🤾🏻‍♂️Human Physiology Engineering Unit 13 – Skin and Integumentary System

Skin Structure and Layers

• Epidermis outermost layer of skin provides protective barrier against environmental factors (UV radiation, pathogens)
  • Stratum corneum uppermost layer of epidermis consists of dead, keratinized cells
  • Stratum basale deepest layer of epidermis contains stem cells responsible for continuous renewal of skin
• Dermis layer beneath epidermis contains connective tissue, blood vessels, and sensory receptors
  • Papillary dermis uppermost layer of dermis consists of loose connective tissue and capillary loops
  • Reticular dermis deeper layer of dermis contains dense irregular connective tissue and larger blood vessels
• Hypodermis subcutaneous layer beneath dermis primarily composed of adipose tissue (fat cells) for insulation and energy storage
• Epidermal-dermal junction undulating interface between epidermis and dermis enhances mechanical stability and nutrient exchange
• Skin thickness varies across body regions (eyelids thinnest, palms and soles thickest) depending on mechanical stress and functional requirements

Functions of the Integumentary System

• Protection skin acts as a physical barrier against external factors (mechanical trauma, chemicals, pathogens)
  • Keratinocytes produce keratin, a tough, fibrous protein that provides mechanical strength to skin
  • Langerhans cells specialized immune cells in epidermis that detect and respond to foreign antigens
• Thermoregulation skin plays a crucial role in maintaining body temperature through various mechanisms
  • Vasodilation and vasoconstriction of cutaneous blood vessels regulate heat loss
  • Sweat glands produce and secrete sweat, which evaporates to cool the skin surface
• Sensation skin contains various sensory receptors that detect touch, pressure, temperature, and pain
  • Meissner's corpuscles respond to light touch and low-frequency vibrations
  • Pacinian corpuscles detect deep pressure and high-frequency vibrations
• Vitamin D synthesis skin synthesizes vitamin D when exposed to UV-B radiation, which is essential for calcium absorption and bone health
• Excretion skin acts as a minor excretory organ, eliminating small amounts of waste products (urea, salts) through sweat
• Immunity skin serves as an immunological barrier, with immune cells (Langerhans cells, T cells) that recognize and respond to pathogens
• Aesthetics and communication skin appearance (color, texture, wrinkles) plays a role in social interactions and non-verbal communication

Skin Appendages and Specialized Structures

• Hair follicles invaginations of epidermis that produce and anchor hair shafts
  • Hair provides insulation, protection, and sensory functions
  • Hair growth occurs in cycles (anagen, catagen, telogen) regulated by hormones and growth factors
• Sebaceous glands holocrine glands associated with hair follicles that secrete sebum, an oily substance that lubricates and waterproofs skin
  • Sebum production is regulated by androgens and plays a role in skin barrier function
• Sweat glands eccrine and apocrine glands that produce and secrete sweat for thermoregulation and excretion
  • Eccrine glands distributed throughout the body, produce a clear, hypotonic sweat
  • Apocrine glands located in specific regions (axillae, groin), produce a thicker, protein-rich sweat
• Nails keratinized structures that protect the distal phalanges of fingers and toes
  • Nail plate hard, translucent layer composed of dead, keratinized cells
  • Nail matrix proliferative region at the proximal end of the nail that generates new nail cells
• Arrector pili muscles small, smooth muscle bundles attached to hair follicles that contract to cause "goosebumps" and erect hair shafts
• Sensory receptors specialized structures (Merkel cells, Ruffini endings) that detect various stimuli (touch, pressure, temperature)

Skin Homeostasis and Thermoregulation

• Skin barrier function maintained by a balance of keratinocyte proliferation, differentiation, and desquamation
  • Lipid matrix in stratum corneum (ceramides, cholesterol, fatty acids) prevents water loss and entry of foreign substances
• pH balance skin surface maintains a slightly acidic pH (4.5-6.5) due to secretions from sweat and sebaceous glands
  • Acidic pH inhibits growth of pathogenic microorganisms and maintains skin barrier integrity
• Melanogenesis process of melanin production by melanocytes in response to UV radiation
  • Melanin pigment provides photoprotection by absorbing and scattering UV rays
  • Skin color determined by the amount, type, and distribution of melanin
• Thermoregulation achieved through a complex interplay of vasomotor responses, sweat production, and evaporative cooling
  • Hypothalamus acts as the body's thermostat, integrating signals from peripheral and central thermoreceptors
  • Cutaneous blood flow regulated by sympathetic nervous system to control heat loss or retention
• Hydration skin maintains proper hydration through a balance of water loss (transepidermal water loss) and water retention (natural moisturizing factors)
• Microbiome diverse community of microorganisms (bacteria, fungi, viruses) that colonize the skin surface and contribute to skin health
  • Commensal microbes compete with pathogens, produce antimicrobial substances, and modulate immune responses

Wound Healing and Tissue Repair

• Hemostasis initial phase of wound healing that involves the formation of a fibrin clot to stop bleeding
  • Platelets adhere to damaged blood vessels, aggregate, and release growth factors and cytokines
  • Coagulation cascade activated to convert fibrinogen into fibrin, stabilizing the clot
• Inflammation second phase of wound healing characterized by the recruitment of immune cells to the wound site
  • Neutrophils first responders that remove debris, bacteria, and damaged tissue
  • Macrophages arrive later, phagocytose pathogens, and secrete growth factors to promote tissue repair
• Proliferation third phase of wound healing marked by the formation of granulation tissue and re-epithelialization
  • Fibroblasts migrate into the wound, proliferate, and synthesize new extracellular matrix (collagen, elastin)
  • Keratinocytes at the wound edges migrate and proliferate to restore the epidermis
• Remodeling final phase of wound healing that involves the maturation and reorganization of the newly formed tissue
  • Collagen fibers align along tension lines to increase the tensile strength of the scar
  • Scar tissue gradually becomes less cellular and vascular over time
• Factors affecting wound healing age, nutrition, diabetes, smoking, and medications can influence the speed and quality of wound repair
• Chronic wounds non-healing wounds (pressure ulcers, diabetic foot ulcers) that fail to progress through the normal stages of healing due to underlying pathologies or infections

Skin Disorders and Diseases

• Acne common inflammatory disorder of the pilosebaceous unit characterized by the formation of comedones, papules, and pustules
  • Multifactorial etiology involves sebum production, bacterial colonization (Propionibacterium acnes), and inflammation
• Atopic dermatitis chronic, relapsing inflammatory skin condition associated with pruritus, xerosis, and lichenification
  • Pathogenesis involves a complex interplay of genetic, immunological, and environmental factors
• Psoriasis chronic, autoimmune disorder characterized by the formation of erythematous, scaly plaques
  • Keratinocyte hyperproliferation and immune dysregulation (T cells, cytokines) contribute to the pathogenesis
• Skin cancer malignant neoplasms arising from the various cells of the skin
  • Basal cell carcinoma most common type, rarely metastasizes, associated with UV exposure
  • Squamous cell carcinoma second most common type, can metastasize, associated with UV exposure and immunosuppression
  • Melanoma most aggressive type, arises from melanocytes, associated with UV exposure and genetic factors
• Infections skin can be affected by various bacterial (cellulitis, impetigo), fungal (dermatophytosis, candidiasis), and viral (herpes simplex, varicella) infections
• Autoimmune disorders skin can be a target of autoimmune reactions, leading to conditions such as vitiligo, scleroderma, and bullous pemphigoid

Integumentary System in Biomedical Engineering

• Tissue engineering strategies to develop artificial skin substitutes for wound healing and regenerative medicine
  • Scaffolds biomaterials (collagen, hydrogels) that provide a 3D structure for cell attachment and growth
  • Cell sources autologous or allogeneic keratinocytes, fibroblasts, and stem cells used to populate the scaffolds
• Transdermal drug delivery systems designed to deliver medications through the skin, bypassing the gastrointestinal tract
  • Patches adhesive devices that contain a drug reservoir and a rate-controlling membrane
  • Microneedles arrays of microscopic needles that create transient channels in the stratum corneum to enhance drug permeation
• Skin-on-a-chip microfluidic devices that mimic the structure and function of human skin for drug testing and disease modeling
  • Microchannels allow for the perfusion of nutrients and the removal of waste products
  • Co-culture of different skin cell types (keratinocytes, fibroblasts, melanocytes) to recapitulate the skin microenvironment
• Skin imaging technologies non-invasive methods to visualize and analyze skin structure and function
  • Optical coherence tomography uses near-infrared light to generate high-resolution, cross-sectional images of the skin
  • Confocal microscopy provides real-time, in vivo imaging of the skin at the cellular level
• Skin-interfacing wearable devices sensors and actuators that monitor skin health and deliver therapeutic interventions
  • Flexible electronics conform to the skin surface and measure various parameters (hydration, pH, temperature)
  • Closed-loop systems monitor skin conditions and provide targeted therapies (drug delivery, electrical stimulation)

Clinical Applications and Future Innovations

• Skin grafting surgical procedure that involves the transplantation of healthy skin to cover wounds or burns
  • Autografts skin taken from another site on the patient's own body
  • Allografts skin obtained from a genetically non-identical donor
• Skin cancer detection artificial intelligence (AI) algorithms that analyze skin lesion images to aid in the early detection and diagnosis of skin malignancies
  • Convolutional neural networks (CNNs) deep learning models that can classify skin lesions with high accuracy
  • Smartphone apps enable users to capture and analyze skin images for self-monitoring and remote consultation
• Skin microbiome modulation strategies to maintain or restore a healthy balance of skin microorganisms
  • Probiotics topical application of beneficial bacteria (Staphylococcus epidermidis) to compete with pathogens and modulate immune responses
  • Prebiotics substances (oligosaccharides) that selectively promote the growth of beneficial skin microbes
• Skin aging interventions therapies aimed at preventing or reversing the signs of skin aging (wrinkles, sagging, pigmentation)
  • Retinoids vitamin A derivatives that stimulate collagen production and improve skin texture
  • Antioxidants (vitamin C, E) protect the skin from oxidative stress and promote collagen synthesis
• Personalized skincare customized approaches that tailor skincare regimens based on an individual's genetic profile, skin type, and environmental factors
  • Genetic testing identifies variations in genes (MC1R, FLG) that influence skin pigmentation, barrier function, and aging
  • Skin microbiome analysis guides the selection of skincare products that support a healthy microbial balance
• 3D bioprinting emerging technology that enables the precise deposition of skin cells and biomaterials to create personalized skin constructs
  • Bioinks cell-laden hydrogels that provide a supportive matrix for skin cell growth and differentiation
  • In situ bioprinting direct printing of skin cells onto wounds to accelerate healing and reduce scarring