Hair follicle

Small parts in your skin grow hair.

These parts live under your skin. They make your hair grow long. Hair helps keep you warm. It also protects you from the sun.

Your hair grows in a cycle. First, the hair grows fast. Then, it stops growing. Next, it rests for a while.

Some hair grows on your head. Other hair covers your arms. Some babies have hair on their whole body.

Tiny muscles are near the hair. They can make goose bumps. This happens when you feel cold.

Your skin does not have these parts on your palms. It does not have them on your feet. Hair grows on many other parts of your body.

Hair follicles are tiny organs in your skin.

They live in the dermal layer. This is a deep part of your skin. These organs make your hair grow. They also help protect you from sun rays. Hair can keep you warm, too.

Your hair grows in a set of steps. First is anagen. This is the active growth phase. During this time, hair grows about 1 cm every 28 days. Next is catagen. This is a short transition stage. Then comes telogen. This is a resting stage that lasts about three months.

Finally, exogen is when hair sheds. There is also kenogen. This is the time between a hair falling out and a new one growing.

Small parts help the follicle work. A sebaceous gland makes sebum. This is an oily or waxy substance. There is also a tiny muscle called the arrector pili. This muscle can make goose bumps. The shape of your follicle changes how hair looks. Curvy follicles make curly hair. Circular follicles make straight hair.

A hair follicle is a tiny organ found inside the skin of mammals.

It lives deep in the dermal layer of your skin. These organs are very complex and use 20 different types of cells. Each cell type has its own special job to do. The follicle helps control how hair grows using hormones and immune cells. This process allows different kinds of hair to grow on different body parts. For example, humans have long terminal hairs on their scalps. Babies may also have soft lanugo hairs covering their bodies.

Hair growth happens in a series of specific steps. The first step is called anagen, which is the active growth phase. During anagen, the hair root divides quickly to add to the hair shaft. In this stage, hair can grow about 1 cm every 28 days. Next is catagen, a short transition stage that lasts about 2 to 3 weeks. This stage turns the hair into a club hair. Then comes telogen, which is a resting phase that lasts around three months. Finally, exogen is the phase where the hair actually sheds. There is also kenogen, the time between a hair falling out and a new one growing.

Many small parts work together to make the follicle function. At the base sits the papilla, which contains a loop of tiny blood vessels. Around it is the hair matrix and a root sheath. A special area called the bulge holds stem cells that provide new cells. These stem cells also help heal the skin after a wound. Attached to the follicle is a tiny muscle called the arrector pili. This muscle can pull the hair upright to create goose bumps. A sebaceous gland is also attached to produce sebum, which is an oily substance.

Different people have different types of hair based on their follicle shape. The shape of the follicle determines if hair is straight or curly. People of European descent often have elliptical follicles that make wavy hair. People of African descent often have curvy follicles that make curly hair. Asian hair often comes from circular follicles, which makes it straight. The size of the hair also changes between different animals. A chimpanzee has hair with a diameter of 101 to 113 micrometers. An orangutan has much thicker hair, measuring 140 to 170 micrometers.

Understanding hair follicles is very important for medicine and science. Changes in the hair can be an early sign of a disease. Some known conditions include alopecia, which is hair loss, and hirsutism, which is excess hair. There is also a condition called lupus erythematosus. Doctors can also look at the microbiome, or tiny living things, on the scalp. Changes here can lead to problems like dandruff or psoriasis. Scientists even use hair follicles for hair restoration through transplants. They move small groups of hairs, called follicular units, to new areas.

A hair follicle is a complex organ located within the dermal layer of mammalian skin.

It is composed of 20 different cell types, each performing a distinct biological function. These follicles regulate hair growth through intricate interactions involving hormones, neuropeptides, and immune cells. This regulatory system allows different types of hair to grow on various parts of the body. For instance, terminal hairs grow on the scalp, while lanugo hairs cover the bodies of fetuses and some newborns.

Understanding these organs is vital for developmental biology, medicine, and studying human appearance.

The hair follicle functions as part of a larger structure called the pilosebaceous unit. This unit consists of a triad: the hair follicle, a sebaceous gland, and the arrector pili muscle. At the base of the follicle lies the papilla, which contains connective tissue and a capillary loop. Surrounding the papilla is the hair matrix and a root sheath. The root sheath includes an external layer and an internal layer made of three parts: Henle's layer, Huxley's layer, and an internal cuticle. A critical area called the bulge is located in the outer root sheath. This bulge houses LGR5+ stem cells that supply the follicle with new cells and assist in epidermal healing.

Hair growth occurs through a series of distinct, sequential stages. The first stage is anagen, the active growth phase, where the hair root divides rapidly. During anagen, hair can grow approximately 1 cm every 28 days. This is followed by catagen, a short transition or regression phase lasting 2 to 3 weeks. During catagen, the follicle converts the hair into a club hair, which is a dead, fully keratinized structure. Next is telogen, a resting phase that lasts about three months. The cycle also includes exogen, the active shedding phase, and kenogen, the period between shedding and new growth.

The length of these growth cycles varies significantly depending on the body part. On the scalp, the anagen phase can last between 2 and 7 years, which is why scalp hair grows so long. In contrast, the cycle for eyebrows is much shorter, with anagen lasting only 4 to 7 months. Typically, about 85% of hair follicles are in the anagen phase at any time. Between 10% and 14% are in telogen, and only 1% to 2% are in catagen. If a person experiences extreme stress, up to 70% of hair may prematurely enter the telogen phase. This specific condition of sudden hair loss is known as telogen effluvium.

Physical characteristics of hair, such as texture and shape, are determined by the follicle's structure. The shape of the dermal papilla, the position of the hair bulb, and the curvature of the follicle all play roles. For example, people of European descent often have elliptical follicles that produce wavy or straight hair. People of African descent often have more curved follicles, resulting in tightly curled hair. Those of Asian descent often have circular follicles that produce straight hair. Diameter also varies by species; a chimpanzee's hair is 101–113 micrometers, while an orangutan's hair is much thicker at 140–170 micrometers.

Changes in the hair follicle can signal various medical conditions or systemic illnesses. Abnormalities in growth or texture can be early indicators of diseases like alopecia, which is hair loss, or hirsutism, which is excess hair growth. Other conditions include lupus erythematosus and various scalp diseases linked to the microbiome. Some disorders, known as the follicular occlusion tetrad, involve chronic inflammation and scarring. This group includes hidradenitis suppurativa, acne conglobata, dissecting cellulitis of the scalp, and pilonidal disease. These conditions are characterized by follicular hyperkeratinization, which leads to the blockage or occlusion of the follicle.

Scientists and doctors also study the follicle to improve hair restoration techniques. Two primary methods used in hair transplantation are Follicular Unit Transplantation (FUT) and follicular unit extraction (FUE). In these procedures, surgeons extract follicular units, which are natural groupings of one to four hairs. These units are taken from donor areas and surgically implanted into a recipient area, such as a balding patch on the scalp. By understanding the biology of the follicle, researchers continue to explore how to treat hair loss and other dermatological issues.