The hair and nails are considered to be extensions of the skin and are both made of a hard, fibrous protein called keratin. As a part of the Integumentary System, the hair helps regulate body temperature, keeps the skin cool and moist, and responds to touch, while the nails protect the fingers and toes, and assist with grasping and holding.
Keratin is the building block of human skin, hair and nails, as well as hooves, claws, and horns in animals. It comes from the Greek keras, meaning horn. As with all proteins, Keratin has large molecules made up of smaller amino acids that join together to create the extremely hard protein.
Keratin is a family of fibrous structural protein (scleroproteins) with main duties to protect and support the body by forming connective tissue, tendons, bone, and muscle fiber.[1] The only other biological substance known to be as hard as keratin is the chitin that makes up the exoskeletons of anthropods.[2]
As a protein, keratin is made up of chains of amino acids, with high concentrations of the amino acid cystine. Each unit of cystine is made up of two cysteine amino acids in different chains which are linked together by two sulphur atoms. This chemical bond is called a disulphide linkage and many of them appear as rungs on the keratin ladder.[3] The disulphide bond is one of the strongest bonds that exists in the natural world.[4]
Keratin molecules assemble into bundles of intermediate filaments (a family of related proteins that share common structural and sequence features) which are extremely tough and insoluble. These filaments are the main component in keratinocytes (cells that have undergone keratinization) in the cornified layer of the epidermis.
Keratinization
Keratinization involves three kinds of epithelial (outer layer) cells: undifferentiated, differentiating, and terminal cells.[5] (Differentiation is the process by which cylindrical basal cells lose their nuclei, change their shape and composition, and become flattened, cornified cells).
- Undifferentiated cells are mitotically active. In the case of nails and stratum corneum, these undifferentiated cells continuously renew the keratinizing tissues. In the case of hair, they renew in cycles.
- Differentiating cells carry out four activities: synthesizing pre-keratin substances like tonofibrils (cytoplasmic protein structures), losing their nucleus and other elements in the cytoplasm, losing water, and uniting the cells into a horny mass.
- Terminal cells consist primarily of keratin and are completely inactive. They make up the final structures of hard keratin for the hair’s cuticle and cortex, and the soft keratins that make up the medulla and stratum corneum.
There are two types of keratin involved in keratinization: soft keratin and hard keratin.
- Soft Keratin (also known as beta type) is part of the stratum corneum of the epidermis (top layer of skin). It also creates the hair’s internal root sheath and medulla. Soft keratin is made of desquamating cells (desquamation: the process by which cells from the stratum corneum split apart, loosen, and fall away as they reach the top of the epidermis). Soft keratin has a high lipid (fat) content and lower sulphur content (less than 3%). Soft keratin contains more of the amino acid cysteine and less cystine than hard keratin, making it less stable in high temperatures.
- Hard Keratin (alpha type) makes up the hair and nails (as well as feathers and horns in animals). As its name suggests, it is tough and hard and doesn’t desquamate. Hard keratin has a lower fat content and higher sulphur content (more than 3%). It has good cell structure and is able to withstand heat.
In the nails, keratin is formed and hardened in the Matrix, the part of the nail bed beneath the nail root that contains nerves, lymph and blood vessels. Hard keratin formation for the hair
takes place in five stages and areas:[6]
1. Germinal Matrix: Cells in the hair bulb’s germinal matrix are constantly dividing during anagen, or the active stage of hair growth. These cells move in rows to the upper part of the bulb where they become longer and larger and will eventually become the hair cortex.
2. Differentiation Zone: The area of the upper hair bulb where the cells in the elongate and the nuclei shape changes from round to oval.
3. Fibrilization Zone: Area above the upper bulb where the cells elongate.
4. Keratinization Zone: Area of the hair shaft where the cells reach their maximum size. This zone has two distinct parts:
(a) The lower zone: where the fibrous protein is complete but the structure is unstable (also called the keratogenous zone). Hydrogen bonding of amino acids takes place followed by disulphide bonds (the amino acid cysteine is oxidized to cystine), making the protein extremely strong.
(b) The upper zone: Cells rapidly become stable and are able to withstand heat and chemicals.
5. Keratinized Zone: By the time the cells reach this zone, they have lost approximately 80% of their water weight, which has been replaced by pockets of air.
[1] Wikipedia.com, Scleroprotein, http://en.wikipedia.org/wiki/Fibrous_protein
[2] How It Works Daily, http://www.howitworksdaily.com/science/question-of-the-day-what-are-our-fingernails-made-of/
[3] P&G Beauty & Grooming, Hair Strength, http://www.pgbeautygroomingscience.com/hair-strength.html
[4] &G Beauty & Grooming, Hair Strength, http://www.pgbeautygroomingscience.com/hair-strength.html
[5] The International Association of Trichologists, The Hair and Scalp, p. 57.
[6] International Association of Trichologists, The Hair and Scalp, p. 58-59.
Sources
About.com Dermatology, Skin Anatomy, http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm
Buzzle.com, Integumetary System Functions, http://www.buzzle.com/articles/integumentary-system-functions.html
Dawber, Rodney, Ed., Diseases Of The Hair And Scalp (Third Edition), Blackwell Science Ltd., 1997.
International Association of Trichologists, The Hair and Scalp, I.A.T., Kalamazoo, 1993.
Natgeo.com, Have You Seen Him? http://www.thelifefiles.com/2011/03/05/have-you-seen-him/
Online Cosmetology School, Anatomy & Physiology of Skin, Hair, and Nails, http://www.cosmetologyschool.org/resources/anatomy-physiology-of-skin-hair-and-nails/
P&G Beauty and Grooming Science, The World of Hair, http://www.pgbeautygroomingscience.com/the-world-of-hair1.html
Sandia Corporation, Structure of the Skin, http://www.nmsl.chem.ccu.edu.tw/tea/SKIN_910721.htm.
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