Integumentary Accessory Structures: Hair

Within the integumentary system, four main accessory structures aid in thermoregulation, sense of touch, protection, and much more. These four main accessory structures are:

  1. Hair Follicles
  2. Nails
  3. Sebaceous Glands
  4. Sweat Glands
Hair Location And Function

Hair can be found from your head to your toes. It is on your entire body except for your lips, palms, soles of the feet, sides of the toes and fingers, and parts of your genitalia.

This accessory structure might be the most important of all. It functions to insulate and keep you warm when it is cold, stop particles in the environment from entering your nose and ears, protect against UV light, and sense the environment using the root hair plexus. The root hair plexus is a network of nerves that surround the base of the hair. When movement occurs, signals are sent to the brain via an afferent nerve.

One of the other functions that hair plays a role in but, is not used anymore by humans is the ability to make your hair erect or give yourself goosebumps. Some believe that these muscles were once used by our ancestors millions of years ago as a defense mechanism. Currently, it is known that the contraction of all the arrector pili muscles will help your body stay warm by generating heat.

Hair Structure

Hair is a complex structure made of many parts that allow it to function properly. At the base of the entire hair, there is a structure known as the hair bulb. This hair bulb houses the hair papilla and functions as an area to receive nutrients from surrounding blood vessels. The hair papilla is responsible for the growth and formation of hair. Within the hair papilla there are cells called mesenchymal cells. These cells are a type of stem cell that functions to create new generations of hair cells so that the hair cycle can be continued.

Above the hair papilla and hair bulb is the hair matrix. This area functions to strengthen and provide nutrients to the hair as a whole. Within the hair matrix there are three layers:

  1. Medulla
  2. Cortex
  3. Cuticle

The medulla is the most internal layer of the hair matrix. It is made of soft keratin. This layer can often be found in thicker hair on your body such as your face and scalp.

The cortex is the middle layer of the three and is also the thickest. It is mostly made of hard keratin. Within this layer, you can find the pigment that gives the hair its color. It is also the layer that contains the most fibers. This helps create the strength and texture of your hair.

The outermost layer is known as the cuticle of the hair. Similar to the layers of the skin, the outermost layer is made of flattened cells that function to protect the internal layers. A healthy cuticle is smooth and shiny.

The hair root is an area in the skin that helps anchor and stabilize the hair. Surrounding the hair root is the hair follicle. The hair follicle is a sheath made of connective tissue and skin.

Finally, the hair shaft is the part of the hair that is visible on the external surface of your body. This part of your hair is not living. It is made of keratinized cells from the living tissue that is within the epidermis, dermis, and hypodermis layers.

Types Of Hair

If you examine the hair around your body, you will begin to notice that it is not all the same. Some grow faster, thicker, and longer while others you can barely see. There are three types of hair you can have on your body:

  1. Lanugo
  2. Vellus
  3. Terminal

Lanugo hair is a special type of hair that is only found three months after embryonic development. This hair is unpigmented, very soft, and thin. It will cover most of the body of the fetus while it is in the womb and functions to keep the fetus warm and protected while it develops. This hair is usually not seen on a baby when birth is given but, if a child is born prematurely it can be seen.

Vellus hair is the most common hair on your body. It is very fine, soft, and sometimes unpigmented hair. It is also known as peach fuzz for its soft and light texture and color.

Terminal hair is the type of hair people think of right away. This type of hair can be found on your head, eyelashes, and eyebrows. It is usually thicker, more pigmented, course, and longer than all other hair types. After puberty, terminal hair will cover about 30% of the average person’s body.

Hair Color

Your hair color is determined by the same pigment that creates your skin color, melanin. Variations in the concentration of melanin that is produced by melanocytes can alter the color of your hair to make it darker or lighter in color. Increasing the concentration of melanin leads to darker hair while a decrease in production will lead to gray and lighter hair.

So, how do people have different colors of hair? Within your melanocytes, there are different melanin that is produced. Eumelanin is the melanin that causes black and brown hair while pheomelanin causes red hair. People that have a phenotype of red hair experience genetic variations that lead to more pheomelanin being produced than eumelanin.

In fact, your hair color is mostly influenced by your genetics, hormones, and your environment. Things such as genetic variations that lead to an increase of one type of melanin over the other are an example of this. Hormones like estrogen lead to more eumelanin production in a woman’s hair specifically. Finally, things such as the sun can change the production of melanin and lead to a change in hair color.

Hair Replacement Cycle

Within the cycle of hair replacement and growth, there are four main stages that we can identify. The first is the active phase. This phase will typically last for 2-5 years and will grow at a rate of 0.33 mm/day.

After the active stage is over, the hair follicle will move into the regression phase. Here, the follicle will decrease in growth rate and begin to transition to the third phase.

In the third phase, the resting phase, the follicle will no longer be attached to its follicle. At this point, we can describe it as club hair.

Finally, the last stage is called the reactivation phase. After the club hair is lost, the hair matric will begin to produce new cells that will form the replacement hair. When the new hair is produced, the cycle is started over from the growth phase.

This cycle can be stopped if something such as male pattern baldness were to occur. In this case, male pattern baldness is due to a change in sex hormones that leads to decreased hair production and growth.