• largest organ
• accounts for 7% of body weight
• divided into 2 layers: epidermis & dermis
• cushions and insulates deeper organs
• protects body from bumps, scrapes, cuts, chemicals, heat, cold

• stratum basale
• stratum spinosum
• stratum granulosum
• stratum corneum

• deepest layer of epidermis
• attached to underlying dermis
• cells actively divide
• contains 2 types of cells: Merkel cells (associated with sensory nerves) and Melanocytes (secrete the pigment melanin)

• spiny appearance
• contains thick bundles of intermediate filaments (tonofilaments)
• contains Langerhans cells (associates with immune system)

• consists of keratinocytes and tonofilaments
• tonofilaments contain keratohyaline granules (help form keratin) and lamellated granules (contain waterproofing glycolipid)

• thick layer of dead keratinocytes and thickened plasma membranes
• protects skin against abrasion and penetration

• second major layer of the skin (beneath the epidermis)
• strong, flexible connective tissue richly supplied with blood vessels and nerves
• 2 layers: Papillary (has dermal papillae) and Reticular (deeper layer, 80% of thickness of dermis)

• also called superficial fascia
• below the dermis
• contains areolar and adipose connective tissue
• anchors skin to underlying structures
• helps insulate body

• melanin: most important pigment, made from tyrosine (white people have minimal amount)
• carotene: yellowish pigment from carrots & tomatoes
• hemoglobin: allows red color of blood to show

• scale-like modification of epidermis
• made of hard keratin
• parts of the nail: free edge (the tip), body, eponychium (cuticle), nail fold (skin above cuticle), root

• flexible strand of dead, keratinized cells
• root: imbedded in the skin
• shaft: projects above surface
• a hair follicle has 3 concentric layers: medulla (central core), cortex (surrounds medulla), cuticle (outermost layer)
• hair stands erect when arrector pili contracts

Types and Growth of Hair

1. Vellus hairs: body hairs of women and children
2. Terminal hairs: hair of scalp, axillary and pubic area
3. Hair thinning and baldness: due to aging, pattern in males

• occur over entire body except palms and soles
• secrete sebum - an oily substance that functions to collect dirt, soften and lubricate hairs and skin

• widely distributed on the body
• a blood filtrate
• 99% water with some salts
• contains traces of metabolic wastes (2% urea)

1. Eccrine gland: most numerous; produce true sweat
2. Apocrine gland: confined to axillary, anal, and genital areas; produce a special kind of sweat (musky odor attracts mate)

• first degree burn: only upper epidermis is damaged
• second degree burn: upper part of dermis is also damaged (blistering, skin heals with little scarring
• third degree burn: consumes thickness of skin; burned area appears red or blackened

1. Basal cell carcinoma: least malignant and most common
2. Squamous cell carcinoma: arises from keratinocytes of stratum spinosum
3. Melanoma: cancer of the melanocytes; most dangerous

• the epidermis develops from the ectoderm
• the dermis and hypodermis develop from the mesoderm