Integumentary Accessory Structures: Nails

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

The nails can be found on the tips of your toes and fingers. The main function of these structures is to protect these areas from damage and the surrounding environment. They also function as a tool and aid in gripping things for picking them up, climbing, and most other things you do with your fingers.

The structure of your nail has two major parts, the area that is below the skin and the area above (visible). On the most distal portion of the finger, you have the free edge. This is the white area of the nail that is usually cut. This is the weakest part of the nail and can be broken or damaged very easily. Where the free edge meets the finger you have the hyponychium. The hyponychium is an area of skin that connects to the bottom of the free edge. The purpose of this structure is to prevent harmful things from entering the nail bed (germs, debris, etc.)

The major part of the external surface of your nail is called the nail body. It is sometimes referred to as corpus unguis and the nail plate. The nail body is the part of the nail that ranges from the free edge to the skin where your nail begins. This nail body is made of a specific type of keratin that makes it translucent as well as layers of dead cells that makes it strong and flexible.

Below the nail body is the nail bed. The nail bed functions to support the nail body during growth. The nail bed is made of tissues such as the hyponychium and the onychondermal. The onychondermal is the strongest attachment point between your nail and the underlying tissue. Both the onychondermal and hyponychium function for attachment and protection against pathogens and the surrounding environment.

The area that is at the base of the nail body is called the lunula. This structure is usually lighter and more white in color. It is also described as a half-crescent moon. The function of the lunula is to provide a defining change in structure between the nail edge and the root. Having a lunula usually means that your root matrix is working properly and well.

At the lateral edges of the nail body, where the nail meets your skin, the lateral nail fold begins. The lateral nail fold functions to aid in the protection of the finger on the lateral edges as well as aid in guiding the proper growth of the nail.

The area where the lunula meets the proximal nail fold is called the eponychium. This structure functions similarly to the hyponychium. It seals the passageway that would be present into the proximal nail fold and into the nail plate. This functions to prevent diseases, pathogens, and other harmful debris from entering parts of the nail that can become infected.

The proximal nail fold works in combination with the eponychium to prevent debris from entering the nail plate. It also functions similar to the lateral nail folds to guide and support nail growth. Finally, the nail fold protects the nail root and matrix that is present below this area.

The nail root is located within the epidermis several millimeters below the surface. The nail root place at which most of the nail is produced and where growth begins.

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.

Subcutaneous Layer

The first layer below the cutaneous membrane is called the subcutaneous layer. It is also known as the hypodermis and superficial fascia. Although this layer helps stabilize the integumentary system and shares some characteristics, it is not technically a part of the integumentary system.

Within the subcutaneous layer, you can find major blood vessels, adipose tissue, connective tissue, lymphatic vessels, nerves, and glands.

The function of the hypodermis layer is to protect vital organs that are deeper in your body. The hypodermis is also heavily affected based on your sex and the hormones you produce. An increased concentration creates a hypodermis that is thicker in areas like the back, arms, and shoulders. If you have a higher concentration of estrogen then you can expect the hypodermis to be thicker in the butt, thighs, and hips.

The hypodermis is also a well-known area for medical injections. Things such as the hypodermic needle and subcutaneous injections target this area for the contents within this layer.