Integumentary System Cutaneous membrane (skin) – our largest organ Accounts for 7% of body weight Divided into two distinct layers Epidermis Dermis Accessory structures Subcutaneous layer (hypodermis)

Functions Of The Integument Cushions and insulates deeper organs Protects body from abrasion, trauma, chemicals, pathogens, temperature extremes and UV rays Excretion and secretion Contains sensory receptors associated with nerve endings Synthesis and storage of nutrients (vitamin D3)

Integumentary Structures Components of the Integumentary System Figure 5-1

Skin Structures

The Epidermis- Four Main Cell Types Melanocytes - found in basal layer, manufacture and secrete pigment Merkel cells - basal layer, attached to sensory nerve endings Keratinocytes – Arise from deepest layer of epidermis to stratum spinosum Produce keratin – a tough fibrous protein Produce antibodies and enzymes Keratinocytes are dead at skin's surface Langerhans cells - stratum spinosum, part of immune system macrophage-like

The Epidermis Stratified squamous epithelium Several distinct cell layers Thick skin—five layers on palms and soles Thin skin—four layers on rest of body

Layers of The Epidermis Stratum corneum Most superficial layer Dead, flattened (squamous) cells Abundant keratin - Keratinized (cornified) tough, water-resistant protein Protects skin against abrasion and penetration Stratum lucidum (clear layer) Occurs only in thick skin – palms and soles Composed of a few rows of flat, dead keratinocytes

Layers of The Epidermis Stratum granulosum (grainy layer) Consists of keratinocytes Tonofilaments Keratohyaline granules – help form keratin Lamellated granules – contain a waterproofing glycolipid Stratum spinosum (spiny layer) “Spiny” appearance caused by artifacts of histological preparation Contains thick bundles of intermediate filaments (tonofilaments) Resist tension Contain protein prekeratin Contains star-shaped Langerhans cells

Layers of the Epidermis Stratum basale Deepest layer of epidermis Attached to underlying dermis Stem cells actively divide Merkel cells – associated with sensory nerve ending Melanocytes – secrete the pigment melanin

Epidermal Cells and Layers of the Epidermis Figure 5.3

Sources of Skin Color Melanocytes Make melanin from tyrosine Melanin provides UV protection Gives reddish-brown to brown-black color Carotene Contributes orange-yellow color Provided from diet (carrots and tomatoes) Hemoglobin - blood pigment Caucasian skin contains little melanin Allows crimson color of blood to show through

Dermis Second major layer of the skin Provides mechanical strength, flexibility, and protection for underlying tissues Highly vascular and contains a variety of sensory receptors that provide information about the external environment Two layers Papillary layer – includes dermal papillae Reticular layer - d eeper layer – 80% of thickness of dermis Flexure lines - creases on palms

Layers of the Dermis Papillary layer Underlies epidermis Named for dermal papillae Aerolar connective tissue Supports, nourishes epidermis Provides sensory nerves, lymphatics, and capillaries Reticular layer Tough, dense, fibrous layer Dense irregular connective tissue Collagen fibers - limit stretch Elastic fibers - provide flexibility Blends into papillary layer (above) Blends into subcutaneous layer (below)

Dermal Components Epidermal accessory organs Cells of connective tissues proper Communication with other organ systems Cardiovascular Lymphatic Nervous Sensation Control of blood flow and secretion

Subcutaneous Layer - Hypodermis Composed of loose connective tissue - areolar and adipose Stabilizes skin position Loosely attached to dermis Loosely attached to muscle Contains many fat cells Provides thermal insulation Cushions underlying organs Safely receives hypodermic needles

Hair Filamentous strands of dead keratinized cells produced by hair follicles Contains hard keratin which is tougher and more durable than soft keratin of the skin Chief parts of a hair Root – imbedded in the skin Shaft – projects above skin's surface

Hair Hair Shaft organized into three concentric layers Medulla – central core Cortex – surrounds medulla Cuticle – outermost layer Pigmented by melanocytes at the base of the hair

Cross Section of a Hair Figure 5.7a, b

Hair Follicle Root sheath extending from the epidermal surface into the dermis Deep end is expanded forming a hair bulb Papilla - nipple-shaped indentation with blood vessels and nerves Matrix - germinal layer of cells (actively dividing cells) right above the papilla A knot of sensory nerve endings (a root hair plexus) wraps around each hair bulb Bending a hair stimulates these endings, hence our hairs act as sensitive touch receptors Arrector pili muscle - b undle of smooth muscle contracts to make hair stand erect

Longitudinal Section of Follicle Figure 5.7c, d

Hair Follicles

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

Sebaceous Glands (oil glands) Occur over entire body - Except palms and soles Simple alveolar glands Holocrine secretion – entire cell breaks up to form secretion Secretes an oily substance called sebum Most are associated with a hair follicle Functions of sebum Softens and lubricates hair and skin Skin waterproofing Collects dirt

Sweat (Sudoriferous) Glands Two types: Eccrine (Merocrine) Most abundant sweat gland “True sweat” 99% water with some salts Contains traces of metabolic wastes ~ 2% urea Role in thermoregulation Widely present in skin (up to 500/cm 2 ) Apocrine Odorous secretion Absent before puberty Present in axillary, areolar, anal and genital areas

Eccrine (Merocrine) Gland Figure 5.10b

Nails Scalelike modification of epidermis made of hard keratin Parts of the nail Free edge Body - d ense mass of keratinized cells Root Nail folds Eponychium – cuticle

Skin Injury and Repair Four Stages in Skin Healing Inflammation Blood flow increases Phagocytes attracted Scab formation Cell division and migration Scar formation

Bleeding occurs at the site of injury immediately after the injury, and mast cells in the region trigger an inflammatory response. Epidermis Dermis After several hours, a scab has formed and cells of the stratum germinativum are migrating along the edges of the wound. Phagocytic cells are removing debris, and more of these cells are arriving with the enhanced circulation in the area. Clotting around the edges of the affected area partially isolates the region. Sweat gland Scab Macrophages and fibroblasts Migratory epithelial cells Granulation tissue Skin Injury and Repair

One week after the injury, the scab has been undermined by epidermal cells migrating over the meshwork produced by fibroblast activity. Phagocytic activity around the site has almost ended, and the fibrin clot is disintegrating. Fibroblasts After several weeks, the scab has been shed, and the epidermis is complete. A shallow depression marks the injury site, but fibroblasts in the dermis continue to create scar tissue that will gradually elevate the overlying epidermis. Scar tissue Skin Injury and Repair

Table 5-1

Aging of the Skin Major Age-Related Changes Injury and infection increase Immune cells decrease Sun protection diminishes Skin becomes dry, scaly Hair thins, grays Sagging, wrinkles occur Heat loss decreases Repair slows

Effects of UV Radiation Beneficial effect - activates synthesis of vitamin D 3 Harmful effects Sun burn Wrinkles, premature aging Malignant melanoma Basal cell carcinoma

Types and Growth of Hair Vellus hairs Body hairs of women and children Terminal hairs Hair of scalp Axillary and pubic area (at puberty) Hair thinning and baldness Due to aging Male pattern baldness

Burns Classified by severity First-degree burn – only upper epidermis is damaged Second-degree burn – upper part of dermis is also damaged Blisters appear Skin heals with little scarring Third-degree burn Consume thickness of skin Burned area appears white, red, or blackened

Estimating Burns Using the Rule of Nines Figure 5.11a

Skin Cancer Basal cell carcinoma Least malignant and most common Squamous cell carcinoma Arises from keratinocytes of stratum spinosum Melanoma A cancer of melanocytes The most dangerous type of skin cancer

Skin Cancer Squamous cell carcinoma Figure 5.12 Melanoma Squamous cell carcinoma Basal cell carcinoma

The Skin Throughout Life Epidermis Develops from embryonic ectoderm Dermis and hypodermis Develop from mesoderm Melanocytes Develop from neural crest cells

The Skin Throughout Life Fetal skin Well formed after the fourth month At 5-6 months The fetus is covered with lanugo (downy hairs) Fetal sebaceous glands produce vernix caseosa

The Skin Throughout Life Middle to old age Skin thins and becomes less elastic Shows harmful effects of environmental damage Skin inflammations become more common