13.1 Structure and function of the skin

The skin, our body's largest organ, serves as a protective barrier against external threats and plays a crucial role in maintaining homeostasis. Its complex structure, composed of multiple layers and specialized cells, enables it to perform various functions essential for our survival.

From the outermost epidermis to the deeper dermis, the skin's layers work together to provide protection, regulate body temperature, and facilitate sensation. Understanding the skin's structure and functions is key to appreciating its importance in human physiology and health.

Epidermis Layers and Functions

Epidermal Layers

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• The epidermis is the outermost layer of the skin composed of stratified squamous epithelium
• The stratum basale is the deepest layer of the epidermis
  • Contains stem cells that continually divide to produce new keratinocytes (skin cells)
• The stratum spinosum is the next layer above the stratum basale
  • Keratinocytes begin to produce keratin (a protective protein) and lipids (fats)
• The stratum granulosum is where keratinocytes become flattened and filled with keratohyalin granules
• The stratum lucidum is a thin, clear layer found only in thick skin (palms and soles)
• The stratum corneum is the outermost layer composed of dead, flattened keratinocytes that provide a protective barrier

Melanocytes and Melanin Production

• Melanocytes are found in the stratum basale and produce melanin
• Melanin is a pigment that protects the skin against UV radiation
  • Absorbs and dissipates UV energy, preventing damage to DNA in skin cells
  • Increased melanin production leads to skin tanning, a protective response to UV exposure

Dermis: Structure and Nourishment

Dermal Layers and Connective Tissue

• The dermis is the layer beneath the epidermis composed of connective tissue
• The papillary dermis is the uppermost layer of the dermis
  • Contains blood vessels, nerve endings, and collagen fibers
  • Provides nutrients and oxygen to the epidermis via its rich blood supply
• The reticular dermis is the deeper layer of the dermis
  • Contains thicker collagen and elastin fibers that provide strength and elasticity to the skin
  • Collagen fibers provide tensile strength, while elastin fibers allow skin to stretch and recoil

Dermal Structures and Functions

• Blood vessels in the dermis supply oxygen and nutrients to the epidermis and help regulate body temperature
  • Vasodilation (widening of blood vessels) increases blood flow and heat loss, cooling the skin
  • Vasoconstriction (narrowing of blood vessels) reduces blood flow and heat loss, conserving body heat
• Fibroblasts in the dermis produce collagen, elastin, and other extracellular matrix components
• Hair follicles, sweat glands, and sebaceous glands are embedded in the dermis
  • Hair follicles produce hair and are involved in sensory functions
  • Sweat glands produce sweat for thermoregulation and excretion of wastes
  • Sebaceous glands secrete sebum, an oily substance that lubricates and waterproofs the skin

Skin Functions: Protection, Thermoregulation, Sensation

Protective Functions

• The skin acts as a physical barrier, protecting the body from mechanical damage, chemicals, and pathogens
  • The stratum corneum provides a tough, waterproof barrier against external insults
  • Tight junctions between keratinocytes prevent pathogens and harmful substances from entering the body
• Melanin in the epidermis absorbs UV radiation, protecting the skin from sun damage
  • UV radiation can cause DNA damage, leading to skin aging and skin cancer
• The skin's acidic pH (around 4.5-6.5) and antimicrobial peptides help prevent bacterial and fungal growth
  • Low pH inhibits growth of many pathogens
  • Antimicrobial peptides (defensins, cathelicidins) directly kill or inhibit microorganisms

Thermoregulation

• The skin helps regulate body temperature through vasodilation, vasoconstriction, and sweating
• Eccrine sweat glands produce sweat, which evaporates to cool the skin surface
  • Evaporation of sweat removes heat from the skin, cooling the body
  • Sweat production is controlled by the hypothalamus in response to increased body temperature
• In cold environments, vasoconstriction reduces blood flow to the skin, conserving heat

Sensation and Vitamin D Synthesis

• The skin contains various sensory receptors that detect touch, pressure, temperature, and pain
  • These receptors allow the body to interact with and respond to the environment
• The skin plays a role in vitamin D synthesis when exposed to UV radiation
  • UV-B radiation converts 7-dehydrocholesterol in the skin to vitamin D3
  • Vitamin D is essential for calcium absorption and bone health

Skin Receptors: Types and Sensory Functions

Mechanoreceptors

• Mechanoreceptors detect mechanical stimuli such as touch, pressure, and vibration
• Meissner's corpuscles are found in the papillary dermis and detect light touch and low-frequency vibrations
  • Densely distributed in sensitive areas like the fingertips and lips
  • Rapidly adapting receptors that respond to changes in stimulus intensity
• Pacinian corpuscles are located in the reticular dermis and detect deep pressure and high-frequency vibrations
  • Onion-shaped structures that respond to rapid changes in pressure
  • Important for detecting textures and vibrations transmitted through objects
• Ruffini endings are slowly adapting mechanoreceptors that detect sustained pressure and skin stretching
  • Respond to long-lasting mechanical stimuli and are important for proprioception (sense of body position)
• Merkel discs are found in the stratum basale and detect fine touch and texture
  • Slowly adapting receptors that provide information about object shape and edges
  • Densely distributed in the fingertips and lips

Thermoreceptors and Nociceptors

• Thermoreceptors detect changes in temperature
• Cold receptors are activated by decreases in temperature and respond more strongly than warm receptors
  • Detect temperatures below skin temperature (around 34°C)
  • Important for detecting cool objects and avoiding cold-induced tissue damage
• Warm receptors are activated by increases in temperature
  • Detect temperatures above skin temperature
  • Help maintain body temperature by detecting warm environments
• Nociceptors detect noxious stimuli that can cause tissue damage, such as extreme temperatures, mechanical damage, and chemicals
  • Respond to potentially harmful stimuli and generate the sensation of pain
  • Important for protecting the body from injury and promoting avoidance behaviors