. Over time, however, mutations can reduce the mechanical resilience of the cells and break the cell-to-cell linkages.
Disruption and Potential Damage
Disruption to the keratin scales can change the initially smooth, soft and lustred appearance to a dull appearance with rough, disordered scales, making the tissue susceptible to damage. Other impacts include:
Disrupted cellular links:
Dermatophyte infection - microbial enzymes break cellular links. Dermatophytes consume keratin for nutrition, extending to fresh areas as the source diminishes.
External physical and chemical forces, such as hair styling, can also break cellular links.
Reduced surface lipids - age and exposure causes dryness, diminishing the protective layers of all keratin structures.
Trauma, scars and nail-bed deformity can reduce adherence of nail plate.
Nail matrix damage (a cyst for example) can result in poor nail formation.
Cuticle - emerges from proximal nail fold towards fingertip, sealing the new (soft) keratin until it hardens (providing waterproofing).
Hyponychium -at the free edge of the nail plate, the hyponychium provides a mechanical and immunologic barrier. It can be disrupted from vigorous nail hygiene.
Harsh hygiene measures can diminish SC lipids causing dull, flaking, inflamed or symptomatic skin.
Physical disruption to growth patterns (hormonal, medication - disruption to rapidly multiplying cells eg chemotherapy):
nail ridge patterns.
hair growth cycles disrupted; hair thinning (male or female pattern), and hair loss.
So, what has keratin ever done for us? Lots! It enables life, maintains appearance, reflects our system's health, facilitates fine motor function and sensation and keeps our tissues strong and resilient!
Bragulla, HH & Homberger, DG 2009, ‘Structure and Functions of Keratin Proteins in Simple, Stratified, Keratinized and Cornified Epithelia’. Journal of Anatomy, vol. 214 no. 4, pp. 516–559, viewed 30 September 2015, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2736122/