Hair is one of the most distinctive features that distinguish mammals from other animals. It consists of a keratinous filament that grows out of an epidermal penetration of the dermis called a hair follicle. Most of the hair is a dead filament, but a small amount is alive and contains a layer of mitotically active cells known as the hair matrix. The color of hair comes from a pigment called melanin. There are two kinds of melanin: eumelanin and pheomelanin. Hair growth is controlled by the hormone follicle stimulating hormone (FSH).
The morphological characteristics of the hair are determined by what kind of follicle produces it. There are two main morphological types of hair: vellus and terminal. Vellus hairs are soft and fuzzy and are present all over the body, but they are largely replaced by coarser terminal hair during puberty. During development, the hair follicle decides whether it will produce curly or straight hair. The shape of the follicle is also important, as it determines the pattern and texture of the hair.
As the hair develops, a bulb of living cells is formed at the base of the hair follicle. The bulb houses a nerve and blood vessels, supplying the follicle with nourishment and hormones. There are also glands near the follicle that produce sebum, a natural oil that lubricates and protects the hair.
A smooth muscle in the scalp called the arrector pili contracts in response to nerve signals from the sympathetic nervous system, causing the external hair to rise. This is the famous goose bump phenomenon, visible in people and animals when frightened or excited. Hair can also stand up when a person is hot or cold, and it can trap a layer of air that provides insulation.
In addition to its practical function, hair has become a symbol of status and culture. Many different styles of hair have been used to identify subcultures and groups. For example, a mohawk or other spiked hairstyle is common among punks and Indian sadhus, while a crew cut or shaved head may indicate membership of a military unit. In some cultures, long hair is a sign of virility.
During the development of the hair follicle, the cellular activity that determines hair type is influenced by environmental factors. The follicle is more likely to produce curly hair if the environment is dry and cool, whereas warm and humid conditions encourage straight hair production.
Hair is a very complex structure that can be very versatile. It can be manipulated to achieve a wide variety of styles, from curly to straight and wavy to frizzy. Most people tend to have some combination of these types. The texture of a person’s hair can also be influenced by what shampoos and conditioners they use. For the best results, it is recommended to use a product that contains no sulfates, parabens, silicone, formaldehyde, or phthalates. In addition, using a conditioner that contains essential oils can keep the hair healthy and shiny.