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.

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.

The Epidermis

Epidermis Cell Types

The epidermis is the most superficial layer of the cutaneous membrane. It is composed of stratified squamous epithelium and consists of four types of cells:

  1. Keratinocytes
  2. Melanocytes
  3. Merkel Cells
  4. Langerhans Cells

Keratinocytes are the most abundant and prominent cell type in the epidermis layer. It produces a protein called keratin that helps harden and makes the cells more durable and waterproof. These cells will replicate at the deepest layer of the epidermis and move their way to the surface where they will eventually be shed off.

Melanocytes are responsible for the color of your skin. They do this by producing a protein called melanin. Melanin is a dark brown and black pigment that is present in the skin, hair, and eyes. This is the protein that is produced when exposed to sunlight and your skin becomes tan.

Merkel cells are sensory cells located just below the epidermis and at the base of hair follicles. They are often found near the receptors of sensory nerves. They function in receiving information through physical movements.

Langerhans cells are one of the defense mechanisms of the integumentary system. These cells are wondering macrophages dedicated to defending the epidermis. They can perform their functions and move within the epidermis tissue but, they are also able to move from the tissue to the lymphatic vessels.

Layers Of The Epidermis

The epidermis is made of many layers of stratified squamous epithelium. As you move throughout these layers, however, you will notice distinct differences between some of the layers. In the epidermis, five possible layers can be present. Four of these can be found in all epidermis tissue:

  1. Stratum Basale
  2. Stratum Spinosum
  3. Stratum Granulosum
  4. Stratum Lucidum (sometimes present)
  5. Stratum Corneum

The Stratum Basale, also known as the Stratum Germinativum, is the deepest layer of cells in the epidermis. It consists of basal cells, melanocytes, and Merkel cells in areas of no hair. The cells of this area are the newest and are usually undergoing active reproduction. These cells create an attachment to the basal lamina.

Basal cells, otherwise known as stem cells are responsible for performing and carrying out the active reproduction that occurs in this area. The melanocytes are located in this layer of cells and are responsible for the pigmentation of the skin. Without any pigmentation, the skin will appear pale and white. This is known as albinism.

The Stratum Spinosum is the second deepest layer in the epidermis. It is superior to the Stratum Basale and inferior to the Stratum Granulosum. In this layer, the keratinocytes are bound using maculae adherens. These adherens attach to desmosomes and tonofibrils. Tonofibrils aid in the attachment of desmosomes to the cells.

In the Stratum Spinosum, you can find some keratinocytes dividing, Langerhans cells, and melanocytes. This layer of cells aid in making the epidermis flexible and strong with the “stickiness” of the cell attachments and the polyhedral shape of the keratinocytes.

The Stratum Granulosum is the third deepest layer in the epidermis. It is located superior to the Stratum Spinosum and Inferior to the Stratum Lucidum or Stratum Corneum, depending on the location in the body. This layer is only a few cells thick and the cells often become flattened here. In this layer, keratinocytes produce keratohyalin and keratin.

Keratin will begin to form keratin fibers as the cells become thinner and thinner. These fibers will help strengthen the cell walls of these cells and aid in the integrity and structure of the epidermis. Keratohyalin is responsible for creating an impermeable network using keratin fibers. This helps in the defense against pathogens and unwanted particles by making them impenetrable. When an excess amount of these proteins are secreted it will cause an extreme thickening and strength of the skin. This is known as calluses.

The Stratum Lucidum is the only layer of the epidermis that is not found throughout the entire body. It is found in areas of high mechanical abrasion such as the palms of your feet and hands. Under the microscope, you can observe this layer as a thin layer of thick glassy cells. This layer is responsible for extra protection to the dermis layer but, it also allows for increased movement without friction to the other layers.

The Stratum Corneum is the most superficial layer of the epidermis. It consists of dehydrated, dead, and interlocking epithelial cells. This is the layer at which cells will shed from the skin and into the environment. It is important however because of how thick it is. This is the thickest of all the layers and provides an immense amount of protection.

Thick and Thin Skin

Thick skin is often found on the palms and soles of your feet and hands. Thick skin is most often found in areas with high mechanical abrasion. Thick skin is defined as an area of skin that contains five layers of cells. This means that the Stratum Lucidum is present to provide extra strength, flexibility, and overall protection.

Thin skin can be found on all other parts of the body that thick skin is not. This layer of skin is only made of four layers and does not provide the same strength and overall protection as Thick skin.

Dermal Ridges

One of the most unique characteristics of your skin, especially on your fingers, is the pattern that forms due to underlying layers. The epidermal ridges that form from these layers below the skin are also known as your dermal papillae. These dermal papillae create the fingerprint pattern that is unique to every person.

The reason these ridges and patterns are formed is because of the dermal ridges that are created from the Stratum Basale. The Stratum Basale creates a wave-like surface with the underlying connective tissue. This wave-like pattern helps increase the surface area between the connective tissue and epithelial tissue, ultimately increasing the area of attachment between the two. This helps increase the strength and durability of the cutaneous membrane.

If you look at the skin on your hands, you may begin to notice that there are areas of skin that has more defined dermal ridges. For example, look at your palms and fingertips. You can clearly see the dermal ridges and patterns that form. Now, if you look at the other side of your hand you will find it harder to see those patterns in such detail. This is because areas of thick skin have more defined dermal ridges than areas of thin skin.

Skin Color

The color of your skin is a result of a variety of different variables. These variables can be divided into three main categories:

  1. Dermal Blood Supply
  2. Concentrations of Melanin and Carotene
  3. Thickness of the Stratum Corneum

Dermal blood supply is responsible for the color of your skin through increased and decreased blood volume. If your blood supply decreases, like your hands on a cold day, the color of your skin will become lighter or pale in color. If blood supply is reduced for a prolonged period it will result in cyanosis. This is is the discoloration of your skin due to inadequate oxygen supply to tissue. Cyanosis will result in the skin turning a dark blue color.

Within the epidermis, there are two main pigments, Carotene and Melanin. Depending on the concentration of both of these, the skin will be darker or lighter in color.

Carotene is not a substance produced within the body. Carotene is derived from foods such as corn, squash, and carrots. Carotene can be converted into vitamin A. Vitamin A is vital for the synthesis of visual pigments in the receptors of the eyes. In regards to the color of your skin, carotene is responsible for the yellow color of your skin. If there is an increased concentration of carotene, there will be an increased color of yellow shown in the skin.

Melanin is responsible for creating the natural color and tan of your skin. It is extremely important and functions to protect the skin against UV radiation from the sun. Melanin is stored and produced within the melanosomes of melanocytes. The more melanin that is stored within these structures the darker the skin will appear.

Melanocytes can increase the amount of melanin being produced due to increased exposure to UV radiation. Repeated increases in UV exposure can result in epidermal cancers (sarcomas), abnormal structure of connective tissue, and long-term damage to the skin.