1. Protection
1. From UV/Ionizing Radiation
2. From harmful organisms
3. From water gain/loss
4. From heat gain/loss
2. Reception of sensory stimuli
3. Excretion
4. Blood Reservoir (Stores approx. 5% of Total Blood Volume)
5. Synthesis of Vitamin D₃(cholecalciferol) from 7-dehydrocholesterol

Epidermis - outermost protective layer composed of keratinized stratified squamous epithelium (which is avascular)

Dermis - beneath the epidermis; thicker; vascularized; consists of CT, smooth muscle, nerves, blood vessels.

NOTE: Hypodermis/subcutaneous tissue is NOT part of the skin

1. Stem cells - undifferentiated cells that give rise to keratinocytes, found in the deep (basal) epidermis
2. Keratinocyte (90%) - synthesizes the protein keratin for water-proofing; arises in the deep (basal) epidermis; takes 25-45 days to mature while migrating towards the external (free) surface; as keratin hardens -> cell dies and appears scale-like at the outer surface
3. Melanocyte (8%) - synthesizes the pigment 'melanin' which absorbs harmful UV light and is responsible for skin color; found at the deep (basal) epidermis; transfers melanin onto the free surface of each keratinocyte
4. Remaining (2%)
1. Dendritic AKA Langerhans' cells - modified macrophages that originate in the bone marrow but migrate to the epithelium of the oral cavity, esophagus, and vagina
2. Merkal AKA tactile cells - sensory receptors for touch found at the deep (basal) epidermis

1. Stratum Corneum - 20-30 layers of dead, flat, anuclear, keratinized epithelial cells arranged shingle-like
2. Stratum Lucidum - 3-5 layers of dead, flat, transparent epithelial cells; ONLY found in the palms and soles
3. Stratum Granulosum - 3-5 layers of flat, epithelial cells filled with granules of hardening keratin; alive but dying
4. Stratum Spinosum - 8-10 layers of round cuboidal-like epithelial cells (spiny or prickly in shape); alive and keratin is being synthesized; most Langerhans' AKA Dendritic Cells are found here
5. Stratum Basale (basal cell layer) - AKA stratum germinativum; single row of alive stem cells -> keratinocyte cells undergoing mitosis

Below epidermis; above hypodermis

Anywhere from .2mm thick (eyelids) to 4.0mm in palms and soles. Possesses two distinct dermal layers.

1. Papillary layer - superior 20% of the dermis; composed of areolar CT
1. Has many receptors - pain receptors at free nerve endings and Meissner's corpuscles (light-touch receptors)
2. Dermal papilla(e) - tiny, nipple-like projection(s)/undulation(s) that join to the basal ridges of the epidtermis. These form fingerprints.
2. Reticular layer  inferior 80% of dermis; composed of Dense Irregular CT
1. Thick collagen fibers that run parallel to the skin -> form tension lines AKA lines of cleavage (Incisions made parallel to these lines heal quicker)
2. Blood vessels (minor); smooth muscle; hair; glands
3. Site of deep pressurce receptors AKA Pacinian Corpuscles (which are also w/in sub-Q)
4. Site of penetration for tattoos

• Mostly Adipose CT and some Areolar CT
• Rich with major blood vessels and nerves
• Provides insulation; 8% thicker in females; this is the site that thickens when one gains weight - begins in breasts and thighs of females and abdominal region of males
• Also a site of deep-pressure receptors AKA Pacinian Corpuscles
• Attaches skin to underlying organs (bone; muscle)

1. Melanin - Produced and secreted by melanocytes; yellow-to-reddish/brown-to-black pigment derived from the amino acid tyrosine (albinism is a lack of tyrosine)
1. Melanin absorbs the sun's harmful UV radiation - > protects the skin from being damaged
2. All 'races' have an equal # of melanocytes; variations in skin color are due to differences in the amount and type of melanin produced
2. In RBCs, Hemoglobin/Hb has an iron-containing pigment called porphyrin; changes in the pigment color of porphyrin are reflected through the avascular epidermis (cyanosis or erythema)
3. Carotene - yellow-to-orange pigment from egg yolks and yellow/orange vegetables
1. Increased concentrations of carotene found in the stratum corneum and sub-q fat of the palms and soles
2. NOTE: Asian, Hispanic, Native American skin color mainly due to variations of melanin and NOT carotene

• Epithelial cells rapidly divide and proliferate -> bridge gap -> begin regeneration (replacement of damaged tissue with the original kind of tissue)

• Two phases should normally occur
• Inflammation is the first phase
• Damaged cells, tissues, blood vessels release chemicals that promote
• WBCs, platelets, plasma protein clotting factors to wander into the injured area -> blood clot forms, clot isolates the injured area, stops blood loss, holds the wound together
• Promote dilation plus increased permeability of blood vessel walls -> redness (rubor), swelling (tumor), warmth (calor), pain (dolor)
• Organization is the second phase
• 3-4 days post injury the blood clot is replaced by granulation tissue consisting of: budding capillaries, macrophages (to digest blood clot and debris), and fibroblasts (which deposit EGF plus proteins to form collagen fibers)
• Approx 2 weeks post injury, granulation tissue is replaced by scar tissue: permanent fibrous CT (visible or invisible depending on wound severity)
• surface epithelium regenerates
• remodeling (maturation) of the repaired tissue begins a few weeks post injury and can last as long as two years

1. ↓ fibroblasts → ↓ collagen fibers → lose skin tone
2. ↓ elastic fibers → ↓ elasticity → skin wrinkles
3. atrophy of fat (adipose CT) → colder extermities
4. ↓ melanocyte activity → gray hair
5. ↑ amount of irregular pigments produced → liver spots
6. atrophy of oil glands → dry skin
7. atrophy of hair follicles → balding or thinning of hair

• Stimulus leads to ↑ body temperature → brain signals:
• dermal blood vessels to dilate → ↑ loss of body heat
• sweat glands to secrete sweat; sweat evaporates → ↑skin cooling
• Response: as a result of the two things above → ↓ body temperature
• Reverse of the above process (vasoconstriction) is also true

Cell death is caused by thermal or nonthermal sources (electrical, radioactive, chemical) → ↑ fluid loss, ↓ blood flow to tissues, ↑ risk of infection

The clinical outcome mainly depends upon two factors: depth of the burn, extent of the surface burned

• First degree burn (partial thickness burn)
• Only the surface epidermis is burned
• Painful, appears red and slightly swollen (like a mild sunburn)
• Tx - local cold or aloe vera → heals in 2-3 days; systemic hydration if Nausa/Vomitting (N/V) occur in the elderly or in an infant

• Second degree burn (partial-thickness burn)
• The entire epidermis plus some dermis is burned; has two categories
• Superficial Second Degree Burn
• Epidermis plus superficial dermis are burned
• Painful, red, hallmark is thin-walled fluid-filled blisters that develop within just a few minutes post-injury (blister develops for any short-term but acute trauma that leads to the separation of dermal-epidermal layers by a fluid-filled vesicle)
• Heals in 3-4 weeks provided no complications develop
• Deep Second Degree Burn
• Epidermis plus most of the deep dermis are burned
• Fluid filled blisters may or may not appear; often present is flat dehydrated 'tissue paper' skin
• Usually is less painful than a superficial 2nd degree burn
• May scar → loss of function
• Wound takess > 4 weeks to heal and typically requires medical attention (silvadene) or a graft

• Full-thickness burn; entire epidermis, dermis, hypodermis plus appendages are burned
• No blisters; no pain; skin may appear white or black; loss of function with ↑ fluid loss
• wound requires escharotomies (cutting away dead flesh which never heals because it's dead) plus skin graft

Skin can be divided into 9% groupings

• Anterior & Posterior head and neck = 9% (total)
• Anterior & Posterior Upper Extremity (UE) = 9% each (18% total)
• Anterior & Posterior Trunk = 18% each side (36% total)
• Anterior & Posterior Lower Extremity (LE) = 18% each (36% total)

25% or greater of a 2nd degree burn

OR

10% or greater of a 3rd degree burn

• Composition: 3 concentic layers of keratinized epithelium
• Inner Medulla
• Middle Cortex - contains the hair pigments made by melanocytes
• Outermost cuticle - highly keratinized cells arranged like shingles on a roof
• 21 million hairs cover the entire body (100k on scalp); only places with no hair are palms, soles, lips, nipples, and external reproductive organs.
• Hair grows at a rate of approx. 2-3mm/week
• Function of hair: sensitive touch receptors plus a minor amount of protection (eyelashes and nasal hair)

• Shaft - portion of a hair that protrudes from and above the skin
• Shape of the shaft opening on the superficial epidermis determines whether one's hair is straight, wavy, or curly
• Root - portion of a hair that is below the epidermis
• Bulb - deepest portion of the root; composed of:
• Hair Papilla - nipple-like opening at the inferior end of the bulb; blood vessels project into this opening feeding the:
• Matrix - an area of rapidly growing epithelial cells that will get keratinized in the shaft
• Hair follicle AKA root sheath - tubular 'bag' enclosing the root; external outlets/openings are one type of skin pore
• Arrector pili AKA piloerector muscle: smooth muscle; attached to a follicle at one end and skin surface at opposite end; contraction of the arrector pili → follicle pulled into upright position and skin surface dimples → 'goose-bumps'

• Body's most numerous gland; present in all skin regions except the palms and soles
• Composed of specialized epithelial cells that secrete sebum (lipids + cell fragments); body's only holocrine gland
• Sebum functions as lubrication, water-proofing, and bacteriocide (on hair and skin)
• Sebaceous gland duct almost always empties into a hair follicle's pore (ALWAYS true as far as our exams go)

• Present in all skin regions except the nipples and external genitalia
• Meocrine AKA Eccrine Sweat Gland
• 3-4 million of these; most numerous type of sweat gland, especially in the palms, soles, and forehead
• Duct empties directly into an epidermal outlet AKA pore
• Excretory function; sweat consists of 99% water, plus salts and waste products (urea and uric acid); cooling function as well
• ↑ glandular activity i.e. ↑ secretion with: heat; exercise; emotions (fright, embarrassment, nervousness)
• Apocrine sweat gland
• Approximately 2,000; mainly in axilla, anogenital areas, and beard area of mature males
• Duct empties into a hair follicle's outlet
• Odorless secretions (sweat + fatty acids) are more viscous and sometimes appear yellow-milky white
• Become odorized when secretions are metabolized by skin's bacteria
• Inactive until puberity, then ↑ glandular activity i.e. ↑ secretion with sexual stimulation

• On fingers and toes; growth of .5-1.0mm/week
• Nail Plate - external keratinized, translucent epithelial tissue; consists of:
• Nail Body - visible attached part
• Free Edge - Overhangs the tip of the finger or toe
• Nail Root - Hidden by the proximal overlying skin, contains modified stratum basale cells that form the (germinal) nail matrix
• Nail bed - Skin that underlies the nail plate
• Lunula AKA lunule - white half-moon at the proximal nail plate; appears white because blood is not reflected through the dermis due to a thick overlying nail matrix
• Cuticle (AKA eponychium) - narrow band of epidermis extending from the proximal-lateral margins of the nail plate
• Function - protection; manual manipulation i.e. grasping

• Most common skin disease of 12-25 y/olds (85% affected)
• Etiology:
• ↑ androgens → ↑ size and activity of sebaceous glands → ↑ sebum secretion → sebum obstructs duct → leading to one of two forms of acne vulgaris lesions
• Noninflammatory Acne - comedones are closed (whitehead) or open (blackhead)
• Inflammatory Acne - obstructed duct → follicle wall ruptures → sebum expelled into dermis → inflammation
• Superficial pustules develop or deeper cystic nodules or papules develop → may lead to scarring

• Result of trauma
• After trauma, blood from broken capillaries leaks into dermis → skin filters out the red color as Hb's pigment undergoes color changes

• Most common congenital anomaly
• Caused by developmental defects whereby normal skin components are arranged in an abnormal pattern

• May occur during any phase of healing, including a reactivation of the inflammatory process → a delay of normal healing

1. Wound sepsis/Infection - most important Tx is organism-specific antibiotics + debridement of necrotic tissue (Elase, Panafil, sharp debridement, wet-to-dry dressings)
• Cellulitis - inflammation due to infection of dermis and sub-q; infected area is erythematous, swollen, painful
2. ↑ collagen synthesis with ↓ collagen lysis (breakdown, not necessarily cell lyses) → elevated overgrowth of immature scar tissue
• keloid - elevated overgrowth of immature scar tissue extending beyond/outside of the scar line
• hypertrophic scar - elevated overgrowth of immature scar tissue that remains within the scar line
3. Wound dehiscence (separation of margins) can result from microangiopathy, metabolic disturbances, inadequate collagen, iatrogenic (physician caused) → ↓ tensile strength of the wound → deficient scar formation

• Fungal infection of the nail's matrix
• Follows trauma
• Thick, discolored, disfigured nail

• Inflammatory abnormality resulting from skin cells migrating from the Stratum Basale to the Stratum Corneum in only 4 days instead of usual 25-45 days
• Keratin and cells do not mature, resulting in superficial skin lesions present as white, oily, 'flaky' papules, patches or scales. 
• Scalp and nails can also be affected

• Most common, least malignant skin cancer.
• Arises from keratinocytes within stratum basale
• Presents most often in a male >40 years old on sun-exposed areas of the face as a shiny, dome-shaped nodule with a pearly, beaded border that later develops into a bleeding ulcer
• Rarely metastasies but can destroy an ear or nose

• Arises from keratinocytes within Stratum Spinosum
• Presents most often in a male >60 years old on sun-exposed areas of the head (scalp, ear, lower lip) as a flat or slightly elevated firm keratotic plaque or as a long-standing ulcer that bleeds easily
• Can metastasize 20% of the time - Dx via a Bx is crucial

• Originates from melanocytes or from cells that synthesize melanin
• 1/2 originate from intact skin
• 1/2 originate from freckles or a nevus
• Highly metastatic and resistant to chemotherapy → poorest prognosis/Px of all skin cancers
• Presents most often in females

• Associated with immunodeficiency states
• Originates from dermal blood vessels and surrounding CT cells
• Lesions appear flat and purplish-brown → develop into plaques and nodules with antioproliferation; typically pruritic (itchy) and painful
• Classic form initially aappears over Lower Extremity
• A rapidly progressive form (seen with AIDS) tends to spread over the upper body, especially the face and oral mucosa

• Overstretched dermis (obesit or pregnancy) → collagen and elastic fibers getting torn
• Stretch marks are signs of repairing scar tissue of the torn fibers

• Caused by contracting the HPV/human papilloma virus
• Presents single or in groups, elevated or flat
• Disappear spontaneously with or without Tx (keratolytics, laser, Sx)
• Subtype condyloma(ta) acuminata AKA veneral warts: cauliflower-like lesions along the glans penis, vulva, and/or anus