HumanA&P 6: Integumentary System

Integumentary System

• Anatomy of the skin

• Functions of Skin

• Accessory Structures

Skin

• Largest organ

• 16% total body weight

• 1.5-2m²

• 2 Major Components:

  • Cutaneous Membrane

  • Accessory Structures

Skin Structure

1. Epidermis: Superficial region

   • Consists of epithelial tissue, avascular

2. Dermis: Underlines epidermis

   • Mostly fibrous connective tissue, vascular

3. Hypodermis: superficial fascia

   • Subcutaneous layer

   • Mostly adipose tissue

   • Anchors skin to underlying structures

4 Cells of the Epidermis

1. Keratinocytes ~90%

2. Melanocytes ~8%

3. Dendritic (Langherhans) 1-2%

4. Tactile (Merkel) <1%

Keratinocytes

• ~90% of epidermis

• Stratified squamous epithelium

• Produce keratin

• Tightly connected by desmosomes

• Millions slough off every day

Melanocytes

• ~8% of epidermis

• Located in deepest epidermis

• Produce melanin packaged into melanosomes

  • Transfers to keratinocytes, protects from UV

Dendritic (Langherhans) Cells

• ~1-2% of epidermis

• Star-shaped macrophages that patrol deep epidermis

• Key activators of the immune system

Tactile (Merkel) Cells

• <1% of epidermis

• Sensory receptors that sense touch & pressure

5 Layers of the Epidermis

1. Stratum Basale

2. Stratum Spinosum

3. Stratum Granulosum

4. Stratum Lucidum

5. Stratum Corneum

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Stratum Basale

• Deepest epidermal layer

• Cuboidal or columnar keratinocytes & stem cells

• 1 row of stem cells

• AKA stratum germinativum

• 10-25% composed of melanocytes

Stratum Spinosum

• 2nd deepest epidermal layer

• 8-10 layers of cells that produce coarser keratin

• Contain intermediate prekeratin filaments

  • Allows resistance to tension & pulling

• Spikey keratinocytes called “prickle cells”

• Abundant melanosomes & dendritic cells

Stratum Granulosum

• 3rd deepest epidermal layer

• 4-6 cells thick

• Cell appearance changes:

  • Flatten

  • Keratinization begins

  • Accumulate lamellar granules

• Cells above this layer die

  • To far from dermal capillaries

Lamellar Granules

Water resistant glycolipid that slows water loss

Stratum Lucidum

• 2nd most superficial epidermal layer

• Only in thick skin

• Thin, translucent band of 2-3 rows of clear, flat dead keratinocytes

Stratum Corneum (horny layer)

• Most superficial epidermal layer

• 20-30 rows of flat dead keratinocytes

• ¾ of dermal thickness

• Cell functions:

  • Protection

  • Prevent water loss

  • Barrier against biological, chemical & physical assaults

Apoptosis

• Controlled cell death

• Cells change by undergoing this process

• Dead cells are dandruff

• Humans can shed ~50,000 cells/minute

Psoriasis

Increased rate in cell division & sloughing (7-10 days)

Formation of New Epidermal Cells

• Formed by cell division in stratum basale

  • ~4-6 weeks from mitosis to sloughing

• Keratinocytes moving superficially accumulate more keratin become less metabolically active

Epidermal Growth Factor

Allows basale keratinocytes to divide faster due to scrapes & burns

Dermis

• Strong, flexible connective tissue

• Cells:

  • fibroblasts

  • macrophages

  • mast cells

  • leukocytes

  • adipocytes

• Contains nerves, blood vessels, lymphatic vessels

• Hair follicles, oil glands, sweat glands

• 2 regions:

  1. Papillary

  2. Reticular

Papillary Layer

• Superficial dermal layer

• Composed of:

  • Areolar connective tissue

  • Collagen

  • Elastic fibers

  • Blood vessels

• Dermal papillae

Dermal Papillae

• Superficial region of dermis that sends fingerlike projections up into epidermis

• Contains:

  • Capillary loops

  • Free nerve endings

  • Touch receptors

Papillary Layer in Thick Skin

• Dermal papillae lie on top of dermal ridges, give rise to epidermal ridges

• Collectively ridges are called friction ridges

  • enhance gripping ability

  • Contribute to sense of touch

  • Sweat pores in ridges leave unique finger prints

Dermis Reticular Layer

• Deep dermal layer

• ~80% of dermal thickness

• Consists of coarse, dense fibrous connective tissue

  • Elastic fibers

  • Collagen fibers

• Cutaneous plexus

• Extracellular matrix contains pockets of adipose cells

Cutaneous Plexus

Network of blood vessels between reticular layer and hypodermis

Cleavage (Tension) Lines

• Collagen fibers in reticular layer running parallel to skin surface

• Externally invisible

• Indicate direction skin is most stretch resistant

• Surgeons make incisions parallel so they heal more readily

Flexure Lines

• Dermal folds in reticular layer at or near joints

• Dermis is secured to deeper structures

• Skin’s inability to slide easily for joint movement causes creases

Striae

• Extreme stretching of skin causing dermal tears leaves striae

• Acute, short-term traumas can cause blisters

3 Pigments that Contribute to Skin Colour

1. Melanin

2. Carotene

3. Hemoglobin

Melanin

• Pigment that contributes to skin colour

• Made in skin by melanocytes

• Melanosomes sent to keratinocytes to shield from UV

• 2 Types:

  • Pheomelanin: reddish-yellow

  • Eumelanin: brownish-black

Carotene

• Pigment that contributes to skin colour

• Yellow-orange pigment

• Palms & soles

• Accumulates in stratum corneum & hypodermis

• Vitamin A

Hemoglobin

• Pigment that contributes to skin colour

• Pinkish hue of fair skin is due to lower levels of melanin

• More O² attached to hemoglobin, the redder the skin appears

9 Functions of Skin

1. Chemical Barrier

2. Physical Barrier

3. Biological Barrier

4. Protection

5. Body Temperature Regulation

6. Cutaneous Sensations

7. Metabolic Functions

8. Blood Resevoir

9. Excretion of Wastes

Functions of the Skin: Chemical Barrier

• Secretes chemicals

  • Sweat: antimicrobial proteins

  • Sebum & defensins: kill bacteria

• Acid mantle: low pH of skin slows bacterial multiplication

• Melanin

Functions of the Skin: Physical Barrier

• Flat, dead, keratinized cells of stratum corneum surrounded by glycolipids, block most water & water soluble substances

• Some chemicals have limited penetration of skin

  • Lipid soluble

  • Plant oleoresins

  • Organic solvents

  • Salts of heavy metals

  • Drugs

  • Drug agents

Functions of the Skin: Biological Barrier

• Epidermis contains phagocytic cells

  • Engulf cells & activate immune system

• Dermis contain macrophages

  • Activate immune system

• DNA can absorb UV converting it to heat

Functions of the Skin: Temperature Regulation

• Sweat glands produce ~ 500ml/day of insensible perspiration

• In body temp rises, dilation of dermal vessels increase sweat gland activity to produce 12L of noticeable perspiration

• In cold, they constrict

Functions of the Skin: Cutaneous Sensations

• Cutaneous sensory receptors are part of the nervous system

• Exteroceptors respond to stimuli outside the body

• Free nerve endings sense painful stimuli

Functions of the Skin: Metabolic Functions

• Synthesize vitamin D

• Chemicals can disarm some carcinogens

• Keratinocytes can activate some hormones

• Collagenase aids natural turnover of collagen to prevent wrinkles

Functions of the Skin: Blood Reservoir & Excretion

• Skin can cold up to ~5% of blood volume

• Skin vessels can shunt blood to other organs

• Can secrete limited amounts of nitrogenous wastes

  • Ammonia, urea, uric acids

• Sweating causes salt & water loss

4 Accessory Structures

1. Hair

2. Nails

3. Sudoriferous Glands

4. Sebaceous Glands

Hair

• Hairs(pili): flexible strands of dead, keratinized cells

• Produced by hair follicles

• Functions:

  • Prevent heat loss from scalp

  • Protects from sun

  • Senses light touch with root plexus

• Anatomy

  • Shaft: extends above scalp

  • Root: area within scalp

Structure of Hair

• Medulla: central core of large cells

• Cortex: several layers of flattened cells surrounding medulla

• Cuticle: outer layer consisting of overlapping layers of single cells

Structure of Hair Follicle

• Hair bulb: expanded area at deep end of follicle

• Follicle Receptor: sensory nerve endings that wrap around bulb

• Hair Matrix: actively dividing area of bulb

• Arrector Pili: small smooth muscle band attached

• Hair papilla: dermal tissue containing knot of capillaries that supplies nutrient to grow hair

Hair Pigment

• Made by melanocytes in follicles

• Combinations of different melanins create colours

  • Black/brown = eumelanin

  • Red = pheomelanin

  • Blonde = a mix of low concentrations of eumelanin
    and/or pheomelanin

  • Gray/white = melanin production decreases and air bubbles replace melanin in shaft

Types & Growth of Hair

• Types of hair:

  • Lanugo Hair: downy fetal hair

  • Vellus Hair: fine body hair

  • Terminal Hair: coarse long hair

• Follicles cycle between active & regressive phases

Nails

• Scale-like modifications for epidermis that contain hard keratin

• Protective cover for distal, dorsal surface of fingers/toes

• Consists of:

  • free edge

  • nail plate: epidermis

  • root

  • nail folds

  • Eponychium

  • Hypochondrium

• Nail Matrix: thickened portion of bed responsible for nail growth

Eponychium

• Nail fold that projects onto surface of nail body

• Also called cuticle

Hyponychium

• Area under free edge of nail plate that accumulates dirt

Sweat Glands

• Sudoriferous glands

• All skin except nipples, external genitalia

  • 3 million/person

• 2 types:

  • Eccrine

  • Apocrine

• Contain myoepithelial cells

  • Contract upon nervous system stimulation to force sweat into ducts
    

Eccrine (Merocrine) Sweat Glands

• Most numerous

• Palms, soles, forehead

• Thermoregulation

• Secretes sweat

Apocrine Glands

• Axillary & anogenital areas

• Secrete viscous milky sweat that contains fatty substances and proteins

• Larger than eccrine glands

• Function at puberty

• Modified apocrine glands:

  • Ceruminous glands: lining of ear canal, earwax

  • Mammary glands

Sebaceous Glands

• Secrete sebum

  • oily holocrine solution

  • bactericidal

• Widely distributed except for palms, soles

• Most develop from hair follicles/secrete into follicles

• Inactive till puberty