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.

Dermis

Layers Of The Dermis

The dermis layer is located inferior to the epidermis layer in the cutaneous layer. This layer is made of connective tissue and can be divided into two layers:

  1. Papillary Layer
  2. Reticular Layer

The papillary layer of the dermis is the most superficial layer of the two. It is inferior to the epidermis and superior to the Reticular layer. Within the papillary layer there are:

  • Dermal Papillae
  • Axons of Neurons
  • Loose Connective Tissue
  • Capillaries

This layer’s main function is to provide attachment to the epidermis layer but, it also provides nutrients through capillaries and sense through neurons.

The reticular layer of the dermis is inferior to the papillary layer and superficial to the subcutaneous layer. This layer consists of:

  • Sebaceous Glands
  • Hair Follicles
  • Networks of Dense Irregular Connective Tissue
  • Sweat Glands

The dermis layer functions to provide structure and integrity to the dermis layer. The networks of dense irregular tissue results in strong and elastic tissue within the skin. It also houses and supports accessory structures such as hair follicles and vital glands.

Organization Of The Dermis

Within the dermis, two major fibers contribute to the structure and organization of your skin. Collagen fibers are responsible for providing tensile strength to the skin or the resistance of something to break under tension. Elastic fibers are responsible for providing elasticity to the skin. Not only does it allow the skin to be elastic, but it also allows your skin to return to its original shape. Bother of these fibers can be seen in use when you stretch the skin on any part of your body.

So, if there are fibers responsible for keeping your skin strong and not distorted, why do we get wrinkles? Wrinkles can be attributed to three major factors:

  1. Age
  2. UV light
  3. Hormones

As a person gets older, their body begins to slow down and produce less of the essential materials needed for good health. One of the things the body reduces in producing as you get older is natural oils that keep your skin moisturized and elastic. Thus, with a decreased amount of oil that helps keep elastic fibers and collagen fibers healthy, your skin will become weaker and form wrinkles.

UV light is directly responsible for the majority of the damage that is caused to the dermis layer. UV light will penetrate the skin and damage collagen fibers. As a result of damaged collagen fibers, elastic fibers will have an increased chance of being produced abnormally. This creates wrinkles in the skin.

Hormones, believe it or not, play a role in the development of wrinkles and the overall health of your skin. One of the major hormones that have been linked to wrinkles being formed is estrogen. A decrease in estrogen can trigger an increased deterioration of elastic fibers in the dermis.

Stretch Marks

Stretch marks can be caused by large amounts of growth in a short period or over-stretching of the skin. This can be done during a growth spirt of puberty, pregnancy, and obesity. When this overstretching happens, the reticular fibers that are holding the layers of skin together break. These fibers do not recoil and do not return to normal after this. Instead, these broken fibers create wrinkles, creases, and stretch marks.

Tension Lines

Tensions lines, also known as cleavage lines, are patterns of collagen fibers that are made within the dermis. Since collagen fibers and elastic fibers usually organize themselves in a parallel pattern, the body has a grid of tension lines that is used heavily in the medical field. These patterns are formed from the mechanical stress of the body. Areas that need more structural support will have tension lines pulling on them in a certain direction.

These tension lines are used heavily in the medical field to avoid the formation of scars. Cutting perpendicular to these tension lines increases the risk of scarring on the body. Thus, surgeons will cut parallel to these lines to make the recovery of the patient as easy as possible. The reason for this is that cutting a tension line perpendicular causes more stress and damage to the fibers than if you cut parallel.

Blood Supply To The Skin

Within the skin, there are two major networks of arteries, veins, and smaller blood vessels. These networks form a plexus or a network of interconnecting blood vessels:

  1. Cutaneous Plexus
  2. Subpapillary Plexus

The cutaneous plexus is composed of arteries and veins. This network supplies the hypodermis, hair follicles, fatty tissue, glands, and deep areas of the dermis. This network is about 1.5 mm from the surface of your skin.

The subpapillary plexus is made of smaller blood vessels. It is located below the dermal papillae and superficial areas of the dermis. Within each dermal papilla, there is a capillary loop that helps supply blood to those areas of the skin.

Both the cutaneous plexus and the subpapillary plexus are direct contributors to thermoregulation or the ability to control the temperature of your body using the cardiovascular system. When the surrounding environment is hot and the body temperature is too high, the vessels will dilate to allow more blood flow to the surface. This helps dissipate some of the heat that is trapped in the body. When it is too cold, the vessels will constrict and focus blood on vital parts of the body such as the heart, lungs, and internal organs.

Nerve Supply To The Skin

The integumentary system is a vital part of sensing the surrounding area. The nerves that are embedded inside our skin can tell us if it is hot, cold, windy, rainy, if you are in pain, and so much more. Most of these senses are sent back to the central nervous system where the body will decide on how to maintain homeostasis.

As well as sensing the surrounding area, the nervous system is responsible for adjusting secretion rates and controlling the flow of blood.

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.