General Facts: A.First line of defense, skin is an organ B.Vital to homeostasis C.Largest body organ

4 Types of Membranes: a. Serous Membrane: 1. Lines the body cavities that lack external openings. 2. Lubricates membrane surfaces

b. Mucous Membranes: 1. Line caviteis that open externally 2. Secrete mucous

c.Synovial Membranes 1.Line joints cavities 2.Lubricate ends of bones at joints

D. Cutaneous Membrane 1. External body covering 2. Protection

Functions of the skin: 1. protection 2. Regulate body temperature 3. Retard water loss 4. House sensory receptors 5. Synthesize biochemicals 6. Excrete waste

Regions Epidermis – outermost superficial region Dermis – middle region Subcutaneous (superficial fascia) – deepest region

Epidermis Composed of stratified squamous epithelium, consisting of four distinct cell types and four or five layers and lacks blood vessels Cell types include keratinocytes, melanocytes, Merkel cells, and Langerhans’ cells Outer portion of the skin is exposed to the external environment and functions in protection

Cells of the Epidermis Keratinocytes – produce the fibrous protein keratin Melanocytes – produce the brown pigment melanin Langerhans’ cells – epidermal macrophages that help activate the immune system Merkel cells – function as touch receptors in association with sensory nerve endings Touchy, Touchy

Layers of the Epidermis

Layers of the Epidermis: Stratum Basale (Basal Layer) Deepest epidermal layer firmly attached to the dermis Consists of a single row of the youngest keratinocytes Cells undergo rapid division Well nourished by dermal blood vessels

Cells are pushed upward as new cells are formed and become keratinized as they die. Protects against water loss, injury, harmful chemicals & bacteria. Melanocytes lie deep in the epidermis & the underlying dermis, produce a pigment called melanin that protects deeper cells from the sun’s UV rays. Melanocytes pass melanin to nearby cells through cytocrine secretion.

Skin Color Three pigments contribute to skin color Melanin – yellow to reddish- brown to black pigment, responsible for dark skin colors Freckles and pigmented moles – result from local accumulations of melanin Carotene – yellow to orange pigment, most obvious in the palms and soles of the feet Hemoglobin – reddish pigment responsible for the pinkish hue of the skin Genetic differences result from different amounts of melanin & in the size of pigment granules. Sunlight causes the melanin production to increase. Circulation with dermal blood vessels affects skin color.

Dermis Second major skin region containing strong, flexible connective tissue (collagen fibers) and blood vessels that carry nutrients to upper layers of skin and help regulate body temperature Cell types include fibroblasts, macrophages, and occasionally mast cells and white blood cells

Also consists of nerves, hair follicles, sweat glands, oil glands and muscles.

Subcutaneous (Hypodermis) Subcutaneous layer deep to the skin Composed of adipose (fat layer) and areolar (loose) connective tissue It binds the skin to underlying organs and contains the blood vessels that supply the skin. There is no sharp boundary between the dermis and the hypodermis

Accessory Organs of the Skin

Hair Function and Distribution Hair is distributed over the entire skin surface except: Palms, soles, and lips Nipples and portions of the external genitalia

Hair Follicle Individual hairs develop from cells at the base of the hair follicle, an invagination of the lower epidermis that dips down into the dermis As new cells are formed, old cells are pushed outward and become keratinized, forming the hair shaft A bundle of smooth muscle cells, called the arrector pili muscle, is attached to each hair follicle Hair color is determined by genetics; melanin from melanocytes is responsible for most hair colors, but red hair also contains the pigment trichosiderin

Hair Function and Distribution Functions of hair include: Helping to maintain warmth Alerting the body to presence of insects on the skin Guarding the scalp against physical trauma, heat loss, and sunlight

Sebaceous Glands (Oil Glands) Simple ~alveolar glands found all over the body Soften skin when stimulated by hormones Secrete an oily secretion called sebum

Structure of a Nail Scalelike modification of the epidermis on the distal, dorsal surface of fingers and toes Consist of stratified squamous epithelial cells overlying the nail bed, with the lunula as the most actively growing region of the nail root As new cells are produced, older ones are pushed outward and become keratinized.

Sweat Glands Different types prevent overheating of the body; secrete cerumen~ and milk Eccrine sweat glands – found in palms, soles of the feet, and forehead. Respond to body temperature Apocrine sweat glands – found in axillary and anogenital areas. Respond to body temperature, stress, & sexual arrousal ~Eruminous glands – modified apocrine glands in external ear canal that secrete cerumen (wax) ~Mammary glands – specialized sweat glands that secrete milk

Functions of the Integumentary System: Regulation of Body Temperature Body temperature regulation is accomplished by: Dilation (cooling) and constriction (warming) of dermal vessels Increasing sweat gland secretions to cool the body Excessive cooling: inactivates sweat glands, shivering

Healing of Wounds & Burns

Wounds Inflammation, in which blood vessels dilate and become more permeable, causing tissues to become red and swollen, is the body’s normal response to injury. Superficial cuts are filled in by reproducing epithelial cells. Deeper cuts are closed off by clots, covered by scabs, and eventually filled in by fibroblasts, making connective tissue. Blood vessels extend into the area, injured tissues are replaced, and the scab falls off. Large wounds leave scars and healing may be accompanied by the formation of granulations.

Burns First-degree – only the epidermis is damaged –Symptoms include localized redness, swelling, and pain Second-degree – epidermis and upper regions of dermis are damaged –Symptoms mimic first degree burns, but blisters also appear Third-degree – entire thickness of the skin is damaged –Burned area appears gray-white, cherry red, or black; there is no initial edema or pain (since nerve endings are destroyed)

Skin Cancer Most skin tumors are benign and do not metastasize A crucial risk factor for nonmelanoma skin cancers is the disabling of the p53 gene Newly developed skin lotions can fix damaged DNA

Skin Cancer The three major types of skin cancer are: –Squamous cell carcinoma –Basal cell carcinoma –Malignant Melanoma

Squamous Cell Carcinoma Arises from keratinocytes of stratum spinosum Arise most often on scalp, ears, and lower lip Grows rapidly and metastasizes if not removed Prognosis is good if treated by radiation therapy or removed surgically

Basal Cell Carcinoma Least malignant and most common skin cancer Stratum basale cells proliferate and invade the dermis and hypodermis Slow growing and do not often metastasize Can be cured by surgical excision in 99% of the cases

Malignant Melanoma Cancer of melanocytes is the most dangerous type of skin cancer because it is: –Highly metastatic –Resistant to chemotherapy

Melanoma (cont.) Melanomas have the following characteristics (ABCD rule) –A: Asymmetry; the two sides of the pigmented area do not match –B: Border is irregular and exhibits indentations –C: Color (pigmented area) is black, brown, tan, and sometimes red or blue –D: Diameter is larger than 6 mm (size of a pencil eraser)

Prognosis and Treatment Treated by wide surgical excision accompanied by immunotherapy Chance of survival is poor if the lesion is over 4 mm thick