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 - deeper 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 - bundle 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/cm2)

• 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 - dense 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 circulationin the area. Clotting around the edgesof the affected area partially isolatesthe region.

Sweat gland

Scab

Macrophagesand

fibroblasts

Migratoryepithelialcells

Granulationtissue

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.

Scartissue

Skin Injury and Repair

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 D3

• 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 5.12Melanoma

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