The Skin: Structure, Function, and Adaptations, Lecture notes of Biology

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The Integumentary System

The epi means “on” and hypo means “under.” Therefore, the. 3 skin layers

on the dermis, and the layer under the dermis

The Epidermis

• composed of stratified squamous epithelium.
  • called keratinized stratified squamous
  • lining of structures that are open
• the skin is dry organ and required special protection called keratinized
• all epithelia , it is avascular (does not contain blood vessels)
• epithelium is made of 4 or 5 layers of epithelial cells, depending on the location.

Layers of Skin

• skin is composed of many layers
• Epidermis is the major outermost layer of the skin (composed of epithelial tissue)
• Dermis which lies deep to the epidermis, is composed of connective tissue
• Hypodermis has different composition of connective tissues than dermis
• Fascia is the term for connective tissue that anchors the skin to the muscle beneath
• 3 main layers of skin: epidermis, dermis and hypodermis

Layers of the Epidermis

• has four layers in most areas of the body
  • stratum basale
  • stratum spinosum
  • stratum granulosum
  • stratum corneum

Layers of the Epidermis

• Cells in all epidermal layers except the stratum basale are called keratinocytes
• Keratinocyte is a cell that manufactures and stores the protein keratin
  • Skin on the palms of the hands and the soles of the feet has fifth layer
    • stratum lucidum, located between the stratum corneuk and granulosum

Keratin

• Keratin is a intracellular fibrous protein that gives hair, nails and skin their hardness and water-resistant properties
• composed of long alpha helices
• the main component of hair shafts
  • referred to as “thick” skin, and all other skin is referred to as “thin” skin.
  • example: the thick skin of back vs the very thin skin of eyelids, contributed to much more by the dermis than the epithelium

Thin vs Thick Skin

• (A and B) thin skin has four distinguishable cell layers in the epidermis
• (C and D) thick skin, found only on the palms of the hands and soles of the feet, has additional layer, the stratum lucidum

Stratum Basale

• deepest epidermal layer
• attaches the epidermis to basement membrane
• 2 types of cells that found: Merkel cell and melanocyte Merkel cell, which functions as a sensory receptor and connected to sensory nerves and send signals about touch to the brain. These cells are abundant on the surface of hands and feet. Melanocyte is a cell that produces the pigment melanin. Melanin is a protein that can be made in two forms, functions to protect cells from UV

Stratum Spinosum

• young keratinocytes that have an american football-shaped appearance
• these cells are joined to each other through desmosomes
• composed of about 8 to 10 layers of
• interspersed among the keratinocytes of this layer is a type of dendritic cells calles langerhans cells
• the keratinocytes begin the synthesis of keratin and release water repelling
• As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.

Stratum Granulosum

• named for its granular appearance
• the cells are characterized by the presence of granules, intercellular protein- filled vesicles
• keratinocytes begin to accumulate so much protein in their cytoplasm that their nuclei and other cell organelles slowly disintegrate
• at the most superficial of granulosum the cells begin to die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.

Stratum Lucidum

• present in the skin of the palms of the hands and soles of the feet
• the keratinocytes are dead and flattened

Stratum Corneum

• the most superficial layer of every epidermis
• the layer of the skin that is exposed to the outside environment
• composed of 15 to 30 layers of dead skin cells
• cells in this layer are shed periodically and replaced by cells pushed up from the underlying strata
• entire layer is replaced during a period of about four weeks The skin has structural adaptations to endure daily friction, such as desmosomes connecting keratinocytes in the epidermis and the shedding of dead cells in the stratum corneum to protect underlying cells. The dermal papillae interlock with the epidermis, similar to interlaced fingers, to strengthen the connection between the layers and prevent them from detaching under friction. This structure, forming during the fetal period, is visible externally as fingerprints, which are unique and remain unchanged throughout life.

The Dermis

• Epidermis lacks blood vessels and receives nutrients from the underlying dermis.
• Composed of connective tissue proper with an interconnected mesh of elastin and collagen fibers, produced by fibroblasts.
• Two layers within the dermis, with varying thickness based on location (thickest in the back of the torso, thinnest on eyelids).
• Provide structure and tensile strength, extending throughout the dermis and into the hypodermis.
• Provide elasticity, allowing the skin to stretch and recoil during movement.
• Collagen binds water, keeping the skin hydrated.
• Scattered cells responsible for producing collagen and elastin fibers.
• Decreased fibroblast activity with age leads to reduced collagen and elastin, resulting in thinner skin, slower healing, and wrinkle formation. Stretch marks form when the skin is stretched too much, causing the supportive fibers to break. This usually happens during times of rapid growth, like puberty or pregnancy.

Papillary Layer

• Made of loose areolar connective tissue
• Forms the papillae that project into the stratum basale
• Contains fibroblasts
• Rich in small blood vessels
• Contains nerve fibers and touch receptors (tactile/Meissner corpuscles)
• The collagen fibers are finer (smaller in diameter) compared to those in the deeper reticular layer

Reticular Layer

• Made of dense, irregular connective tissue
• Lies beneath the papillary layer
• Well vascularized (rich blood supply)
• Contains a rich sensory and sympathetic nerve supply
• Has a denser weave of collagen fibers, with thicker individual fibers compared to the papillary layer

The Hypodermis

• Located directly below the dermis
• Connects the skin to underlying fascia (fibrous covering of muscles and organs)
• Collagen fibers in the hypodermis interweave with fascia collagen fibers, connecting the structures
• Made of loose areolar connective tissue and adipose tissue
• Provides fat storage, insulation, and cushioning
• Not technically part of the skin, but closely connected to it

The papillary and reticular layers of the dermis can be distinguished since

the word papillary means “small projections.”

Tactile corpuscles (Meissner corpuscles)

Hypodermis (subcutaneous layer)

Melanosomes

The relative coloration of the skin depends on the amount of melanin stored

in melanosomes inside the keratinocytes

Dermal Papillae

(A) The bumps and ridges of the dermis and epidermis interlock to form a combined structure that rarely pulls apart when exposed to friction or shear force. (B) The interdigitation can be likened to the interlocking of fingers. Bedsores, also called pressure ulcers, are caused by constant, long-term, pressure necrosis (tissue death) in the epidermis. Bedsores are most common in patients who have conditions that cause them to have reduced mobility.

Accessory Structures of the Skin

The skin's accessory structures include hair, nails, sweat glands, and oil glands. They start in the epidermis and can go down into the deeper layers. Glands are made of living cells that create fluids, while hair and nails are made of dead cells filled with keratin, like the outer skin.

Hair

• The visible portion of hair, made of keratin.
• long filament of keratin, is the hair shaft
• This hair is grown within a epidermal structure called hair follicle
• The deepest portion of follicle is the hair bulb
• The central core of the hair is medulla, the fragile inner core made up of living cells
• Each hair follicle is associated with an oil gland known as sebaceous gland
• Each hair root is connected to small muscle called arrector pili

Hair Growth Cycle

Hair follicles do not grow continuously, Rather, they alternate between growth and rest cycles. When a new growth phase begins, it pushes the old hair out of the follicle.

Nails

• nail bed is the living components of nails
• The nail body functions to protect the tip of our fingers and toes
• The nail body forms at the nail root (a protected region at the proximal side of the nail bed)
• Nail cuticle is a narrow and thin strap of just epidermis (also called as eponychium)
• The thick layer of epithelium called lunula

Sweat Glands

• Eccrine sweat gland are the type of sweat gland found all over the body. Abundant on palms, sole of the feet and forehead.
• Apocrine sweat gland are associated with hair follicles and found in densely hairy areas, such as armpits.

Sebaceous Glands

• type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and hair.
• Produce an oily mixture called sebum
• As we age the skin.s accessory of structures also have lowered activity, generating thinner hair and nails and reduced amounts of sebum and sweat

Acne

Acne is the name for disorder produced by immune reaction that can occur when the hair follicle becomes inflamed. The bacteria that feed off sebum can overgrow, triggering inflammation.

Functions of the Integumentary System

The skin is the largest organ in the body and its accessory structures are vital to the maintenance of homeostasis. The skin also performs a vital role in connecting other body system to the outside world so they can perform their own homeostatic functions.

Protection

The skin is designed to withstand environmental conditions and protect the body from wind, water, and UV light. Its layers of keratin and glycolipids prevent water loss and provide a barrier against friction, grit, microbes, and harmful chemicals. Sweat also helps protect the skin with its antimicrobial ingredient, dermcidin.

Sensory Function

The skin is highly sensitive and can detect small changes in the environment, such as a mosquito landing. Hair follicles and their surrounding nerve fibers sense touch and transmit signals to the brain, enabling a quick response. The skin contains various sensory receptors, including tactile corpuscles for light touch, lamellated corpuscles for pressure and vibration, Merkel cells for touch, nociceptors for pain, and thermoreceptors for temperature changes. This rich network of nerve fibers helps us sense and respond to environmental stimuli.

Thermoregulation

The integumentary system regulates body temperature with the help of the brain and spinal cord, particularly through the sympathetic nervous system, which also controls the fight-or-flight response.

Sensory Innervation of the Skin

The skin provides the central nervous system with environmental information through various receptors: thermoreceptors for temperature changes, tactile corpuscles for touch, nociceptors for pain, and lamellated corpuscles for pressure and vibration.

The sympathetic nervous system regulates body temperature by controlling sweat glands and blood vessels. It continuously monitors body temperature, maintaining homeostasis by secreting about 500 mL of sweat daily, even when not visibly sweating. During heat or exercise, sweat production can increase to 0.7-1.5 L per hour, cooling the body through evaporation. Blood vessels in the dermis also dilate to release heat. When body temperature drops, these vessels constrict to retain heat, but extreme cold can cause frostbite.

Vitamin D Synthesis

When a person is hot, dermal

blood vessel vasolidate.

When s person is cold, dermal

blood vessel vasoconstrict.

Blood vessels in the dermis vasodilate or vasoconstrict in order to control heat loss at the skin’s surface.

• The epidermis produces a precursor to vitamin D from cholesterol when exposed to UV radiation; this is completed in the liver and kidneys.
• Vitamin D is crucial for calcium and phosphorus absorption, essential for healthy bones.
• Lack of sun exposure can lead to vitamin D deficiency, causing rickets in children (bone deformities) and osteomalacia in adults (bone softening).
• Vitamin D is added to foods like milk and orange juice to address common deficiencies.
• Vitamin D supports immunity against infections and may be linked to reduced risks of tuberculosis, COVID-19, and some cancers.

Rickets

Rickets is a disease in which insufficient vitamin D is present during bone development. The bones become soft and do not have sufficient strength and rigidity to hold the body's weight. This normal presentation shows an X-ray of the lower limbs of a healthy child.

Healing the Integument

Injuries

The skin, being the body's primary interface with the environment, is prone to injuries like burns, wounds, and blisters. Healing begins with a blood clot to stop bleeding, followed by the formation of granulation tissue, which includes collagen deposited by fibroblasts and new blood vessels that provide nutrients and oxygen. Immune cells clear debris and pathogens. Vitamin D, crucial for calcium and phosphorus absorption, supports bone health. Deficiency can lead to rickets in children and osteomalacia in adults. Vitamin D also plays a role in immune defense against infections and may be linked to cancer. In the U.S., vitamin D is added to many foods due to insufficient sun exposure.

Wound Healing

(A) Wound healing begins with blood clotting and an inflammatory response that attracts white blood cells and fibroblasts. (B) Fibroblasts create a collagen-rich matrix called granulation tissue, which supports the migration of epithelial cells to close the wound. (C) The new epithelium and dermis may differ in consistency from the surrounding tissue.

Burns

Burns damage the skin through intense heat, radiation, electricity, or chemicals, leading to cell death and potential serious complications like dehydration, electrolyte imbalance, and infection. They are treated with intravenous fluids to address these issues. Burns are assessed using the "rule of nines" to determine the affected surface area and are classified by depth:

First-degree burns: affect only the epidermis, causing pain and swelling but typically heal in a few days. Second-degree burns: impact both the epidermis and part of the dermis, causing swelling, pain, and blistering. They require sterile care and heal in several weeks. Third-degree burns: extend through the epidermis and dermis, destroying tissue and nerve endings. They may appear white, red, or black and require medical attention for proper healing. These burns may be less painful due to nerve damage.

Estimating the Size of a Burn

The amount of body area affected by a burn is a critical piece of data in the calculation of treatment options.

Skin Cancer

Cancer refers to diseases where cells divide uncontrollably, often named after the organ or tissue where they originate. Skin cancer is particularly common in the U.S., with one in five people affected, partly due to increased UV radiation from ozone layer degradation. UV radiation can damage DNA, leading to cancerous changes. While many tumors are benign, cancers can metastasize, spreading to other organs. Skin cancers differ based on the type of skin cell they originate from.

Cancer Type

Basal cell carcinoma- its is a common skin cancer that affects stem cells in the stratum basale of the epidermis, often in areas with prolonged sun exposure. Early detection leads to better treatment outcomes. Squamous cell carcinoma- it is the second most common skin cancer, affecting keratinocytes in the stratum spinosum. It is more aggressive than basal cell carcinoma and can metastasize. Melanoma- it is a dangerous skin cancer caused by uncontrolled growth of melanocytes, often developing from a mole. It appears as asymmetrical brown or black patches with uneven borders and a raised surface. Melanoma is the most fatal skin cancer due to its high metastasis and difficulty in early detection.