Hair Follicle Structure
The average human scalp contains between 90,000 and 140,000 terminal hairs. These hairs grow approximately 1 cm per month, while hair loss is a continuous, ongoing process, with people typically losing about 100 hairs per day.
The pilosebaceous unit, commonly known as the hair follicle unit, is made up of the hair follicle along with an attached sebaceous gland and an arrector pili muscle. Hair follicles vary in size and shape depending on their location on the body, but they all share the same basic structure. The number and distribution of hair follicles is established during fetal development, and no additional follicles are added after birth.
The hair follicle begins at the surface of the epidermis and extends into the dermis. Fine, vellus hairs may extend only into the upper dermis, while thicker terminal hairs extend deeper, sometimes into the subcutaneous layer.
Hair Layers
The hair shaft has an inner core called the medulla, surrounded by the cortex, which makes up most of the hair’s structure. Moving outward, a single layer of cells forms the shaft cuticle, which is encased in layers forming the inner root sheath. The inner root sheath helps shape the hair shaft as it grows upward from the hair matrix, and it gradually breaks down around the level of the isthmus. The outer root sheath encases the entire hair shaft along its length.
Sebaceous glands are appendages of the epidermis and an important part of the pilosebaceous unit. They are found throughout the body, especially on the face, and open onto the hair follicle in most areas (an exception is the lips, which do not contain hair follicles). When stimulated by hormones such as androgens, sebaceous glands produce and release sebum, an oily material that contributes to the skin’s natural barrier.
The arrector pili muscle is a small band of smooth muscle attached to the follicle and connected to the autonomic nervous system. In cold conditions, this muscle contracts, slightly raising the skin and causing hair to stand up, commonly known as goosebumps.
The Hair Growth Cycle
Hair growth happens in a cyclical pattern, and each follicle follows its own timeline, independent of surrounding hairs. A typical hair growth cycle has three phases: anagen, catagen, and telogen.
- Anagen phase: the active growth stage, typically lasting two to seven years. At any given time, approximately 85 to 90 percent of scalp hair is in this phase.
- Catagen phase: a transition phase lasting about two weeks, during which the hair follicle undergoes changes that form a club hair with a hardened node at the end.
- Telogen phase: a resting phase in which club hairs remain in the scalp for about 100 days before being released and shed, allowing the anagen phase to begin again with a new hair.
Hair Loss and the Growth Cycle
When hair loss occurs, the hair growth cycle is typically affected in some way, regardless of the underlying cause. This can include a shortened anagen phase, where the growth phase ends earlier than typical, an early transition into the catagen phase, or a prolonged telogen phase, where more hairs enter the resting phase prematurely, increasing shedding and slowing regrowth.
Several factors can influence the hair growth cycle, including:
- Genetics
- Androgens (including testosterone and its active metabolite, dihydrotestosterone, or DHT)
- Estrogens
- Thyroid hormones
- Glucocorticoids
- Retinoids
- Prolactin and growth hormone
- Certain medications
- Nutritional status
- Stress
Androgens, particularly testosterone and DHT, have one of the largest impacts on hair follicles. Acting through androgen receptors in the dermal papilla, these hormones are associated with androgenic alopecia later in life in genetically susceptible individuals, causing follicles on the scalp to gradually miniaturize. Earlier in life, these same hormones can increase the size of hair follicles in androgen-dependent areas, such as the beard area.
If you are noticing changes in your hair growth cycle, a consultation with a physician-directed provider can help review your history and discuss options that may be appropriate for you. Individual results vary.
