I. The Skin (pp. 192 – 196) – the body’s largest organ A. Consists of three layers, the epidermis, dermis and hypodermis: 1. Epidermis a. outermost superficial region (epithelial tissue) b. keratinized stratified squamous epithelium c. Cells of the Epidermis i. Keratinocytes - ii. Melanocytes - iii. Langerhans’ cells - iv. Merkel cells - d. Layers of the Epidermis i. Stratum basale (basal layer) is the deepest epidermal layer and firmly attached to the dermis. 1. Consists of a single row of mostly the youngest keratinocytes 2. 10-25% of cells are melanocytes 3. Site of mitosis. Cells undergo rapid division, hence its alternate name, stratum germinativum. ii. Stratum spinosum (prickly layer) 1. Cells contain a weblike system of intermediate filaments (spiny projections) attached to desmosomes to hold cells together, provides strength and flexibility 2.Several layers thick 3. Contains keratinocytes, melanin granules, and the highest concentration of Langerhans’ cells. iii. Statum granulosum (granular layer) 1. Thin: three to five cell layers 2. Contains keratinocytes that are undergoing a great deal of physical changes, turning them into the tough outer cells of the epidermis. 3. Keratohyaline and lamellated granules accumulate in the cells of this layer, which provides for water proofing (lipid-rich) and moisturizing. iv. Stratum lucidum (clear layer) 1. Thin, transparent band superficial to the stratum granulosum 2. Is found only in thick skin and composed of dead keratinocytes. v. Stratum corneum (horny layer) is the outermost protective layer of the epidermis composed of a thick layer of dead keratinocytes. 1. Accounts for three quarters of the epidermal thickness 2. up to 30 layers of dead keratinocytes continuously shed and replaced from lower layers 3. Keratinization and growth of epidermis-progressive accumulation of keratin as cells rise to higher layers 4. Functions include: a. Waterproofing b. Protection from abrasion and penetration c. Rendering the body relatively insensitive to biological, chemical and physical assaults 2. Dermis a. middle region (strong, flexible connective tissue) b. Cell types include fibroblasts, macrophages, and occasionally mast cells and white blood cells. c. Nutrients reach the epidermis by diffusing through the tissue fluid from blood vessels in the dermis. d. Layers of the Dermis i. Papillary layer 1. thin, superficial layer 2. areolar connective tissue with collagen and elastic fibers 3. Superficial surface contains peg-like projections that indent the epidermis called dermal papillae ii. Reticular layer 1. thick, deeper layer 2. Dense irregular connective tissue 3. Collagen fibers present strength and resiliency to the skin 4. Elastin fibers provide stretch-recoil properties 5. Contains hair follicles, nerves, sebaceous glands, sweat glands 3. Hypodermis - deepest region (areolar and adipose tissue); not considered the skin B. Skin is determined by three pigments: melanin, hemoglobin, and carotene 1. Melanin – in epidermis i. color ranges from yellow to reddish-brown to black ii. Melanosomes – iii. Made from tyrosine by tyrosinase, which is an enzyme in melanocytes that is needed to synthesize melanin. 2. Hemoglobin - 3. Carotene – II. Functions of the Skin (pp. 199 – 200) A. Protection 1. Chemical barriers include skin secretions and melanin 2. Physical and chemical barriers are provided by the continuity of the skin, and the hardness of the keratinized cells. a. Lamellar granules – retard dehydration and reduce water entry b. Substance that DO penetrate the skin:

3. Biological barriers a. The Langerhans’ cells of the epidermis b. The macrophages of the dermis c. Acid mantle B. Body Temperature Regulation 1. sweat glands of the skin cool the body 2. dermal capillaries vasoconstrict to prevent heat loss C. Cutaneous Sensation - cutaneous sensory receptors (exoreceptors) make this possible with their convenient placement in the layers of the skin D. Vitamin D Synthesis 1.Vitamin D regulates blood calcium and phosphate levels 2.Occurs in the epidermal keratinocytes when exposed to sunlight. E. Blood Reservoirs - can hold up to 5% of the body’s blood supply, which may be diverted to other areas of the body should the need arise. F. Excretion – limited amounts of nitrogenous wastes are excreted via the skin. III. Appendages of the Skin (pp. 201 – 207) A. Hairs, or pili, are flexible strands produced by hair follicles that consist of dead, keratinized cells. 1. Three hair types: a. Lanugo – b. Vellus – c. Terminal hair –

2. Structure of hair follicle: a. the main regions of hair are the shaft and the root. b. Has three layers of keratinized cells: the inner core is the medulla, the middle layer is the cortex, and the outer layer is the cuticle. c. Hair follicles fold down from the epidermis into the dermis and occasionally into the hypodermis. d. Deep end is expanded, forming a hair bulb, which is surrounded by a knot of sensory nerve endings called a hair follicle receptor, or root hair plexus. e. The wall of a hair follicle is composed of an outer connective tissue root sheath, a thickened basement membrane called a glossy membrane, and an epithelial root sheath. f. Associated with each hair follicle is a bundle of smooth muscle cells called an arrector pili muscle. 3. Hair pigments - made by melanocytes at the base of hair follicle. 4. Hair Growth – a given hair grows in a hair cycle consisting of three stages: a.Anagen – b.Catogen – c.Telogen -

5. Hair Thinning & Baldness a. After age 40, hair is not replaced as quickly as it is lost, which leads to hair thinning and some degree of balding, or alopecia, in both sexes. b. Male pattern baldness, which is a type of true, or frank, balding, is a genetically determined, sex-linked condition in response to hormones. B. Nails 1. scale-like modification of the epidermis (stratum corneum) that forms a clear, protective covering 2. Made up of hard keratin and has two distinct parts: nail matrix and nail plate C. Cutaneous Glands 1. Sweat Glands – two main types, eccrine (merocrine) and apocrine A. Eccrine a. most numerous of the sweat glands, produce true sweat b. most abundant on the palms of the hands, soles of the feet, and forehead c. located deep in the dermis and exits through skin pores d. formed at birth e. regulate body temperature and removal site of wastes such as urea, uric acid and ammonia

B. Apocrine a. confined to the axillary and genital areas and produce true sweat with the addition of fatty substances, proteins and pheromones (more viscous than eccrine) b. ducts empty into hair follicles c. onset at puberty, becomes more active in females during ovulation and less active at time of menstrual flow d. Respond to emotional stress or sexual excitement 2. Ceruminous glands are modified sweat glands found lining the ear canal that secrete earwax, or cerumen. 3. Mammary glands are modified sweat glands found in the breasts that secrete milk. 4. Sebaceous (Oil) glands – simple alveolar glands found all over the body, except the palms of the hands and soles of the feet that secrete sebum. 1. Function as holocrine glands that secrete their products into a hair follicle or to a pore on the surface of the skin. 2. Soften skin when stimulated by hormones, reduces water evaporation, keeps skin pliable and inhibits bacteria 3. Acne is a condition associated with the sebaceous glands IV. Diseases of the Skin (pp. 197- 198, 208-211) A. Skin Cancer 1.Most skin tumors are benign and do not metastasize 2. A crucial risk factor for nonmelanoma skin cancers is that UV radiation disables the tumor suppressor gene (p53 or the patched gene) 3. Newly developed skin lotions can fix damaged DNA a. Three major types of skin cancer are: i. Basal cell carcinoma – ii. Squamous cell carcinoma – iii. Melanoma – B. Burns – tissue damage inflicted by intense heat, electricity, radiation, or certain chemicals, all of which denature cell proteins and cause cell death. 1. Most immediate threat to a burn patient is dehydration and electrolyte imbalance due to fluid loss. 2. After the first 24 hours have passed. The threat to a burn patient becomes infection to the wound site. 3. Burns are classified according to their severity: a. First-degree burns - b. Second-degree burns – c. Third-degree burns – C. Alterations in skin color 1. Erythema or redness – 2. Pallor or blanching – 3. Jaundice – 4. Bronzing – 5. Black & blue marks – 6. Cyanosis - 7. Hematoma – 8. Albinism –