Hair and Alopecia

by Robert Tallitsch, PhD | February 22, 2024

Hair and Alopecia | BodyViz 3D Anatomy

 Video explaining Hair and Alopecia with a patient example!


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All of us, throughout our lives, lose hair. Although the number varies, the average male and female loses between fifty and one hundred hairs daily. Most of the time, we don’t notice the hair loss. However, the loss of a considerable amount of hair can be a sign of a medical condition.

In this Brain Builder, we will introduce the integumentary system and discuss the anatomy of hair. We will follow this with a discussion of alopecia (which is the medical term for hair loss), some of the types of alopecia, their possible causes, and a variety of treatments for alopecia.

Integumentary System

The integumentary system, or integument, is composed of the skin and its derivatives (nails, hair, and glands). The functions of the integument include

  • Protection

  • Synthesis and storage of lipids

  • Excretion of wastes

  • Regulation of body temperature

  • Synthesis of vitamin D3

  • Coordination of immune responses involving cancers and other pathogens of the skin

  • Sensing environmental stimuli

For a more detailed explanation of the skin, see our previous Brain Builder Integumentary System and Burns.

Anatomy of Hair

There are approximately five million hairs on the human body, with approximately 98% found on the general body surface. Hair is not found on the lips, portions of the external genitalia, the palms of the hands and soles of the feet, and the sides of the fingers, toes, and feet.

There are two types of hair found on the human body; vellus hair and terminal hair. Vellus hair is shorter and thinner than the longer and thicker terminal hairs. Terminal hairs make up chest hair, pubic hair, facial hair, eyelashes, eyebrows, and the hair on the head. Under the influence of male sexual hormones during puberty, vellus hairs on some parts of the body (face, armpit, pubic area) are replaced with terminal hairs.

Each hair has two parts. The hair shaft is the visible part of the hair that emerges from the skin.

The hair root extends down into the dermis of the skin. The hair root is surrounded by the hair follicle, which is composed of portions of the epidermis and dermis. The follicle is the living part of the hair.

The hair root expands at its deepest portion to form the round hair bulb. A structure termed the hair papilla extends into the bottom of the hair bulb, supplying the hair with blood.

New hair cells are constantly being made from stem cells found within the hair bulb. As new cells are formed, older cells are pushed superficially. As these cells move superficially, they no longer receive nourishment from the capillaries within the papilla, and they die.

Hair will continue to grow as long as new hair cells are produced within the hair bulb. The growth cycle of a hair is composed of the following three phases:

  • The growth phase (also termed the anagen phase): The length of the growth phase of an individual hair is determined by where the hair is found on the body. Hair on the head can grow for several years, and can grow to more than a meter in length. However, the growth phase is relatively short for hairs that form the eyelashes and eyebrows, as well as nasal and ear hair.

  • The transitional phase (also termed the catagen phase): This phase lasts between two and four weeks. It begins when the hair root separates from the papilla.

  • The resting phase (also termed the telogen phase) begins when the hair is completely separated from the papilla, thereby completely eliminating the supply of blood to the hair. Within a variable amount of time the hair will no longer remain within the follicle.

As soon as the hair follicle is empty, new hair cells begin to form a new hair, and the growth phase of the new hair begins.

Most hair cells have two layers: an inner medulla and an outer cortex. The inner medulla is composed of a soft and flexible substance termed soft keratin. The outer cortex contains hard keratin, which is responsible for the stiffness of hair. If present, the cuticle forms the third and outermost layer of the hair. This layer is composed of a single layer of dead, overlapping keratinized cells that protect the medulla and cortex.

Hair shape (straight or curly) is determined by the shape of the hair follicle. Straight hair grows in follicles that are oval in shape when viewed in cross section. Curly hair, in contrast, grows in follicles that are curved when viewed in longitudinal section. These follicles are c-shaped throughout most of the length of the follicle. In addition, these follicles may have smaller, secondary curvatures near the bottom of the follicle.

The color of hair is due to the presence of two types of melanin within the hair: pheomelanin and eumelanin. Pheomelanin is responsible for lighter hair color, while eumelanin is responsible for darker hair colors. Melanocytes adjacent to the hair follicles inject the keratinized cells of the hair shaft with melanin.

Structures Associated with Hair Follicles

  • Arrector pili muscles. Each hair follicle is associated with specialized smooth muscle cells termed arrector pili muscles. These muscles contract due to stimulation of the sympathetic branch of the autonomic nervous system. This causes the hair to “stand on end”.

  • Sebaceous (oil) glands. These glands are located in the dermis of the skin and empty into the superior 1/3 of the hair follicle. Sebaceous glands release an oily substance termed sebum into the hair follicle. Sebum lubricates the skin immediately adjacent to the hair follicle, acts as a bactericide for both the skin and hair, and keeps the hair soft. Sebum is released into the hair follicle following stimulation of the sympathetic branch of the autonomic nervous system, as well as by contraction of the arrector pili muscles. When the arrector pili muscle contracts it squeezes the gland, forcing sebum into the hair follicle.


Hair loss (alopecia) can affect any part of your body or just the scalp. Alopecia can also be temporary or permanent. Some of the numerous forms of alopecia include the following:

  • Hereditary and/or age-related alopecia

    • This appears with increasing age and is the most common form of alopecia.

    • Male pattern baldness is an example of this type of alopecia.

  • Autoimmune forms of alopecia

    • Alopecia areata

      • This form of alopecia, with is an autoimmune disorder, is manifested by hair falling out in patches, typically on the scalp. It can, however, also involve the eyebrows, eyelashes, and other regions of the body.

      • This autoimmune form of alopecia may be hereditary.

    • Alopecia totalis

      • This condition involves hair loss from the entire scalp.

      • This type of alopecia appears to be an autoimmune disorder, but the exact cause has yet to be determined.

    • Alopecia universalis

      • As the name implies, this type of alopecia involves the loss of hair from the entire body.

    • Alopecia barbae

      • In this form of alopecia facial hair falls out in small, circular patterns on various locations of the face.

  • Inflammatory form of alopecia

    • Cicatricial alopecia

      • This form sometimes develops after the skin has been damaged due to burns or a severe infection.

  • Postpartum alopecia

    • This form of alopecia manifests itself shortly after childbirth, and is believed to be due to the sudden drop in the female hormone estrogen.

    • Most researchers do not consider this to be a “typical” form of alopecia, in that postpartum alopecia is believed to be the shedding of the increased hair growth that is typically seen during pregnancy.

    • Hair growth typically resumes its normal cycle in the female within months of childbirth.

  • Alopecia without known causes

    • Centric centrifugal cicatricial alopecia (also termed CCCA)

      • This type of alopecia is almost exclusively seen in women of color (with the highest percentage being seen in Black women)

      • Hair loss starts at the crown of the head and progresses posteriorly over the scalp.


Treatments for Alopecia

Alopecia has no known permanent cure. However, there are several treatment options that may give short- or long-term relief from alopecia. Some of these options include treatment with corticosteroids, minoxidil (Rogaine), behavior modification to reduce daily stress, immunotherapy, or platelet-rich plasma injections.

Most forms of alopecia occur without warning, and the spread of the hair loss is unpredictable. Although some form of short-term or long-term treatment is required for most forms of hair loss, hair growth may return to normal on its own. Regardless, consultation with a dermatologist is strongly urged to determine treatment strategies and outcomes.

Key Terms

Terminal hair - The longer and thicker of the two types of hair found on the human body. Terminal hairs make up chest hair, pubic hair, facial hair, eyelashes, eyebrows, and the hair on the head.

Eumelanin - Pigment that is responsible for darker hair color.

Anagen phase - The growth phase of hair.

Telogen phase - Resting phase of hair growth. This phase starts when the hair is completely separated from the papilla, thereby eliminating all blood flow to the hair.

Sebaceous glands - Also termed oil glands. These glands are located in the dermis of the skin and empty into the superior 1/3 of the hair follicle.

Alopecia - Clinical term for several types of hair loss.

Postpartum alopecia - A form of alopecia that manifests itself after childbirth, and is believed to be caused by the sudden drop of the female hormone estrogen.

Arrector pili muscles - A smooth muscle that is associated with a hair follicle.

Pheomelanin - Pigment that is responsible for lighter hair color.


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