Introduction- Chapter 6

• Skin (integument) is body’s largest organ

• Approximately 1.6 to 1.9 m2 in average-sized adult

• Integumentary system describes the skin and its appendages— the hair, nails, and skin glands

Structure of the Skin

• Skin classified as a cutaneous membrane

• Two primary layers—epidermis and dermis; joined by dermal-epidermal junction

• Hypodermis lies beneath dermis

Structure of the Skin

• Thin and thick skin

– “Thin skin”—covers most of body surface • 1 to 3 mm thick

– “Thick skin”—soles and palms • 4 to 5 mm thick

– Makes fingerprints– No hair

Structure of the Skin

• Epidermis–Cell types

• Keratinocytes

• Melanocytes

• Langerhans cells

Keratinocytes

• constitute over 90% of cells present• principal structural element of the outer

skin• Arranged in strata, or layers

Melanocytes pigment-producing cells (5% of the total) contribute to skin color filter ultraviolet light – decrease the amount

of UV light that can penetrate into the deeper layers of the

skin

Langerhans cells

• they play a role in immune response

• Originate in the bone marrow

• Function with specialized leukocytes called helper T cells, to trigger immune response

Structure of the Skin• Epidermis

–Cell layers- 5 Layers (deep superficial)• Stratum germinativum (growth layer)—

describes the stratum spinosum and stratum basale together

1.Stratum basale (base layer)2.Stratum spinosum (spiny layer)3.Stratum granulosum (granular layer)4.Stratum lucidum (clear layer)5.Stratum corneum (horny layer)

• Epidermis– The cells form in the basale layer and

degenerate and fill with keratin as they move up to the surface

– This is keratinization

Structure of the Skin

section reveals epidermis containing basal, spinous, granular keratinocytes and stratum corneum. Dermis contains numerous viable fibroblasts. 400X)

1. Stratum basale (base layer- deepest)

• single layer of columnar cells• only these cells undergo mitosis

migrate through the other layers until they are shed

The renewal of the human

epidermis takes

about 3 to 4 weeks.

2. Stratum spinosum (spiny layer)

• cells arranged in 8 to 10 layers with desmosomes (bridges) that pull cells into spiny shapes

• cells rich in RNA protein synthesis to produce keratin

3. Stratum granulosum (granular layer)

• cells arranged in two to four layers

• filled with granules makes keratin

• contain high levels of lysosomal enzymes they’re starting to degenerate

• Nuclei absent

• May not be in regions of thin skin

4. Stratum lucidum (clear layer)

• Closely packed and clear keratinocytes

• cells filled with eleidin (ay lee din) blocks water penetration or loss (eventually makes keratin)

• absent in thin skin (in thick skin- hands and feet)

• Nuclei absent

5. Stratum corneum (horny layer)

• Dead, squamous cells• most superficial layer• dead cells filled with keratin water-repellent

protein• Cell membranes are thick and chemically resistant• Also called barrier area because it functions to

prevent water loss

Structure of the SkinThin Skin Thick Skin

Wrinkled Fingers and Toes

• FYI:

• dead cells in stratum corneum absorb water

• (deeper layers do NOT)

• When swollen water-filled cells expand over the normal sized cells below, the skin wrinkles

Structure of the Skin• Epidermis

– Epidermal growth and repair• Turnover or regeneration time refers to time

required for epidermal cells to form in the stratum basale and migrate to the skin surface— about 35 days

• Shortened turnover time will increase the thickness of the stratum corneum and result in callus formation

– Epidermal growth and repair continued• Normally 10% to 12% of all cells in

stratum basale enter mitosis daily• Each group of 8 to 10 basal cells in

mitosis with their vertical columns of migrating keratinocytes is called an epidermal proliferating unit, or EPU

Structure of the Skin

Structure of the Skin• Dermal-epidermal junction

– Definite basement membrane

– “glues” the epidermis to the dermis below

– Partial barrier to the passage of some cells and large molecules

Specific staining of the indicated protein is displayed as a red band localized at the dermal/epidermal junction.

Blisters

• FYI:• Caused by

injury to cells in epidermis OR

• From separation of dermal-epidermal junction

Structure of the Skin

• Dermis (corium)–Sometimes called “true skin”—much

thicker than the epidermis and lies beneath it

–Gives strength to the skin

Sunburn cell formation in EpiDerm-FT following solar UV-irradiation. H&E stained paraffin sections were prepared from EpiDerm-FT 24 hr after irradiation. Sunburn cells are indicated by arrows.

Structure of the Skin

• Dermis (cont.)–Contains various structures:

• Arrector pili muscles and hair follicles

• Sensory receptors

• Sweat and sebaceous glands

• Blood vessels

–Rich vascular supply plays a critical role in temperature regulation homeostasis

Structure of the Skin• Dermis (cont.)

–2 Layers of dermis:• Papillary layer• Reticular layer

Dermis-Papillary Layer

• composed of dermal papillae that project into the epidermis

fingerprints- used for grasping and gripping tools

• contains loose connective and elastic fibers• contains the dermal-epidermal junction

• Like egg crate foam mattress pad

Dermis- Reticular Layer

contains interlacing collagenous fibers and elastic fibers

make the skin tough yet stretchable when processed from animal skin,

produces leather Attachment point for muscle fibers

(smooth and skeletal) Contains arrector pili muscles

Arrector pili muscles

• Bundle of smooth muscles on each hair follicle

• Makes the hair “stand on end”• Occurs due to fright or cold• Causes erection of nipples and elevation of

testes

1. Where are the arrector pili?

2. Where is the adipose tissue?

3. Is adipose deep or superficial to the stratum lucidum?

Dermis–Dermal growth and repair:

• The dermis does NOT continually shed and regenerate

• During wound healing, begins forming a dense mass of new connective fibers scar

• If elastic fibers in dermis stretch too much, they TEAR and form stretch marks

»Eventually lose color- NOT due to cocoa butter

FYI• If an incision

“cuts across” cleavage lines (Langer’s lines), stress tends to pull the cut edges apart and may retard healing.

• Surgical incisions parallel to cleavage lines are subjected to less stress and tend to heal more rapidly.

Hypodermis

– Also called subcutaneous layer or superficial fascia

– Deep to the dermis

– forms connection between the skin and other structures

– NOT part of the skin

Skin Color• Melanin

–Basic determinant of skin color

–Melanin formed by melanocytes

–Albinism—

congenital

absence of

melanin

Functions of the Skin (Table 6-1 Page 171)

• Protection– Physical barrier– Prevents dehydration

• Sensation– Pain– Heat and cold– Pressure and touch

Functions of the Skin cont’d (Table 6-1 Page 171)

• Movement & growth– Imagine moving without elastic skin

• Endocrine (hormones)– vitamin D production

• Excretion (minor role)– Water & Urea/ammonia/uric acid

Functions of the Skin cont’d (Table 6-1 Page 171)

• Immunity

– Phagocytic cells (“phago”- to eat) (-”cyte”- cell)

– Langerhans cells

• Temperature Regulation

– Heat loss or retention

Temperature Regulation- Vasodilation

Heat loss—approximately 80% of heat loss occurs through the skin Increased blood

flow to epidermis

Decreased blood flow to organs

Redness after exercise

Temperature Regulation- Vasoconstriction

• Heat retention

– Decreased blood flow to skin

– Increased blood flow to organs

• Turning white

• Vasoconstriction

How do we sense touch?

• MANY receptors in our skin (mostly dermis) allow for different “touch” stimuli

– Meissner’s corpuscle- detects light touch- superficial dermis

– Pacinian corpuscle- detects pressure- deep dermis

Receptors send signals to the brain (those signals were called?)

Somatosensory Cortex in Brain

• Receptors send action potentials to specialized part of brain somatosensory cortex (“soma”- body) (“cortex”- outermost, superficial)

• Each part of the body

corresponds with a

particular area in the

cortex

• Also a corresponding

Motor cortex

Homunculi (Page 394)• Both sensory & motor cortex

have “homunculi” (little man)

– Hands and face- large part of cortical area• Most important for survival

Two point discrimination

• The density of receptors varies

• If the hands and the face have the most cortical area, where do you think the densest area of receptors will be?

Skin Glands

• 3 Types:1.Sweat (2 types)

2.Sebaceous

3.Ceruminous

Skin Glands- Sweat Glands

1.Eccrine glands–Most numerous sweat glands; very

small–Distributed over total body surface

except a few places (ear canal, lips)

–Simple, coiled, tubular glands–Secrete sweat constant

core temperature

2. Apocrine glands–Located deep in subcutaneous layer

–Limited distribution—axilla (armpit), areola of breast, and around anus

–Large (> 5 mm in diameter)

–Connected to hair follicle

–Begin to function at puberty

Skin Glands- Sweat Glands

Skin Glands

– Sebaceous glands• Secrete sebum (oil)

– Hair and skin– prevents excessive water loss

from the skin

• antifungal activity• dermis except in palms and

soles• Secretion increases in

adolescence– Oxidated sebum accumulations blackheads

Stages of acne. (A) Normal follicle; (B) open comedo (blackhead); (C) closed comedo (whitehead); (D) ; (E) pustule v

Blackhead

Whitehead

Papules

Skin Glands– Ceruminous glands (se ROO mi nus)

• Modified apocrine sweat glands

• Mixed secretions of sebaceous and ceruminous glands called cerumen (wax)

• protects from dehydration

Cycle of Life: Skin

• Children– Skin is smooth, unwrinkled, and characterized

by elasticity and flexibility– Few sweat glands– Rapid healing

Cycle of Life: Skin

• Adults– Development and activation of sebaceous

and sweat glands– Increased sweat production

• Body odor

– Increased sebum production• Acne

Cycle of Life: Skin

• Old age– Decreased sebaceous and sweat gland

activity• Wrinkling• Decrease of body’s ability to cool itself

The Big Picture: Skin and the Whole Body

• Skin is a major component of the body’s structural framework

• Skin defines the internal environment of the body

• Primary functions are support and protection