-Hair plays two important roles in our lives as part of the human body.
Firstly, hair protects the brain and skull from external shocks, thereby playing a crucial role in maintaining individual life. Secondly, thick and glossy hair increases sexual attractiveness to others, contributing to human prosperity.
Conversely, hair damage or sudden hair loss caused by internal or external issues can not only destroy an individual's healthy and attractive life but also lead to difficulties in dating, marriage, and employment, reducing the quality of life.
As is the case now and increasingly in the future, the need to appeal to personal beauty will grow as marriage ages get older and birth rates drop. The desire for shiny, full, and healthy hair will undoubtedly continue to rise as a means of appealing beauty.
From now on, we will sequentially explore the mechanisms of hair generation, prevention of hair loss, and recovery and treatment methods. We will start by definitively answering the question: What is the definition of hair?
- Definition and Composition of Hair:
Another name for hair is 毛髮, 毛发, or Hair, and it will be referred to as "hair" from now on. Hair is an appendage of the skin and, through the process of cell differentiation, acquires special structures and functions.
The composition of hair is approximately 80-90% keratin, a type of protein, about 3% melanin pigment, 1-8% lipids that give hair its luster, 10-15% moisture that affects elasticity, and 0.6-1% trace elements.
As a protein, keratin is classified into hard keratin, found in nails, wool, and human hair, and soft keratin, found in the skin. Hard keratin is mainly composed of fibrous proteins containing sulfur (S).
When hair is burned, it produces a foul smell due to the decomposition of cystine, a sulfur-ontaining amino acid in keratin. Human proteins are composed of 20 amino acids, 18 of which are involved in the composition of hair.
These 18 amino acids include the acidic amino acids (monoamino dicarboxylic acids) glutamic acid and aspartic acid, as well as neutral amino acids (monoamino monocarboxylic acids) such as alanine, glycine, isoleucine, methionine, leucine, cystine, proline, tryptophan, tyrosine, phenylalanine, valine, threonine, and serine.
Melanin pigment derives from the Greek word for 'black, dark'. Melanin consists of two forms: eumelanin, which is particulate, and pheomelanin, which is diffuse. Eumelanin determines dark colors ranging from black to reddish-rown and is larger in particle size compared to pheomelanin. Eumelanin is predominantly found in Asians and Africans.
Pheomelanin represents lighter shades, such as red and yellow, and is more common in the hair of Westerners. (Details on lipids, moisture, and trace elements are either omitted or will be described later.)
Hair originates and grows from the human head. It is produced in the hair follicle, which is nourished by circulating blood, lymph, and extracellular fluid, providing the necessary nutrients for hair growth. After being produced, hair emerges and grows through the hair pore.
Hair consists of the hair shaft and the hair root. The hair shaft is composed of the cuticle, cortex, and medulla from the outside in.
The cuticle, also known as the hair cuticle, is the outermost layer of hard keratin. It has a tile or fish scale pattern, is transparent, and has lipophilic properties, accounting for about 10-5% of the hair. Depending on race and individual, it is layered 5 to 15 times, preventing moisture evaporation.
The more cuticles there are, the healthier the hair. The cortex surrounds the medulla and is composed of cortical cells and ground substance. It is the middle layer, hydrophilic, making up 85-90% of the hair, where most cosmetic chemical treatments occur. Most of the melanin pigment is distributed here, determining the color, elasticity, strength, feel, texture, and overall properties of the hair.
The medulla is the central part of the hair, hollow in the middle, and has a honeycomb structure containing air. It is not present in vellus hair, children's hair, or hair of alopecia patients but exists in healthy hair, determining the thickness of the hair.
The hair root comprises the hair follicle, inner root sheath, outer root sheath, hair papilla, keratinocytes, sebaceous glands, oil glands, and arrector pili muscle.
Among these, the hair papilla is supplied with nutrients by capillaries and autonomic nerves, playing a crucial role in forming keratin and promoting hair growth.
It is known that East Asians have about 100,000 hairs, while Westerners have about 80,000. The hair thickness of East Asians is generally thicker than that of Westerners.