The Skin and the Hypodermis Skin –our largest organ9 Appendages of the Skin •Hair follicles...

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C H A P T E R

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The Integumentary

System

The Skin and the Hypodermis

• Skin – our largest organ• Accounts for 7% of body weight• Divided into two distinct layers

• EpidermisD i

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

• Dermis• Hypodermis – lies deep to the dermis

Figure 5.2 Gross structure of skin and underlying tissues.

Epidermis

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DermisHypodermisDeep fasciaMuscle

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The Skin and the Hypodermis

• Functions• Cushions and insulates deeper organs• Protects body from bumps, scrapes, and cuts• Protects body from chemicals, heat, and cold


• Acts as a mini-excretory system• Screens out UV rays from the sun• Contains sensory receptors associated with nerve

endings

Skin Structure

Epidermis

Hair shaft

Papillarylayer

Dermal papillae

Pore

Subpapillary vascularplexus

Figure 5.1

DermisReticularlayer

Hypodermis(superficial fascia)

Appendages of skinEccrine sweat glandArrector pili muscleSebaceous (oil) glandHair follicleHair root

Nervous structuresSensory nerve fiberLamellar (Pacinian)corpuscleHair follicle receptor(root hair plexus)

Dermal vascular plexus

Adipose tissue

• Provides mechanical protection• Prevents fluid loss• Keeps microorganisms from invading the body

The epidermis


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Epidermis

• Contains four main cell types• Keratinocytes• Melanocytes• Merkel cells (tactile)• Langerhans (dendritic) cells


Epidermis

• Keratinocytes – most abundant cell type in epidermis• Arise from deepest layer of epidermis• Produce keratin – a tough fibrous protein

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• Produce antibodies and enzymes• Keratinocytes are dead at skin's surface

• The epidermis is composed of layers of keratinocytes• Thin skin = four layers (strata)• Thick skin = five layers

Thin Skin and Thick Skin


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Layers of the Epidermis

• Stratum basale (stratum geminativum) • Stratum spinosum• Stratum granulosum• Stratum lucidum (only in thick skin)


• Stratum corneum

Epidermal Cells and Layers of the Epidermis

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 5.3


• Stratum basale• Deepest layer of epidermis• Attached to underlying dermis• Cells actively divide


• Stratum basale contains• Merkel cells – associated with sensory nerve ending• Melanocytes – secrete the pigment melanin

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• Stratum spinosum (spiny layer)• "Spiny" appearance caused by artifacts of

histological preparation• Contains thick bundles of intermediate filaments

(tonofilaments)


(tonofilaments)• Contains star-shaped Langerhans cells


• Stratum granulosum• Consists of keratinocytes and tonofilaments • Tonofilaments contain

• Keratohyaline granules – help form keratin• Lamellated granules – contain a waterproofing glycolipid



• Stratum lucidum (clear layer)• Occurs only in thick skin• Composed of a few rows of flat, dead keratinocytes

• Stratum corneum (horny layer)


• Thick layer of dead keratinocytes and thickened plasma membranes

• Protects skin against abrasion and penetration

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• Cells accumulate keratin and eventually are shed• Epidermal ridges are interlocked with dermal papillae

• Fingerprints • Improve gripping ability

• Langerhans cells (immunity) in stratum spinosum

Epidermal characteristics:


• Merkel cells (sensitivity) in s. germinativum


Friction ridgesOpenings of sweat gland ducts

Dermal Modifications

(a) Friction ridges of finger tip (SEM 20×)

(b) Cleavage lines in thereticular dermis

(c) Flexure lines of the hand

Flexioncreaseson digitFlexion creaseson the palm

Figure 5.6

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Dermis• Second major layer of the

skin• Strong, flexible connective

tissue• Richly supplied with blood

vessels and nerves


• Has two layers• Papillary layer – includes

dermal papillae• Reticular layer – deeper

layer – 80% of thickness of dermis

Hypodermis• Deep to the skin – also called superficial fascia• Contains areolar and adipose connective tissues • Anchors skin to underlying structures• Helps insulate the body


Skin Color

• Three pigments contribute to skin color• Melanin – most important pigment – made from

tyrosine• Carotene – yellowish pigment from carrots and

tomatoes


tomatoes • Hemoglobin – Caucasian skin contains little melanin

• Allows crimson color of blood to show through

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Appendages of the Skin

• Hair• Flexible strand of dead, keratinized cells• Hard keratin – tough and durable• Chief parts of a hair


• Root – imbedded in the skin• Shaft – projects above skin's surface


• Hair – three concentric layers keratinized cells• Medulla – central core • Cortex – surrounds medulla• Cuticle – outermost layer


Cross Section of a Hair

Hair shaft

Arrector

Connective tissueroot sheath

Follicle wall

Cuticle

Glassy membrane

CortexMedulla

Internal epithelial root sheath

External epithelialroot sheath

Hair

Figure 5.8a, b

ArrectorpiliSebaceousgland

Hair root

Hair bulb

(a) Diagram of a cross section of a hair within its follicle

Hair shaft


Hair root

Hair bulb(b) Photomicrograph of a cross section

of a hair and hair follicle (185×)

Connective tissueroot sheath

Follicle wall

Cuticle

Glassy membrane

CortexMedulla

Internal epithelial root sheath


Hair

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• Hair follicles – extend from epidermis into dermis• Hair bulb – deep, expanded end of the hair follicle• Root plexus – knot of sensory nerves around hair

bulb• Wall of hair follicle

• Connective tissue root sheath


• Epithelial root sheath• Arrector pili muscle – bundle of smooth muscle

• Hair stands erect when arrector pili contracts

Hair shaft


Hair root

Hair bulb

Internal epithelialroot sheath


Connectivetissue root sheath

Follicle wall

Hair matrix

Melanocyte

Hair papilla

MedullaCortexCuticle

Glassy membrane

Hair root

Longitudinal Section of Base of Follicle

Follicle wall

(c) Diagram of a longitudinal view of the expanded hairbulb of the follicle, which encloses the matrix

Subcutaneousadipose tissue

Figure 5.8c, d

Hair shaft


Hair root

Hair bulb

(d) Photomicrograph of longitudinal viewof the hair bulb in the follicle (130×)

Internal epithelialroot sheath


Connectivetissue root sheath

Hair matrix

Hair papilla

Subcutaneousadipose tissue

MedullaCortexCuticle

Glassy membrane

Hair root

Types and Growth of Hair

• Vellus hairs – body hairs of women and children• Terminal hairs – hair of scalp; axillary and pubic

area (at puberty)• Lanugo


• Hair thinning and baldness• Due to aging• Male pattern baldness

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Sebaceous Glands

• Occur over entire body, except palms and soles• Secrete sebum – an oily substance

• Simple alveolar glands • Holocrine secretion – entire cell breaks up to form

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secretion• Most are associated with a hair follicle• Functions of sebum

• Collects dirt; softens and lubricates hair and skin

Sebaceousgland duct

Hair inhair follicle

Dermalconnectivetissue

Sebaceousgland

Sweatpore

Sebaceous Glands

(a) Photomicrograph of a sectioned sebaceous gland (140×)

Secretory cells

Eccrinegland

Figure 5.9a

Sweatpore

Sweat Glands

(b) Photomicrograph of a sectioned eccrine gland (140×)

Secretory cells

Dermal connectivetissue

Duct

Sebaceousgland

Eccrinegland

Figure 5.9b

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Sweat Glands

• Sweat glands (sudoriferous glands) widely distributed on body

• Sweat – is a blood filtrate • 99% water with some salts

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• Contains traces of metabolic wastes

Sweat Glands• Two types of sweat gland

• Eccrine gland (merocrine sweat gland) • Most numerous – produce true sweat

• Apocrine gland • Confined to axillary, anal, and genital areas• Produce a special kind of sweat


Nails

• Nails – scale-like modification of epidermis• Made of hard keratin• Parts of the nail

• Free edgeB d


• Body• Root• Nail folds• Eponychium – cuticle

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Lateralnail fold

Lunule

(a)

Structure of a Nail

Nailmatrix

Root of nailProximalnail fold

Nail bed Phalanx (bone of fingertip)

Eponychium(cuticle)

Bodyof nail

Free edgeof nail

(b)

Figure 5.7

Burns

• Classified by severity• First degree burn – only epidermis is damaged• Second degree burn – upper part of dermis is also

damaged• Blisters appear


• 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


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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


• The most dangerous type of skin cancer

Skin Cancer

Figure 5.11

The Skin Throughout Life

• Epidermis develops from embryonic ectoderm• Dermis and hypodermis develop from mesoderm• Melanocytes develop from neural crest cells


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• Fetal skin is well formed after the fourth month• At 5-6 months, the fetus is covered with lanugo

(downy hairs)• Fetal sebaceous glands produce vernix caseosa



• In middle to old age• Skin thins and becomes less elastic• Shows harmful effects of environmental damage• Skin inflammations become more common


The Skin and the Hypodermis Skin –our largest organ9 Appendages of the Skin •Hair follicles...

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