Integumentary System & Skin Layers

• The integumentary system consists of the skin, hair, nails, and associated exocrine glands.
• The integumentary system is responsible for protecting internal organs, sensory function, thermoregulation, and Vitamin D synthesis.

Skin Layers

• The skin is composed of three layers: the epidermis, dermis, and hypodermis (subcutaneous layer).
• Fascia connects skin to underlying muscle.
• The layers of skin that are cut during abdominal surgery are the epidermis, dermis, hypodermis, and fascia.

Epidermis

• The epidermis is the most superficial layer of the skin
• The epidermis is made of keratinized stratified squamous epithelium.
• It is avascular, meaning it contains no blood vessels.
• It contains four to five layers depending on the location on the body
• Thick skin, which has five layers, is found on the palms of the hands and the soles of the feet.
• Thin skin, which has four layers of epidermis, covers most of the body.
• Thick skin contains an additional stratum lucidum, which is not found in thin skin, and also has a higher number of sensory receptors.
• Cells are produced in the deepest layer of the epidermis, and they migrate from that layer to the superficial layer.
• Mature cells in the epidermis are called keratinocytes.
• Keratin makes cells tough and water-resistant.
• The layers of the epidermis from superficial to deep are: stratum corneum, stratum granulosum, stratum spinosum, and stratum basale.

Stratum Basale

• The stratum basale is the deepest of the epidermal layers.
• It is a single layer of cells that actively divide to replace cells in superficial layers.
• Additional cells found in the stratum basale are Merkel cells, which are sensory receptors used for discriminatory touch
• Melanocytes, which produce melanin to protect cells from UV radiation, are also found in the stratum basale.
• Melanin is produced by melanocytes in the stratum basale.
• Melanin travels to cells within the stratum granulosum, and is held in melanosomes.
• There are two forms of melanin: eumelanin, which is black and brown, and pheomelanin, which is reddish.
• All humans have a similar concentration of melanocytes, but the activity of melanocytes leads to different skin tones.

Stratum Spinosum

• The stratum spinosum is eight to ten cell layers thick.
• Keratinocytes are shaped like footballs.
• Pointed ends that look like spines give rise to the name "spinosum”.
• Langerhans (dendritic) cells provide immune protection.
• Cells are continually pushed toward the stratum granulosum.

Stratum Granulosum

• It is named for its granular appearance.
• Cells begin to flatten and accumulate more keratin.
• Melanin can travel to cells and is contained in melanosomes in this layer.
• Cells begin to die at the most superficial layers.

Stratum Lucidum

• The Stratum Lucidum is only found in thick skin, such as the skin on the palms or the soles of the feet.
• It is composed of tightly packed, dead keratinocytes.
• The cells contain eleidin, a protein that functions as a water barrier.

Stratum Corneum

• It is the most superficial layer of the epidermis.
• It consists of about 15 to 30 layers of dead keratinocytes.
• Cells are shed and lost due to mechanical forces.
• Cells are replaced by cells in deeper layers migrating into the stratum corneum.

Dermis

• The dermis lies deep to the epidermis.
• The dermis forms projections that extend into the epidermis.
• The dermis is composed mainly of connective tissue.
• The dermis also contains blood vessels, hair follicles, and glands.
• It supports the epidermis with nutrients, strength, and elasticity.
• The dermis has two layers: the papillary layer and the reticular layer.
• The papillary layer is the more superficial layer of the dermis.
• The reticular layer is the deeper layer of the dermis.

Papillary Layer

• The Papillary Layer is composed of loose areolar connective tissue.
• The dermis contains dermal papillae that project up into the stratum basale of the epidermis.
• Dermal papillae contain blood vessels, nerve fibers, and tactile (Meissner's) corpuscles.
• Meissner's corpuscles used to detect light pressure.

Reticular Layer

• The reticular layer is much thicker than the papillary layer.
• It is made of dense irregular connective tissue
• The reticular layer contains hair follicles, blood vessels, and nerves.
• Pacinian corpuscles, or cells that sense deep pressure, are also found in the reticular layer.

Hypodermis

• The hypodermis is also known as the subcutaneous layer.
• It lies deep to the dermis.
• The hypodermis is composed of adipose and loose areolar connective tissue.
• The hypodermis provides insulation and cushioning.
• Since it is highly vascularized, high blood flow provides essential nutrients.
• The hypodermis contains brown fat in infants, which aids in thermoregulation.
• The epidermis and dermis would need to be cut in order for a cut to bleed. The hypodermis may be cut as well. If only the epidermis was cut, there would be no bleeding as it is an entirely avascular layer.

Accessory Structures of the Skin

• Structures associated with skin include hair, nails, sweat glands, and sebaceous glands.

Hair

• Hair is found on most body surfaces, except for the palms of hands and soles of feet
• Hair is composed of dead, keratinized cells from the epidermis.
• Structures associated with hair are sebaceous (oil) glands, which secrete sebum, and arrector pili muscles, which contract to make hair stand up.
• Hair provides physical protection, sensory input, thermoregulation, and UV protection.
• Components of hair from deep to superficial are: hair papilla (blood supply to follicle), hair bulb (deepest portion of follicle), hair root (between bulb and shaft), hair shaft (visible portion above skin), medulla (center of hair shaft), cortex (surrounds medulla), and cuticle (surrounds cortex).
• Hair growth is not continuous, where follicles alternate between growth and rest cycles
• New growth pushes old hair out of follicle.

Nails

• Nails are composed of keratinized epidermal cells.
• Nails consist of the nail bed (living component which produces the nail body), nail body (visible hard portion), nail root (proximal side), nail cuticle (eponychium, a thin layer of skin at the base of nail), and the lunula (crescent-shaped region of nail bed)

Sweat Glands

• Sweat glands, also known as sudoriferous glands, produce sweat (perspiration) to aid in temperature regulation.
• There are two types of sweat glands: eccrine and apocrine.

Eccrine Sweat Glands

• Eccrine sweat glands are found all over body, and produce less viscous sweat.
• They're involved in thermoregulation.

Apocrine Sweat Glands

• Apocrine sweat glands are found in the groin and axilla (armpit).
• They produce more viscous secretion, and may be involved in pheromone release.
• Apocrine sweat glands secrete a viscous sweat within hair follicles, and are associated with pubic hair, and only become active during puberty.
• Eccrine sweat glands secrete a less viscous sweat onto the surface of skin.

Sebaceous Glands

• Sebaceous glands are usually associated with hair follicles.
• They secrete an oily mixture called sebum.
• They lubricate and waterproof skin.
• Secretion is stimulated by hormones released during puberty.
• Accumulation of sebum, keratin and dead cells can block hair follicle, which can lead to bacteria feeding on sebum and sweat to grow, which can lead to inflammation of the skin called acne
• Overly active sebaceous glands can contribute to acne, because bacteria can use nutrients in sebum to grow rapidly.
• By decreasing sebaceous gland activity the nutrients available to the bacteria would decrease and that should slow their growth.

Integumentary System Functions

• The integumentary system provides protection against water loss through keratin, sebum, and glycolipids
• The integumentary also creates protection against microbes is provided through dermcidin in sweat and macrophages (immune cells)
• The integumentary system and its production of melanin provides protection against UV radiation.

Sensory Function

• The skin has different types of sensory receptors found in various layers.
• Meissner's corpuscles and Pacinian corpuscles create tactile sensations of touch, pressure, vibration, as well as tickle.
• Thermoreceptors detect hot and cool sensations.
• Nociceptors detect pain.

Thermoregulation

• Sweat assists in keeping the body cool.
• Increased blood flow to the skin cools the body.
• Adjustments of blood flow regulate body temperature.
• On a cold day, blood flow is diverted away from the skin and toward the internal organs to maintain a warmer temperature for those critical organs. Sweat glands remain inactive.
• On a hot day, blood flow through the skin is increased to allow for heat to dissipate from the body. Sweat glands are activated to dissipate heat through the skin.

Vitamin D

• It's synthesis is assisted by ultraviolet (UV) rays, which activate the precursor molecule to initiate vitamin D synthesis.
• It assists in the absorption of calcium from foods in the gastrointestinal tract.
• It is necessary for bone growth and immune function.
• Vitamin D deficiency can cause misshaped bones due to calcium deficiency in children.
• Bones are weak due to lack of calcium
• This may develop into osteomalacia in adults

Injuries

• Skin is highly vulnerable to injury.
• Injuries include abrasions, cuts, burns
• Skin is highly regenerative.
• Wound healing may lead to scars.
• There may be a loss of accessory structures, such as hair.
• Repaired tissue may have a different texture or consistency.

Burns

• Burns occur when damage is caused by heat, radiation, electricity, or chemicals.
• Skin cells die can be replaced.
• First-degree burns only affect the epidermis.
• Second-degree burns affect the epidermis and dermis.
• Third-degree burns affect the epidermis, dermis, and hypodermis.

Severity of Burns

• The "rule of nines" assists with estimations of treatment for the burn.
• The percentage breakdowns are: head and neck - 9%, upper limbs – 9% each, trunk – 36 %, genitalia – 1%, and lower limbs – 18% each
• With third degree burns all of the skin layers are affected

Wound Healing

• The wound healing process in multiple steps.
• It begins with blood clotting.
• This is followed by fibroblasts producing new collagen, in a process called granulation tissue creation
• The final steps include the regeneration of epidermis.
• After healing, a scar may form.

Skin Cancer

• Skin cancer is associated with overexposure to UV radiation.
• Ultraviolet radiation causes mutations in DNA, leading to increased cancer risk.
• Skin cancers vary depending on cell where the cancer originated.
• There are three forms of skin cancer: Basal cell carcinoma, which is caused by cells of stratum basale, squamous cell carcinoma, which is caused by keratinocytes of stratum spinosum, and melanoma, which is caused by melanocytes.
• The effects of basal cell carcinoma involve the cells of the stratum basale, it is the most common cancer in the United States.
• Squamous cell carcinoma is less common than basal cell carcinoma
• Squamous cell carcinoma is more aggressive and can metastasize.
• Melanoma usually begins as a mole, can be difficult to detect, and it is highly metastatic.