Integumentary System
Cutaneous layer – the skin (most superficial)
Cutaneous = “of the skin”
Skin accounts for 7% of body weight
Divided into two distinct layers:
Epidermis
Dermis
Subcutaneous layer (hypodermis) –
Subcutaneous = “below the skin”
Consists of areolar (connective tissue) and adipose
(fat) tissues
Functions Of The Integument
Cushions and insulates deeper organs
Protects body from abrasion, trauma, chemicals, pathogens, temperature extremes, and UV rays
Waterproofing; Prevention of water loss
Holds sensory receptors associated with nerve endings
Synthesis and storage of nutrients (vitamin D3)
Excretion and secretion (urea, water, salts via sweat)
Integumentary Structures
The Epidermis - Four Main Cell Types
Melanocytes - produced basal layer
Manufacture and secrete melanin pigment
Melanin is passed from melanocytes to keratinocytes
Merkel cells – located in basal layer
Attached to sensory nerve endings
Serve as tactile (touch) receptors
Keratinocytes – arise from deepest layer of epidermis to stratum
spinosum
Constitute 95% of the cells in the epidermis
Produce keratin – a tough fibrous protein
Produce antibodies and enzymes that detoxify harmful
chemicals
Corneocytes - keratinocytes that have lost their nucleus &
organelles (considered dead cells)
Langerhans cells – located in stratum spinosum
A dendritic cell; part of immune system
Phagocytic
The Epidermis
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 (cornified layer)
Most superficial layer
Corneocytes – dead, flattened squamous cells;
Cornified – filled with keratin, a tough, waterresistant protein
Protects skin against abrasion and penetration
Sheds in flakes (desquamation)
Stratum lucidum (clear layer)
Occurs only in thick skin (palms and soles)
Composed of a few rows of flat, dead
keratinocytes
At times visibly indistinguishable from adjacent
layers
Stratum granulosum (grainy layer)
Contain tonofilaments (intermediate filament)
Bundled at desmosomes
Resist tension
Contain protein prekeratin
Keratohyaline granules – help form keratin in
dying keratinocytes
Lamellated granules – release a waterproofing
glycolipid
Stratum spinosum (spiny layer)
“Spiny” appearance a result of histological preparation
Cells shrink but maintain desmosome connections
Contain tonofilaments
Contains star-shaped Langerhans cells (dendritic immune cells)
Stratum basale (basal layer)
Single row of cells
Deepest layer of epidermis
Attached to underlying dermis
High mitotic rate - stem cells actively divide, forming new keratinocytes
Merkel cells – associated with sensory nerve ending
Melanocytes (10 – 25%) – secrete the pigment melanin
Sources of Skin Color
Melanocytes
Produce melanin from the amino acid tyrosine using the enzyme tyrosinase
Melanin moves from melanocytes to keratinocytes
Melanin provides UV protection
Gives reddish-brown to brown-black color
Carotene
Yellow-orange pigment
Obtained from vegetable sources (carrots and tomatoes)
Hemoglobin - blood pigment
The blood pigment
Caucasian skin contains little melanin- nearly transparent- which 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 types:
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
Appendages of the Skin
Hair
Nails
Glands
Sebaceous Glands
Sweat Glands
Hair
Flexible strands of dead keratinized cells
Produced by hair follicles
Contains hard keratin which is tougher and more durable than the soft keratin of the skin
Chief parts of a hair:
Root – imbedded in the skin
Shaft – projects above skin's surface
Cross Section of a 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
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
Sensory nerve endings – afferent nerve receptors (a root hair plexus) wrapping 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
Shape of Hair Shaft
Hair Color
Melanocytes in the matrix of the hair bulb
Melanin is transferred to the cells in the hair shaft.
Different proportions of brown-black, yellow and reddish melanin combine to produce varieties of hair
color, from blond to black.
Gray hair occurs because of a decline in the enzyme (tyrosinase), inhibiting the conversion of tyrosine to
melanin
White hair is an accumulation of air bubbles in hair shaft combined with lack of pigment (decrease in
tyrosinase)
In red hairs, nearly all the melanin is present in the form of phaeomelanin (a lighter pigment found in red and
blonde hair)
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
Lips
Nipples and portions of the external genitalia
Glands
Holocrine glands
Whole glandular cells break apart to form the product
Used by sebaceous glands to produce sebum (oil) at hair follciles
Eccrine (Merocrine) glands
Vesicles release product via exocytosis
Most numerous type of integumentary gland
Produce “true sweat”
Apocrine glands
Apical portion of glandular cells pinch off as product
Mostly confined to axillary (armpit), anal, and genital areas
Made of true sweat plus fats and proteins
Sebaceous (Oil) Glands
Sebaceous glands (oil glands) secrete oil onto hair shafts
Occur over entire body, except palms and soles
Holocrine secretion
Entire cell breaks up to form secretion
Simple alveolar glands
Secretes an oily substance called sebum
Most are associated with a hair follicle
Sebum (oil)
Composed of lipids, wax, and the debris of dead, fat-producing cells.
Softens and lubricates hair and skin;
Prevents drying & cracking
Waterproofs skin
Collects dirt
Sudoriferous (Sweat) Glands
Two types of Sudoriferous Glands:
Eccrine (Merocrine)
Vesicles release product via exocytosis
Ducts form individual pores directly on skin surface
Produce “True sweat”
o 99% water with some salts
o Traces of metabolic wastes ~ 2% urea
Role in thermoregulation
Most abundant sweat gland (~500/cm2)
Apocrine
Apical portion of glandular cells pinch off as product
Ducts merge with hair follicle, superior to sebaceous (oil) glands
Milky, viscous, sweat made of true sweat, plus fats and proteins
Odorous secretion until digested by bacteria
Secretion begins at puberty
Present in axillary, areolar, anal, and genital areas
Product contains pheromones, responsible for synchrony of menstrual cycle in females
Sudoriferous (Sweat) Glands
Modified Sudoriferous (Sweat) Glands
Ceruminous glands
Localized in outer auditory canal
Produce cerumen (ear wax)
waterproofs the canal
kills bacteria
traps foreign particles (dust, fungal spores) by coating guard hairs, making them sticky
Mammary glands
Localized in anterior thorax
Produce colostrum and milk in pregnant and nursing females
Nails
Scale-like modification of epidermis made of hard keratin
Parts of the nail:
Free edge
Body (dense mass of keratinized cells)
Root
Nail folds
Eponychium (cuticle)
Nail matrix
Skin Injury and Repair
Four Stages in Skin Healing:
Inflammation
Blood flow increases
Phagocytes attracted
Scab formation
Cell division and migration
Scar formation
Skin Injury and Repair
1) Bleeding occurs at the site2 of injury immediately after the injury, and mast cells in the region trigger an
inflammatory response.
2) 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.
3) 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.
4) 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.
Aging of the Skin
Major Age-Related Changes
Telomeres (DNA caps) shorten with every mitotic divide
Cells enter “retirement” stage of the Cell Life Cycle (Stage G0)
Cells eventually stop dividing
Fibroblasts in connective tissue decrease in collagen or elastin
production;
Skin sags, wrinkles, and becomes thin
Skin becomes dry, scaly
Injury and infection increase; Repair slows
Immune cells decrease
Sun protection by melanocytes diminishes
Hair thins, grays
Thermoregulation decreases
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
X-linked recessive
Men (XY) need only one affected X chromosome to cause hair loss
Women (XX) need two affected X chromosomes for hair loss
Burns
Classified by severity:
First-degree burn – only upper epidermis is damaged
Redness
Second-degree burn – upper part of dermis is also damaged
Blisters appear
Skin heals with little scarring
Third-degree burn
Consumes thickness of skin
Burned area appears white, red, or blackened
Estimating Burns Using the Rule of Nines
Head = 9%
Chest (front) = 9%
Abdomen (front) = 9%
Back = 9%
Buttocks = 9%
Left Arm = 9% (front = 4.5%, back = 4.5%)
Right Arm = 9% (front = 4.5%, back = 4.5%)
Front of Left Leg = 9%
Back of Left Leg = 9%
Front of Right Leg = 9%
Back of Right Leg = 9%
Groin = 1%
Total: 100%
Skin Cancer
Basal cell carcinoma
Least malignant and most common
Squamous keratinocytes in basal layer
Squamous cell carcinoma
Arises from keratinocytes of stratum spinosum
Melanoma
The most dangerous type of skin cancer
A cancer of melanocytes
Most serious type, affects melanocytes
Development of Integument
Epidermis
Develops from embryonic ectoderm
Dermis and hypodermis
Develop from mesoderm
Melanocytes
Develop from neural crest cells
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 waxy or cheese-like white substance found coating the skin of newborn babies