Facts about Human Hair Follicles

Facts about Human Hair Follicles


Facts about Human Hair Follicles

The average human body has around 5 million pilosebaceous units capable of producing hair. These units are composed of hair follicles and sebaceous glands, which are found throughout the skin, except on the palms of the hands and the soles of the feet. The density of these units varies across different body regions, with higher concentrations on the scalp, face, chest, and back, and lower concentrations on areas like the arms and legs. The number of pilosebaceous units can also vary from person to person based on genetic factors and individual characteristics.

The part of the body that contains the hair follicle and its appendages (such as sebaceous glands and arrector pili muscles) is called the "dermis." The dermis is the middle layer of the skin, lying between the epidermis (the outermost layer) and the subcutaneous tissue (the deepest layer). It is a thick, supportive layer that contains blood vessels, nerves, hair follicles, sweat glands, and other structures vital for maintaining skin health and function.

The hair follicles in the dermis produce hair, and the sebaceous glands are associated with each hair follicle, secreting sebum (an oily substance) that helps lubricate and protect the hair and skin. The arrector pili muscles are tiny muscles attached to hair follicles and are responsible for causing "goosebumps" when they contract. These structures collectively contribute to the overall health and regulation of hair growth and skin function.

The hair matrix is produced by a specific region within the pilosebaceous unit known as the "hair bulb." The hair bulb is located at the base of the hair follicle, deep within the dermis of the skin. It is the actively growing part of the hair follicle and contains a group of rapidly dividing cells called the hair matrix.

The hair matrix is responsible for producing the cells that form the hair shaft. As new cells are produced in the hair matrix, older cells are pushed upward and eventually harden, forming the visible hair that extends out of the skin's surface. This continuous process of cell division and hair shaft production allows hair to grow and eventually be replaced with new hair over time.

Hair is anchored to skin cells by the "hair follicle." The hair follicle is a complex structure within the skin that serves as the anchor and support for each individual hair shaft. It extends from the epidermis (the outer layer of the skin) into the dermis (the middle layer of the skin).

The hair follicle surrounds the hair root, which is the lower part of the hair shaft embedded in the skin. The hair root is connected to the hair bulb at the base of the hair follicle. The hair bulb contains the hair matrix, a region of actively dividing cells responsible for producing the new hair cells that make up the hair shaft.

The hair follicle is surrounded by several structures, including sebaceous glands (which produce sebum to lubricate the hair and skin) and arrector pili muscles (tiny muscles that cause the hair to stand on end, resulting in "goosebumps" when they contract).

Together, the hair follicle and its associated structures form the pilosebaceous unit, which is responsible for hair growth and maintenance. When the hair follicle is healthy and functional, it keeps the hair firmly anchored to the skin, allowing it to grow, shed, and be replaced continuously throughout the hair's natural growth cycle.

Cell division occurs in the pilosebaceous unit primarily in the region known as the "hair matrix." The hair matrix is a group of rapidly dividing cells located at the base of the hair follicle, deep within the dermis of the skin. These dividing cells are responsible for producing the new cells that form the hair shaft.

As the cells in the hair matrix divide and grow, they push the older cells upward toward the surface of the skin. These older cells eventually harden and become keratinized, forming the visible part of the hair shaft that extends out of the skin. This continuous process of cell division and hair shaft production allows hair to grow and be replaced throughout its natural growth cycle.

Apart from the hair matrix, cell division also occurs in other parts of the pilosebaceous unit, including the sebaceous glands and the outer root sheath of the hair follicle. However, the hair matrix is the primary site of cell division for hair growth.

The part of the pilosebaceous unit most responsible for healthy-looking hair is the "sebaceous gland." Sebaceous glands are small, oil-producing glands that are connected to each hair follicle. They secrete an oily substance called sebum, which lubricates and protects both the hair and the surrounding skin.

Sebum plays a crucial role in maintaining healthy-looking hair by providing natural conditioning and moisture. It helps to keep the hair soft, smooth, and shiny. Sebum also forms a protective barrier that prevents excessive water loss from the hair, keeping it adequately moisturized.

However, an excessive production of sebum can lead to oily or greasy hair, while too little sebum can result in dry and brittle hair. Striking the right balance in sebum production is essential for achieving healthy-looking hair.

Proper hair care, including regular washing and using suitable hair products, can help maintain the optimal balance of sebum on the scalp and hair. This, in turn, promotes healthy-looking hair and a comfortable scalp.

The part of the hair that is visible above the skin is known as the "hair shaft." The hair shaft is the portion of the hair that extends from the surface of the skin outwards. It is composed of dead, keratinized cells that were produced by the actively growing hair matrix located at the base of the hair follicle.

The hair shaft is made up of three layers:

1. Cuticle: The outermost layer of the hair shaft consists of overlapping, protective scales that help protect the inner layers.

2. Cortex: The middle layer is the thickest part of the hair shaft and contains fibrous proteins that give the hair its strength, elasticity, and color.

3. Medulla: The innermost layer is the medulla, which is a less defined and sometimes absent layer in fine hairs.

The appearance and characteristics of the hair shaft can vary based on factors such as hair type, color, texture, and overall hair health. Regular hair care, including washing, conditioning, and protecting the hair from damage, can help maintain the hair shaft's integrity and enhance the overall appearance of the hair.

The waxy substance that lubricates the skin and hair is secreted by the "sebaceous glands." Sebaceous glands are small glands located within the dermis, the middle layer of the skin. They are connected to hair follicles and secrete an oily substance called sebum.

Sebum is composed of lipids, fats, and cell debris. Its primary function is to moisturize and protect the skin and hair. When sebum is produced in appropriate amounts, it helps keep the skin and hair soft, supple, and moisturized. It also forms a protective barrier on the skin's surface, which helps to prevent excessive moisture loss and protect against environmental factors.

However, an overproduction of sebum can lead to oily skin and hair, while underproduction can result in dryness and hair issues. Striking the right balance of sebum production is important for maintaining healthy skin and hair.

Regular cleansing and hair care practices can help maintain the appropriate levels of sebum on the scalp and hair, promoting overall skin and hair health.

The soft and downy hair that grows on a fetus is called "lanugo." Lanugo is a type of fine, unpigmented hair that covers the developing fetus's body while it is still in the womb. It typically appears around the fifth month of pregnancy and is most abundant on the back, shoulders, forehead, and other areas of the body.

Lanugo serves several purposes during fetal development:

1. Thermal Regulation: Lanugo helps regulate the fetal body temperature by providing some insulation.

2. Protection: It may offer some protection to the delicate skin of the developing fetus.

3. Vernix Adhesion: Lanugo helps the vernix caseosa, a waxy, white substance that also covers the fetus, to adhere to the skin.

As the fetus grows and approaches the later stages of pregnancy, the lanugo is usually shed, and most of it is replaced by more mature hair, or vellus hair, which covers the body after birth. In some cases, premature babies may still have lanugo present on their skin at birth, but it usually disappears shortly after birth as the baby's body adapts to the external environment.

The first stage of hair growth is known as the "anagen" phase. The hair growth cycle consists of three main phases: anagen, catagen, and telogen. Each hair on our scalp goes through these phases independently.

1. Anagen Phase: This is the active growth phase of the hair follicle. During the anagen phase, cells in the hair matrix rapidly divide, producing new hair cells that form the hair shaft. The hair grows longer during this stage, and the duration of the anagen phase varies among individuals and different body regions. Scalp hair typically has the longest anagen phase, lasting several years, while hair on other parts of the body has a much shorter anagen phase.

2. Catagen Phase: After the anagen phase, the hair follicle transitions to the catagen phase, which is a relatively short transitional stage. During catagen, the hair follicle shrinks, and hair growth stops. This phase lasts only a few weeks.

3. Telogen Phase: The telogen phase is the resting phase of the hair growth cycle. During this stage, the hair follicle remains inactive, and the hair is held in place without further growth. After the telogen phase, the old hair is shed, and the hair follicle re-enters the anagen phase, starting the hair growth cycle anew.

It's important to note that the hair growth cycle is continuous and not all hair follicles are in the same phase simultaneously. This is why we continuously shed and grow new hairs over time.

The longest anagen phase is found in the scalp hair. The anagen phase for scalp hair can last anywhere from 2 to 7 years or even longer in some cases. This extended growth phase is one of the reasons why scalp hair can grow quite long before it naturally sheds and enters the catagen and telogen phases.

In contrast, hair on other parts of the body, such as the eyebrows, eyelashes, arms, legs, and body, have much shorter anagen phases. For example, the anagen phase for eyebrow hair typically lasts around 4 to 6 months, while body hair may have an anagen phase of only a few months.

The variation in the duration of the anagen phase in different body regions is determined by genetics and hormonal factors. These differences contribute to the varying lengths and growth rates of hair on different parts of the body.

During the catagen phase of the hair growth cycle, several changes occur within the hair follicle and the hair itself:

1. Transition to Resting Phase: The catagen phase is a transitional stage between the active growth phase (anagen) and the resting phase (telogen). During this period, the hair follicle starts to shrink and detach from the blood supply, which ultimately leads to the cessation of hair growth.

2. Hair Growth Stops: The hair stops growing during the catagen phase. The hair shaft is no longer being pushed upwards by the actively dividing cells of the hair matrix, which happens in the anagen phase.

3. Short Duration: The catagen phase is relatively short, lasting only a few weeks. It is a temporary stage in the hair growth cycle before the hair follicle progresses to the telogen phase.

4. Cuticle and Inner Sheath Formation: During catagen, the cells surrounding the hair shaft form a structure called the inner sheath, which encloses the hair shaft. Additionally, the cuticle, which is the outermost protective layer of the hair shaft, starts to thicken.

5. Preparation for Shedding: The catagen phase is a prelude to hair shedding. As the hair follicle transitions from the catagen to the telogen phase, the hair becomes fully disconnected from the blood supply and the dermal papilla, the structure that supplies nutrients to the growing hair.

6. Club Hair Formation: At the end of the catagen phase, the hair is now considered a "club hair." This is the name given to the hair that has completed its growth cycle and is ready to be shed. The club hair is firmly held in place until the telogen phase, during which the hair will eventually fall out and be replaced by a new hair when the anagen phase resumes.

It's important to understand that the catagen phase is a normal part of the hair growth cycle and is necessary for the hair follicles to go through periods of rest and regeneration before new hair growth can begin.

The appearance of two hairs in the same follicle might occur during the transition between the anagen phase and the catagen phase of hair growth. During this transitional period, the hair follicle undergoes changes that can lead to the formation of two hairs within the same follicular unit.

In the anagen phase, the hair follicle is actively producing new hair cells in the hair matrix, and a single hair shaft grows from the follicle. However, as the hair follicle transitions to the catagen phase, it starts to shrink and detach from the blood supply. During this process, the hair shaft may not be fully expelled from the follicle before the catagen phase begins.

This can result in the next hair shaft that is forming in the hair matrix to emerge alongside the older hair shaft that is still present in the follicle. As a result, there might be two hairs growing in close proximity or even side by side within the same follicular unit during this transitional period.

This phenomenon is relatively rare and often temporary since the older hair shaft will eventually be shed during the catagen and telogen phases, and the new hair shaft will continue its growth during the subsequent anagen phase. If you notice two hairs in the same follicle, it's likely a temporary occurrence due to the hair follicle's natural cycle and should not be a cause for concern.

Back to blog