Human Skin Layers

• Human skin is the biggest organ in the body
• Adult skin weighs approximately 5 kg
• The body surface area of skin is 2m^2
• The skin protects underlying structures like muscle and fascia

Distinctive Layers

• The skin contains 3 distinctive layers
• Epidermis: outer layer, stratified and cellular
• Dermis: connective tissue
• Subcutis: innermost layer, contains fat and skin appendages

Epidermis

• The epidermis alone is 0.05-0.1 mm thick
• It has a 30-day cell cycle, where cells move from the basal layer to the surface
• The epidermis is a cellular layer containing keratinocytes, which proliferate and differentiate
• The epidermis is stratified into the stratum corneum, stratum granulosum, stratum spinosum, and stratum basale

Dermo-Epidermal Junction (DEJ)

• The dermo-epidermal junction (DEJ) is a complex network of proteins and glycoproteins
• It includes basement membranes and allows communication betwen basal keratinocytes and the superficial dermis
• The DEJ is important for cell adhesion and blistering
• It facilitates cellular migration during wound healing
• It's further responsible for cell signalling

Dermis

• The dermis provides a supporting matrix and ground substance
• It consists of proteins, polysaccharides, and fibers
• It provides strength, elasticity, and water retention
• The dermis is rich in blood supply in superficial and deep vascular plexuses
• The dermis contains adnexal structures, including pilosebaceous units, eccrine glands, and apocrine glands

Protein Fibres

• The dermis contains two major protein fibres
• Collagens make up 80-85% of the dermis by dry weight
• Elastic tissues constitute 2-4% of the dermis, comprised of elastin and microfibrils
• Collagen gives the skin tensile strength; including, types I and II in the dermal interstitium, and type IV in the DEJ
• Elastin provides elasticity

Polysaccharide

• Polysaccharides make up 0.1-0.3% of the dermis
• They contain hyaluronic acid, which binds water
• Water makes up ~ 60% of the dermis by weight
• Polysaccharides cannot pass through the DEJ

Subcutis

• Subcutis consists of adipose tissue
• It makes up 80% of the fat reserve in the body
• It contains arterioles, venules, and lymphatics
• The main functions are: calorie reserve, cushion effect, and insulation

Skin Appendaes

• The skin's appendages are essential structures:
• Hair
• Nails
• Adnexal structures

Origination

• Hair follicles originate from epidermal downgrowth
• They develop between 10 and 14 weeks of gestation
• Four classes exist, terminal (scalp, beard), vellus (majority of skin), apopilosebaceous (axilla, groin), sebaceous (face, chest, back)

Hair Follicles

• Mainly terminal and vary with size, structure, and density
• Scalp, which contains terminal hair, largest follicle size, extends into the subcutis
• The forehead contains vellus hair, smaller follicle size, and large sebaceous glands
• Number of follicles remains unchanged until middle life
• Terminal and villus proportions can change
• Androgenetic alopecia is a change from terminal to vellus hair
• Hair growth averages 1 cm per month on the scalp

Hair Growth Cycle

• Hair growth undergoes a cyclical pattern
• Anagen is the active growth stage for 3 years and 90% of hair follicles
• Catagen is the regressing stage for three weeks and 1% of hair follicles
• Telogen is the resting stage for three months and 10% of hair follicles
• Hair shedding occurs in the telogen phase
• About 10,000 hair follicles lose 100 strands of hair per day
• Telogen effluvium is caused by physical or psychological stress and causes 70% of hair to prematurely enter the telogen phase
• The complete hair cycle for eyebrows is completed in 4 months

Sebaceous Glands

• Sebaceous glands differentiate at 13-15 weeks
• They are larger and well-developed in foetuses
• Size rapidly reduces after birth, but regain function at puberty
• Examples of sebaceous gland diseases are seborrheic dermatitis (infants) and acne vulgaris (ados)

Skin Types

• Glabrous skin is non-hair bearing, such as palms and soles
• Hair-bearing skin include hair follicles and sebaceous glands
• The glabrous skin is of compact structure

Sweat Glands

• Humans have 1.6 to 4.0 million sweat glands
• Eccrine glands (sweat) directly open to the skin and provide thermoregulatory sweating
• Sweat glands are distributed nearly the entire body surface
• They are identifiable over palms and soles in the 16th week
• These glands are 30-50 μm in diameter and 2-5 mm in length
• Key structures include a bullous secretory coil (lower dermis), secretory duct (dermis), and opening pore (surface)

Apocrine Glands

• Apocrine glands are sweat glands that do secretion by decapitation
• They have low secretory output and lipid-rich production
• They connect to hair follicles and are found in the axilla, genitalia, and mammary areas

Perspiration

• Human perspiration can either be insensible or active
• Insensible perspiration is passive water evaporation influenced by temperature and humidity
• Active sweating happens via eccrine glands stimulated by increase in body temperature, mental/emotional condition mainly on the palms and soles
• Nerve innervation is sympathetic

Pilocarpine and Atropine

• Pilocarpine increases nerve innervation
• Atropine decreases nerve innervation

Nails

• Nail development starts 8-9 weeks of gestation
• Growth rate of finger nails at 3mm/month
• Growth rate of toe nails at 1mm/month
• Main anatomy includes nail plate, proximal nail fold, nail matrix, nail bed, and hyponychium

Skin Functions

• Provides mechanical protection
• Enhances sensory discrimination
• Provides dextering (scratching, and grooming)
• Functions as a cosmetic accessory

Skin Development

• Skin develops from ectoderm and mesoderm
• The nervous system and epidermis originate from the ectoderm
• The dermis and skin appendages originate from the mesoderm

Components

• Structural components include hair follicles and nails at 9 weeks
• Sweat glands begin at 9 weeks (palms, soles) and 15 weeks (others)
• Sebaceous glands begin at 15 weeks

Mechanical functions

• It acts as a mechanical barrier
• Prevents physical injury
• Preserves water and electrolytes
• The stratum corneum's cornified cell envelope is highly insoluble and has glutamul-lysl isodipeptide bonds, rich in ceramides, free sterols, and FFAs

Protection

• UV protection is facilitated via the epidermis and melanin
• Protects against UVA, UVB, and UVC damage
• DNA damage can lead to mutation and carcinogenesis
• UV absorption is facilitated by the stratum corneum, epidermal keratinocytes, and melanin
• DNA repair mechanisms exist
• Melanin is produced by melanocytes located at the basal epidermis
• Eumelanin is brown/black, while pheomelanin is yellow/red pigment
• Melanin is transferred to the keratinocyte/hair via melanosomes

Body Regulation

• The skin aids thermo-regulation (vessel + sweat gland)
• Maintains constant core body temperature
• Is independent of environmental changes
• Thermoreceptors sense warm and cold
• Signal processing occurs in the hypothalamus
• Leads to sweating, shivering, and regulation of circulation

Immunity and Sensation

• The skin creates immunity through innate and adaptive systems
• Antimicrobial peptides such as cathelicidins and beta-defensins act directly as anti-microbials
• Alarmins can enhance the host defence system
• Chemotaxis and angiogenesis are triggered
• Phagocytosis is performed via dermal dendritic cells and macrophages

Sensation

• Skin provides sensation (pain, touch, temperature, etc)
• Afferent sensory
• Efferent autonomic
• Specialized sensory receptors include Meissner's corpuscles and Merkel's receptors
• Sensations include touch, pressure, temperature change, pain, and itch

Synthesis

• Skin is responsible for endocrine production of Vitamin D synthesis
• Vitamin D is produced upon sunlight exposure, cholesterol becomes cholecalciferol (D3)
• Vitamin D is fat soluble, enhances intestinal absorption of Ca and PO4, and is essential for bone metabolism
• Deficiency in vitamin D can cause rickets, osteomalacia, and osteoporosis

Communication

• Skin is used for visual appeal, texture, and smell
• Plays a role in social and sexual interactions
• Is an organ of communication
• Is enhanced by clothing and cosmetics

Homeostasis

• Homeostasis (regenerating skin and appendages)
• Proliferation and differention
• Stem cells found in the bulge area of follicles, basal area of interfollicular epidermis and the base of sebaceous glands