The Hair System

All hair follicles are formed in utero by 22 weeks. There are about five million hair follicles, about one million of those on the head and scalp, and 100,000 on the rest of the body. Hair follicles do not continue to grow during life. As we age, the number of hair follicles decreases as we stretch and grow. 

The integumentary system is the body's first line of defense, consisting of the skin, hair, nails, and associated glands. Hair, a key component of this system, is more than just a cosmetic feature—it plays a vital role in protection, sensation, and regulation.

Each strand of hair grows from follicles embedded in the skin and is composed of keratin, a strong protein that provides durability and structure. Hair helps regulate body temperature, offers a barrier against environmental damage, and even enhances sensory perception. Understanding hair’s role in the integumentary system highlights its importance beyond aesthetics, emphasizing its function in overall health and well-being.

Hair Structure: Hair Follicle

A piece of hair is one of the body’s most complicated structures. Hair has two separate structures: the hair follicle below the skin and the hair shaft above the skin. The hair follicle holds the hair in place and is where hair starts to grow. This stocking-like structure begins in the epidermis or top layer and extends out to the second layer of skin, or the dermis. A piece of tissue called the papilla is at the bottom of the hair follicle. This contains tiny blood vessels, also called capillaries. These nourish the hair root to keep hair growing and healthy. The hair follicle also contains the germinal matrix that produces new hairs. The bud of the follicle is a rounded structure found deep in the skin at the root of the hair. This surrounds the germinal matrix and the papilla. Several types of stem cells here turn into specialized cells that can renew themselves over time. The follicle is lined by an inner and outer sheath that protects and shapes the developing hair. The inner sheath follows the hair and goes to just before the opening of the oil gland, called the sebaceous gland. The sebaceous gland creates sebum, an oil that acts as the body's natural conditioner. Sebum is produced more during puberty and decreases with age. The outer sheath grows all the way up to the gland. Attached to the outer sheath is the arrector pilli muscle. This tiny bundle of muscle fiber causes hair to stand up when the muscle contracts. Hello goosebumps! 

Hair Structure: Hair Shaft

Once the hair grows beyond the surface of the skin, the cells are no longer living. It is made up of keratin, a hardening protein that has three layers. 

The inner layer, called the medulla, that may or may not be present. 

The middle layer, called the cortex makes up most of the hair shaft. Both the medulla and cortex contain pigment cells that give hair color. The outer layer, called the cuticle that is formed by scales packed tightly together in an overlapping structure that looks like the shingles of a roof. 

The Growth Cycle

Hair can be in one of three growth stages. 

Anagen Phase: Stage 1 (2-5 years & 30-45 days)

This is the growth phase of hair, and hair can last several years in this stage. New hair will push old hair out of the follicle. The cells in the root of hair divide rapidly, resulting in the formation of new hair. Scalp hair can grow 1 to 1.5 cm every 28 days and stays in the active growth phase for 2-5 years. While eyelash, eyebrow, leg, arm, and nose hair remain in this phase for 30-45 days. 

Catagen Phase: Stage 2 (3-6 weeks)

The catgen phase is a transitional phase that lasts 3-6 weeks. During this time, the follicles prepare for rest. About 3% of all scalp hair is in this phase at any time. Growth stops, hair follicles shrink, and portions of the hair follicles collapse and form the club hair. 

Telogen Phase: Stage 3 (3-5 months)

This is the resting phase, and it can last for about 3-5 months. About 10-15% of hair is in the phase at any time. During this phase, the hair follicle is at rest, and the club hair is fully formed. 

Exogen Phase: Stage 4

This is the shedding phase. During this time, the older hairs have finished their life cycle and will push out to make way for new growth. You can shed 25-100 telogen hairs every day during this cycle.