The Human Skin

• The skin is the largest organ in the human body, with a surface area of 1.5 to 2.0 m² in the average adult.
• Most skin is 2 to 3 mm thick, constituting 16% of a person's total body weight, heavier than the brain or liver.
• One square centimetre of skin contains approximately 100 sweat glands, three blood vessels, 9000 melanocytes, and over 150 nerve endings.
• Throughout life, the skin continuously sheds outer layers and replaces them with inner layers, resulting in about 900 "skins" shed per lifetime.
• The skin supports a complex ecosystem of microorganisms, including fungi, bacteria, and viruses, collectively known as the skin microbiome.
• Even thorough washing cannot remove the skin microbiome.

Bacteria on the Skin

• Estimates place over 7 million bacteria on 1 cm² of skin, with oily areas like the face containing over 75 million bacteria per cm².
• Despite the large quantity, all surface bacteria would fit into a pea-sized volume.
• The four main groups of bacteria that predominate on all skin are diphtheroids, micrococci (staphylococci), streptococci, and enterococci.
• In addition to bacteria, skin hosts yeasts like Candida and fungi.
• Variations in pH, oxygen, water levels, and secretions cause the skin microbiome to vary across the body.

Fungi on the Skin

• Genome sequencing has identified over 80 fungal genera on the skin, far more than traditional culturing methods detected.
• Arms have high bacterial diversity but low fungal diversity, while the reverse is true for other body sites.
• Malassezia fungus is found on all healthy skin surfaces.
• The heel hosts about 80 genus-level types of fungi, with many types found in toenail and toe web samples.
• To avoid contamination from fungi, it is recommended to wear personal flip flops in locker rooms.
• In general, microorganisms in healthy skin keep one another in check, with deep skin containing bacteria and bacterial components, even in dermal and subcutaneous compartments.
• These bacteria interact with skin cells, influencing immune defences and inflammation.

Skin and Disease

• The skin is affected by the greatest number of primary or systemic cutaneous diseases, estimated at over 2000.

Skin pH

• Skin pH is approximately 5.4.

Skin Thickness

• Skin thickness varies significantly depending on the specific site, sex, age, and race of the person.
• Male skin is usually thicker than female skin.
• Children's skin is relatively thin.
• Epidermal skin is thickest on the palms and soles (1.6 mm), and thinnest on the eyelids and postauricular region (0.04 mm).
• From about age eleven, the skin thickens until the fourth or fifth decade of life, when it begins to thin due to loss of dermal elastic fibers, epithelial appendages and ground substance.

Skin Structure

• The skin comprises of the epidermis, basement membrane, dermis, subcutis, and cutaneous tissue.
• Epidermis: outermost layer which acts as a barrier against moisture and damage.
• Basement membrane: thin layer that provides adhesion between epidermis and dermis.
• Dermis: a structural layer made of collagen and elastin that supports the epidermis
• Subcutis: a loose connective tissue layer below the dermis
• Cutaneous tissue: fat layer providing insulation.
• The epidermis is the outer, thinner layer derived from ectodermal epithelium (5% of the skin).
• The dermis is the thicker, inner layer derived from dense mesodermal connective tissue (95% of skin).

Classification by Epidermal Thickness

• Thick or acral skin: found on palms, soles, protrusions and extremities.
• Thin skin: found on the remainder of the body, with significant regional differences, such as on the face.
• The thickness of the epidermis varies from 0.04 mm on the eyelids to 1.6 mm on the palms and soles.

Epidermal Layers

• The epidermis is stratified squamous epithelium, primarily of keratinocytes in progressive stages of differentiation.
• The epidermis has no blood vessels, receiving nutrients through diffusion from the dermis
• The epidermis is composed of four to five layers.
• Stratum corneum is made up of horny cells (corneocytes) and is the skin surface.
• Stratum lucidum is found in areas of thick skin and helps reduce friction.
• Stratum granulosum is the granular cell layer.
• Stratum spinosum contains prickle (spinous) cells
• Stratum basale is the basal cell layer.
• The epidermis contains keratinocytes, melanocytes, Langerhans cells, and Merkel cells.

Keratinocytes and Melanocytes

• Keratinocytes are the key cells of the epidermis, comprising approximately 80%.

• They progress through differentiation from the basal layer to the stratum corneum over 14–30 days.

• The epidermis protects the body from the outside and withstands wear because keratinocytes bind together with desmosomes.

• Keratinocytes produce 30 different keratin proteins and secrete cytokines to regulate wound healing and combat infection.

• Defects in keratins can result in a broad array of dermatoses.

• Basal cells are the source of epidermal regeneration.

• Basal cells are stimulated by trauma, UV, growth factors, oestrogens, and tumour promoters, and can be inhibited by protein deprivation.

• The spinous layer has dense desmosomal proteins for adhesion and discharges lipid precursor products to prevent water loss

• This layer mediates stratum corneum adhesion. Suprabasal keratinocytes down-regulate in conditions such as actinic keratoses, wound healing, and psoriasis.

• As keratinocytes migrate superficially, they form the granular layer and the cornified cell envelope (CE), which is the source of epidermal barrier strength and function

• It prevents water loss, maintains hydration, and prevents overhydration.

• Disruption of the skin leads to reparative mechanisms: an inflammatory response, increased mitotic activity, more stratum corneum, and a cytokine response to up-regulate immune defences.

Melanin

• Melanin is a class of compounds derived from tyrosine that act as pigments, dissipating 99.9% of UVR as heat.

• There are three melanin pigments: eumelanin/no red pigment (black/brown), pheomelanin/red-brown pigment (for red hair and freckles), and neuromelanin.

• Eumelanin oxidizes in response to UVB, while pheomelanin oxidizes in response to both UVB and UVA and exhibits more oxidative stress.

• Eumelanins are photo-stable and neutralize free radicals, whereas pheomelanins decompose under UVR.

• Melanocytes are large cells interspersed among keratinocytes, producing melanin, which absorbs radiant energy from the sun.

• Melanin accumulates in organelles termed melanosomes, which are then transferred to the surrounding keratinocytes

• Melanocytes are found in the hair follicles, retina, uveal tract and leptomeninges.

• They produce and transport melanin (in melanosomes) to neighboring keratinocytes.

• Pigmentation differences relate to melanosome size rather than cell number.

• Sun exposure, melanocyte-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), oestrogens, and progesterones stimulate melanin production.

• With age, the number of melanocytes decreases.

• Melanocyte cells originate from Schwann cell precursors (SCPs).

Langerhans and Merkel Cells

• Langerhans cells are dendritic antigen-presenting cells of the prickle layer from two embryonic lineages
• Langerhans cells "see" and "alert" other immune cells to harmful foreign invaders in the skin.
• Merkel cells originate in and specialize in the perception of light touch.

Epidermis Function and Repair

• The epidermis externally protects against microorganisms, chemicals, and other abuse, but is not effective against UVR (80-90% damage).
• It internally holds essential fluids to prevent dehydration.
• The stratum corneum contains keratinocytes arranged in overlapping layers with a lipid coating (ceramides, fatty acids, cholesterol).
• Modern demands and lifestyles lead to long-term detrimental results
• Low humidity, wind, sun, UVR, hot water, detergents, solvents, mechanical trauma, occupational chemicals, smoke and pollution allow water loss so that skin enzymes can no longer shed the squames.
• Moisturizers hold water, provide lipids, restore elasticity, and reverse the effects of drying
• Petroleum jelly bathes the cells, forming an anti-inflammatory, hydrating, waterproofing and insulating milieu but does not enter the cells.

Dermal-Epidermal Junction

• The basement membrane zone (BMZ) is the interface between the epidermis and dermis that allows for communication between cell types.

• The BMZ functions as a vehicle for cell attachment, a matrix for cellular migration, a gatekeeper and moderator of cellular activities.

• The dermal-epidermal junction interlocks the epidermis to the dermis with finger-like rete ridges, which increase the epidermal surface area.

• Ultrastructural studies identify distinct structural zones in the BMZ: plasma membrane and hemidesmosome (zone 1), lamina lucida (zone 2), lamina densa (zone 3), sub-lamina densa/papillary dermis (zone 4).

• The lamina lucida and densa are the site of immunological injury in bullous pemphigoid and epidermolysis bullosa.

Epidermal Skin Differences

• The epidermis on the face differs from that on the trunk.
• The trunk pigment network is not found on the face, which has a pigment pseudo-network.
• Facal skin under dermoscopy shows dilated blood vessels that mimic this pseudo-network.

Acral Skin

• Acral skin lacks hair follicles and has a thickened stratum corneum.
• It includes body protrusions: fingertips, knuckles, elbows, knees, buttocks, toes, heels and ears.

Dermis

• The dermis is the structural support of the epidermis (0.3 mm on the eyelid to 3.0 mm on the back) with collagen and elastic fibers that form the connective tissue.

• The fibroblast is the main cell type and produces procollagen and elastic fibres

• Procollagen is cleaved into collagen.

• The dermal tightly cross-linked collagen fibres provide tensile strength and resistance to mechanical forces, there are 28 types of collagen.

• Collagen makes up 70% of the dermis is made of type I (85%) and type III (15%) collagen.

• The dermis has two layers: papillary and reticular.

• The papillary dermis is the outermost, superficial and thinner layer that pushes up to invaginate the basement membrane contains capillaries, elastic fibers, and some collagen.

• Reticular dermis: consists of a thicker mesh of dense connective tissue with interwoven capillaries, elastic fibres and coarse bundles of collagen. Age affects the recovery rate of epidermal barrier.

Composition of the Dermis

• Contains fibroblasts, mast cells, nerve endings, lymphatics and epidermal appendages.
• It is surrounded by ground substance, composed of mucopolysaccharides (primarily hyaluronic acid), chondroitin sulfates and glycoproteins.
• The deep surface of the dermis is irregular.

Collagen and Elastic Fibers

• Collagen is heat labile (becomes hydrated and turns into gelatin when boiled.)
• Elastic fibers constitute less than 1% of the weight of the dermis, but resist deformational forces.
• Mast cells release histamine and heparin.

Histiocytes

• Histiocytes are wandering macrophages that accumulate the debris of inflammation, melanin and haemosiderin.
• Previously, cosmetics were designed to sit on the skin surface, formulas are now being developed to penetrate.

Subcutaneous Tissue

• The subcutaneous layer is the fat layer providing insulation.

Appendageal Structures

• The appendageal structures include sebaceous glands, sweat glands, apocrine glands, mammary glands and hair follicles.
• The appendages provide any and all re-epithelialisation if the epidermis is injured

Sebaceous (Holocrine) Glands

• Sebaceous glands are spread over the entire surface of the body except the palms, soles and dorsum of the feet, found in highest concentration on the scalp and face.
• The glands produce and secrete sebum, a complex mixture of triglycerides, fatty acids breakdown products, wax esters, squalene, cholesterol esters and cholesterol.

Sweat (Eccrine) Glands

• Produce sweat, cools body by evaporation; are over the entire surfaces of the body except the vermillion border of lips, external ear canal.

Apocrine Glands

• Found in the axillae, the anogenital region, the external ear canal as modified glands (ceruminous glands), the eyelid (Moll's glands) and the breast (mammary glands.)
• Produce odor
• Do not function prior to puberty
• May serve a vestigial sexual function.

Hair Follicles

• Complex structures formed by the epidermis and dermis.

• Found over entire surface of the body except soles of the feet, palms, glans penis, clitoris, labia minora, mucocutaneous junction and portions of the fingers and toes.

• Sebaceous glands often open into the hair follicle rather than directly onto the skin surface.

• Hair follicles of white people are oriented obliquely to the skin surface Hair follicles of black people are oriented almost parallel to the skin surface.

• Asian people have vertically oriented follicles which produce straight hairs.

• Important consideration to avoid alopecia when being cut

Hair Growth

• Hair exhibits cyclical phases of growth (anagen), resting state (telogen), and the transition between anagen and telogen (catagen).
• Follicles are found in all three phases of hair growth
• Explains why multiple treatments by intense pulsed light (IPL) or laser are necessary.

Blood Supply

• Blood supply is extremely important for cutaneous surgery.
• Flap survival is dependent on the blood supply to skin.

Function of Skin

• Humans lose heat in the body directly through the skin.
• Evaporating sweat cools blood circulating through the papillary and subpapillary plexi Animals other than the pig get their sunburnt/cannot lose heat in this way.

Lymphatics

• Lymphatic vessels parallel the blood supply.
• Their disruption may contribute to flap oedema.
• Conserve plasma proteins and scavenge foreign material
• Superficial dermal lymphatic capillaries arise within the interstitial spaces and drain into valved deep dermal and subdermal plexuses, to ultimately join the venous circulation.

Skin Innervation

• Numerous specialised structures are present in the skin to detect various stimuli.

• Merkel cells of the epidermis detect light touch.

• Meissner corpuscles also detect light touch and are found in the dermal papillae.

• Pacini corpuscles are deep within the dermis or subcutaneous tissue and are specialized to detect pressure.

• Naked nerve endings located in the basal layer transmit pain.

• Krause bulbs detect cold and Ruffini corpuscles detect heat.

• Heat, cold and proprioception sensors are located in the superficial dermis

• There are type III and IV fibers that lack specialized receptors, are mostly the papillary dermis, respond to multple stimuli including inflammatory and chemical.

• Low intensity inflammation cause these naked nerve endings and cause itching, high intensity inflammatory stimulation causes main.

• Increased sensitization can occur following injury

• Normal things become abnormal such as a lack of moisture, soaps and exposure for long term use.

• By re established with a good skin milueu with proper mouisturizer.

• Cutaneous nerves follow the routes of blood vessels to the skin.

• The area supplied by a single spinal cord is called a dermatome

Skin Behavior

• Stress is the force applied per cross sectional area
• Strain is the change in the length with a given force, dived by original length.
• Creep increase in strain when a constant force is applies
• Skin is not elastic, and be stretched and be quite forgiving.
• Some pullback is called "elasticiy" and recoil.
• the length will increase with stress relaxation

The skin not elastic because the collagen relaxed state is randomly arranged with elastin loosely wrapped on with multiple points.

• Elastin stretch easily.
• If no good, it will require more force and pull on collagen causes it to realign
• Pulling wounds doesn't help it stretch and may cause wound necrosis; design is important.
• Practical applications: edges can be stretched and relax for 5/10mins to allow the wound to be closed. but tension of the wound should be limited

Wound Healing

• While dog ears must be excised, they may settle with time

• Series of exision can be preformed

• Cytochrome P450 system mainly is concerned with detoxification and coverting waht ever is going into to body that are soluble.

• Stem cells withinthe stem will help regenerate and cell defereciation to form either the surface or from it the stem.

• In additon there stem cells will replenish themselves continuisly to rejuvenate skin and hair.

• The overall sequence is immediate-vasospasm & local blood coagulation,15mins-14days for contraction, 15minsto3days inflammatory

• 12-48 hours epithelialzation,days-7weeks-Scar collagen matrix,6/18months-Neve repair.

• Healing is continous event of three overlapping/haemostasis/proliferation and remodeling:

• Haemo-Constriction/Spasm with plating aggerating.

• The platelets relase mutliple growth factors for tubbing endothelial profileration.

• inflame- Rubor tumour swelling/fluid leakage to block lypthmics

• inflmatore reaspose in which leukocytes invades within six hours.

• Wound derbreiement phagcytosits will be initially by 24-72hours then by mycroghapes

• angio- initaiised upon wounding mediaating thought entire

• capillaries buds spew to bloodstream and forming to plexus and angiogenesis.

• proliferation fibroplaisa with neavasculazion.

• 15 mints contractions in drection in relaxed skin.tensition depth.

• 48-72 hours population enhanced growth oxo and lactate levles 8 (occlusive)

• Epi- 14-48 hours reephiterlaziatio reateed related .

• base membrand re epiithilation, if not there will be from wound margins and will still appedegendes as line by viable.

• these apparagnges more destroyed not 29-37 base cells begin and diffeerancete in the same skinty and do their morppholgi

• remoddeling out the matrix fibronectin collagen and blood cells what miximize.

• collagen synethis and lysis of old collagen with less cells

• remodeling is going increase more than 40 percent at year but neve rmroe than pretury inury levels 90 percent (

• dermis remains presstabliised 1year, dyas apprximated d edge contrcation in morforblasts

• weeks remordleing in 20 prercsent,matrix is laid down by fibrin, 5 weeks 50 prrecenter and monoth s80 percent

• Biological bandage and sbilsteers I regard blosters which is lifteded epidermis that its not iofnected are better than commer

• sterial needl conpres the eidderiesis and what left and proect the ssking underneath itchting and stem histtmanise

• Scabs combinaot of platelets with fibruing

• 30 persxcent at 30 pecent

• 60 percent 60 and 9 at a year