Integumentary System Notes

Integumentary System

Overview

• The integumentary system, also known as the skin, is the body's most accessible organ system.
• It accounts for 16% of total body weight and covers a surface area of 1.5–2 square meters.
• It serves as the body’s first line of defense against the environment.
• The integumentary system has two major components:
  • Cutaneous membrane
  • Accessory structures

Components of the Integumentary System

• Cutaneous Membrane:
  • Epidermis:
    • Composed of stratified squamous epithelium.
    • Located above the dermis.
  • Dermis:
    • Papillary Layer:
      • Made of areolar connective tissue.
    • Reticular Layer:
      • Made of dense irregular connective tissue.
• Accessory Structures:
  • Hairs
  • Nails
  • Exocrine Glands:
    • Sebaceous glands
    • Sweat glands
  • Sensory receptors and nerve fibers
  • Arrector pili muscles
  • Cutaneous plexus (network of blood vessels)

Functions of the Integumentary System

• Protects underlying tissues and organs against:
  • Impact
  • Abrasion
  • Fluid loss
  • Chemical attack
• Excretes salts, water, and organic wastes through integumentary glands.
• Maintains normal body temperature through insulation or evaporative cooling.
• Produces melanin to protect underlying tissue from ultraviolet radiation.
• Produces keratin for protection against abrasion and as a water repellent.
• Synthesizes vitamin D3, a steroid converted to calcitriol, important for calcium metabolism.
• Stores lipids in adipocytes in the dermis and hypodermis.
• Detects touch, pressure, pain, and temperature stimuli, relaying information to the nervous system.

Epidermis

• The epidermis is composed of multiple layers of cells called strata.
• The primary cell type in the epidermis is the keratinocyte, the body’s most abundant epithelial cell.
• Keratinocytes are continuously produced by stem cell division in the deepest layers and shed at exposed surfaces.
• Deeper layers of the epidermis form epidermal ridges adjacent to dermal papillae, increasing surface area for better attachment.

Fingerprints

• Fingerprints are patterns of epidermal ridges on the surface of fingertips.
• They are determined by genes and the intrauterine environment during fetal development.
• Fingerprints provide a unique pattern that does not change during a lifetime, used to identify individuals.

Thin vs. Thick Skin

• "Thin" and "thick" refer to the thickness of the epidermis.
  • Thin Skin:
    • Covers most of the body surface.
    • Contains four strata (layers).
    • Approximately 0.08 mm thick (as thick as a plastic sandwich bag).
  • Thick Skin:
    • Found on the palms of the hands and soles of the feet.
    • Contains five strata (layers).
    • Approximately 0.5 mm thick (as thick as a paper towel).

Epidermal Layers Overview

• The entire epidermis lacks blood vessels.
• Cells receive oxygen and nutrients from capillaries in the dermis.
• Cells with the highest metabolic demand are closest to the dermis.
• It takes approximately 7–10 days for cells to move from the deepest stratum to the most superficial layer.
• Cells in the surface layer (stratum corneum) remain for about 2 weeks before being shed or washed away.

Epidermal Layers (Deep to Superficial)

• Stratum Basale (Basal Layer):
  • The bottom layer of the epidermis.
  • Attached to the basement membrane by hemidesmosomes.
  • Most cells are basal cells, stem cells that divide to replace superficial keratinocytes.
  • Merkel cells, which respond to touch, are also found here.
• Stratum Spinosum (Spiny Layer):
  • Composed of 8–10 layers of keratinocytes bound together by desmosomes.
  • Appears spiny when on a prepared slide.
  • Contains dendritic (Langerhans) cells, which are part of the immune response defending against microorganisms and superficial skin cancers.
• Stratum Granulosum (Grainy Layer):
  • Composed of 3–5 layers of keratinocytes.
  • Most cells have stopped dividing and started producing keratin and keratohyalin.
  • Cells grow thinner and flatter.
  • Cell membranes thicken and become less permeable.
• Stratum Lucidum (Clear Layer):
  • Found only in thick skin.
  • Separates the stratum corneum from underlying layers.
  • Contains flattened, densely packed dead cells filled with keratin and keratohyalin.
• Stratum Corneum (Horny Layer):
  • Outermost, protective region with 15–30 layers of keratinized cells filled with keratin.
  • Dead cells are tightly connected by desmosomes.
  • Water resistant, but not waterproof.
  • Loses water through insensible perspiration (unfelt) and sensible perspiration (sweat).

Factors Influencing Skin Color

• Presence of pigments in the skin
  • Carotene
  • Melanin
• Degree of dermal circulation
• Thickness and degree of keratinization in the epidermis
• Exposure to ultraviolet (UV) radiation, which can increase pigmentation (even though skin color is genetically determined)

Skin Coloration - Primary Pigments

• Carotene:
  • Orange-yellow pigment.
  • Accumulates in the epidermis and fatty tissues of the dermis and hypodermis.
  • Most apparent in the stratum corneum of light-skinned people.
  • Found in orange vegetables (e.g., carrots and some squashes).
• Melanin:
  • Brown, yellow-brown, or black pigment.
  • Produced by melanocytes in the stratum basale.
  • Manufactured from tyrosine (an amino acid).
  • Packaged into melanosomes (vesicles).
  • Melanosomes are transferred to keratinocytes.
  • Provides some protection against the effects of UV radiation by shading cell nuclei.

Skin Coloration Variations

• Melanin is transferred to keratinocytes and maintained until melanosomes are destroyed by fusion with lysosomes.
  • Paler skinned people: transfer occurs in deeper layers; more superficial layers lose pigmentation.
  • Darker skinned people: transfer occurs in more superficial layers (up to the stratum granulosum), so more pigmentation is visible. They also have larger melanosomes.

Melanocytes

• The Ratio of melanocytes to basal cells varies by body region between 1:4 to 1:20.
• Most areas have about 1000 melanocytes/mm^2.
• Differences in skin pigmentation result from the amount of melanin produced, not the number of melanocytes.
• Albinism is a deficiency or absence of melanin production.
  • Individuals with Albinism are not lacking melanocytes; they just can’t produce melanin.

Effects of Blood Supply on Skin Color

• Hemoglobin is a red pigment found in red blood cells.
• Blood flows to the dermis through the subpapillary plexus.
  • More blood flow to the region results in a redder color.
    • Example: flushed skin with fever from dilated superficial blood vessels.
  • Less blood flow to the region initially results in a pale color.
  • Sustained reduction of blood flow decreases available oxygen.
    • Lower oxygen levels makes hemoglobin darker red.
    • From surface view, skin has bluish color (cyanosis).
    • Most apparent in very thin skin (lips, beneath nails).

Skin Cancers

• Basal Cell Carcinoma:
  • Most common form of skin cancer.
  • Originates in the stratum basale due to mutations caused by overexposure to UV radiation.
  • Virtually no metastasis; most people survive.
• Malignant Melanoma:
  • Most serious form of skin cancer.
  • Extremely dangerous.
  • Cancerous melanocytes grow rapidly and metastasize through the lymphatic system.
  • If detected early and removed surgically, the 5-year survival rate is 99%.
  • If not detected until after metastasis, the 5-year survival rate drops to 14%.

Dermis

• Located between the epidermis and hypodermis.
• Contains two fiber types:
  • Collagen fibers:
    • Strong, resist stretching, bend and twist easily.
    • Limit flexibility in the dermis and prevent damage.
  • Elastic fibers:
    • Stretch and recoil to original length.
    • Allow flexibility in the dermis.
    • Reduced due to aging, hormonal changes, exposure to UV radiation.

Dermis Layers

• Papillary Layer:
  • Named for dermal papillae in this region.
  • Composed of areolar tissue.
  • Contains capillaries, lymphatic vessels, and sensory neurons.
• Reticular Layer:
  • Interwoven meshwork of dense irregular connective tissue.
  • Contains both collagen and elastic fibers.
  • Collagen fibers from this layer blend into both the papillary layer above and the hypodermis below.
  • Contains blood vessels, lymphatic vessels, nerve fibers, and accessory organs (hair follicles, sweat glands).

Hypodermis

• Separates skin from deeper structures
• Stabilizes position of skin relative to underlying tissues.
• Allows independent movement.
• Dominated by adipose tissue, making it an important energy storage site.

Sensory Receptors in the Integument

• ~400 cm of nerve fibers per cm^2 of skin
• Receptors in the Epidermis:
  • Free nerve endings
    • Sensitive to touch and pressure
    • Between epidermal cells
  • Tactile discs
    • Detect texture and steady pressure
    • Found in deepest layer of epidermis
• Receptors in the Dermis:
  • Tactile (Meissner’s) corpuscles
    • Detect light touch, pressure, and vibration
    • Found in the papillary layer
  • Lamellated (Pacinian) corpuscles
    • Detect deep pressure and vibration
    • Found in both dermal layers and in hypodermis
  • Bulbous (Ruffini) corpuscles
    • Sensitive to pressure and stretching of skin
    • Found in the reticular layer

Burns

• Burns are significant injuries resulting from exposure to heat, friction, radiation, electrical shock, or strong chemical agents.
• They can damage large areas of skin, compromising many essential functions.
• Dehydration and electrolyte imbalance can lead to kidney impairment and circulatory shock.
• Severity depends on:
  • Depth of penetration
  • Total area affected

Burn Classification

• Partial-Thickness Burns:
  • First-degree burn
    • Only the surface of the epidermis is affected.
    • Example: most sunburns
    • Painful
    • Skin redness (erythema) results from inflammation
  • Second-degree burn
    • The entire epidermis and maybe some of the dermis are damaged.
    • Accessory structures are not affected.
    • Blistering, pain, and swelling occur.
    • Infection can develop from ruptured blisters.
    • Healing takes 1–2 weeks.
    • Some scar tissue may form.
• Full-Thickness Burns:
  • Third-degree burns
    • Destroy the epidermis, dermis, and damage extends into the hypodermis.
    • Less painful than second-degree burns because sensory nerves are destroyed.
    • Extensive burns of this type cannot repair themselves.
      • Epithelial cells are unable to cover the injury.
    • Skin grafting is usually necessary.

Homeostatic Functions Threatened by Burns

• Fluid and electrolyte balance
  • Skin loses effectiveness as a barrier.
  • In full-thickness burns, the fluid loss rate increases up to five times the normal level.
• Thermoregulation
  • Increased fluid loss = increased evaporative cooling.
  • It takes more energy to maintain body temperature.
• Protection from infection
  • Dampness (from fluid loss) encourages bacterial growth.
  • If skin is broken, infection is likely to occur.
  • Sepsis (widespread bacterial infection) is a leading cause of death in burn victims.

Emergency Treatment of Burns

• Replacing lost fluids and electrolytes
• Providing sufficient nutrients (increased metabolic demands for thermoregulation and healing)
• Preventing infection
  • Cleaning and covering burn
  • Administering antibiotics
• Assisting tissue repair with skin grafts
  • Areas of intact skin are transplanted to cover the burn site.

Skin Grafts

• Split-thickness graft: transfer of epidermis and superficial portions of dermis
• Full-thickness graft: transfer of epidermis and both layers of dermis
• Source of material
  • Autograft – patient’s own undamaged skin
    • Best choice if possible; no rejection by the immune system
  • Allograft – frozen skin from a cadaver
  • Xenograft – animal skin

Accessory Structures of the Skin

• Hair follicles:
  • Produce hairs that protect the skull.
  • Produce hairs that provide delicate touch sensations.
• Exocrine glands:
  • Sweat glands (assist in thermoregulation and excrete wastes).
  • Sebaceous glands (lubricate epidermis).
• Nails:
  • Protect and support tips of fingers and toes.

Hair Overview

• Found almost everywhere on the body except:
  • Palms of hands
  • Soles of feet
  • Sides of fingers and toes
  • Lips
  • Parts of external genitalia
• The body has about 2.5 million hairs, where 75% is not on the head.
• Nonliving structures.
• Each hair is produced by a hair follicle.
  • Complex structure composed of epithelial and connective tissue that forms a single hair.

Two Types of Hair

• Terminal hairs:
  • Large, coarse, darkly pigmented.
  • Examples: hairs found on the scalp or in the armpit.
• Vellus hairs:
  • Smaller, shorter, delicate.
  • Found on the general body surface.

Hair Regions and Associated Structures

• Hair shaft: begins deep within the hair follicle but can be seen on the surface.
• Hair root: anchors the hair into the skin.
  • Extends from the base of the follicle to the point where the hair shaft loses connection with follicle walls.
• Root hair plexus: collection of sensory nerves surrounding the base of the follicle.
• Arrector pili: smooth muscle attached to the hair follicle; contraction pulls hair erect.

Hair Formation

• Begins at the hair bulb: an expanded base of the hair follicle.
• Bulb surrounds hair papilla: peg of connective tissue filled with blood vessels and nerves.
• Hair matrix: actively dividing basal cells in contact with hair papilla.
• Medulla: layer of daughter cells formed at the center of the matrix.
• Cortex: intermediate layer deep to the cuticle.
• Cuticle: daughter cells produced at the edges of the matrix; forms the surface of the hair.

Keratin in Hair

• Medulla or core of hair
  • Contains flexible, soft keratin
• Cortex
  • Contains thick layers of hard keratin
  • Gives hair stiffness
• Cuticle
  • Contains hard keratin
  • Thin, but very tough

Hair Growth

• Hairs grow and shed in a hair growth cycle.
  • Active phase
    • Lasts 2–5 years
    • Hair grows at a rate of 0.33 mm/day
  • Resting phase
    • Hair loses attachment to follicle; becomes a club hair
  • Club hair is shed when the follicle is reactivated and new hair formation begins
• Variations in growth rate and duration of cycle result in different lengths of uncut hair.

Sebaceous Glands

• Holocrine glands that discharge an oily lipid secretion.
• Contractions of the arrector pili muscle cause the release of sebum onto the follicle and skin surface.
  • Sebum: mixture of triglycerides, cholesterol, proteins, and electrolytes.
    • Lubricates hair shaft and is antimicrobial.

Sweat Glands

• Produce watery secretion by merocrine (eccrine) secretion
• Myoepithelial cells
  • Squeeze gland to discharge secretion
• Function to wash epidermal surface
• Two types
  • Apocrine sweat glands
  • Merocrine sweat glands

Types of Sweat Glands

• Apocrine sweat glands
  • Limited distribution (axillae, groin, nipples)
  • Secrete into hair follicles
  • Produce viscous/sticky, cloudy, odorous secretion with complex composition
    • Possible function in olfactory communication
  • Strongly influenced by hormones
  • Include ceruminous glands and mammary glands
• Merocrine sweat glands
  • Found in most areas of skin
    • Highest number found on palms and soles
  • Produce watery secretions with electrolytes
  • Controlled primarily by the nervous system
  • Important in thermoregulation and excretion
    • Some antibacterial action
  • Secrete directly onto the surface of the skin

Nails

• Thick sheets of keratinized epidermal cells.
• Protect exposed dorsal surfaces of tips of fingers and toes.
• Help limit distortion of digits under physical stress.
• The nail body is composed of dead cells packed with keratin.

Structures Associated with Nails

• Eponychium (cuticle): portion of stratum corneum of nail root extending over the exposed nail.
• Hyponychium: area of thickened stratum corneum under the free edge.

Age-Related Changes to the Integument

• Fewer melanocytes
  • In light-skinned people, skin becomes very pale
    • Increased sensitivity to sun exposure, more likely to sunburn
• Drier epidermis (decreased sebaceous gland activity)
• Thinning epidermis (declining basal cell activity)
  • Connections between epidermis and dermis weaken
    • More prone to injury, skin tears, and skin infection
  • Reduced vitamin D3 production
    • Causes muscle weakness and brittle bones
• Diminished immune response
  • Declining numbers of dendritic cells
    • Increased chance of skin damage and infection
• Thinning dermis
  • Fewer elastic fibers
    • Sagging and wrinkling are the results
• Decreased perspiration
  • Sweat glands are less active
    • Greater risk of overheating
• Reduced blood supply
  • Cools skin and stimulates thermoreceptors
    • Makes person feel cold even in warm room
  • Decreased ability to lose heat
• Slower skin repair
  • Example: blister repair 3–4 weeks in young adult takes 6–8 weeks in a 65- to 75-year-old
• Fewer active follicles
  • Thinner, finer hairs (gray or white from decreased melanocyte activity)
• Altered hair and fat distribution (decreased sex hormone levels)

Endocrine and Integumentary System Interactions

• Circulating hormones
  • Allow communication between skin and the rest of the body
• Thyroid hormones
  • Maintain normal blood flow to subpapillary plexus
• Sex hormones
  • Increase epidermal thickness
  • Accelerate wound healing
  • Increase the number of dendritic cells protecting against cancer cells and pathogens
• Growth factors
  • Stimulate cell growth and cell division

Vitamin D3 Production

• Inadequate supply of calcitriol leads to impaired bone growth and maintenance
  • In children, leads to rickets
    • Condition resulting in flexible, poorly mineralized bones
    • From not enough sunlight or not enough dietary cholecalciferol (vitamin D3)
    • Bone matrix has insufficient calcium and phosphate

Integumentary System Repair

• Four phases in skin regeneration after injury
  • Inflammatory phase
    • Initial injury causes bleeding and mast cell activation
    • Mast cells stimulate inflammation
      • Produces swelling, redness, heat, and pain
  • Migratory phase (after several hours)
    • Blood clot (scab) forms at surface
    • Cells of stratum basale divide and migrate along wound edges
    • Macrophages remove debris and pathogens
    • If damage into the dermis, a combination of fibroblasts, blood clot, and capillary network form granulation tissue as part of the repair process
  • Proliferation phase (one week after injury)
    • Scab undermined by migrating epidermal cells
    • Phagocytic activity almost complete
    • Blood clot disintegrating
    • Fibroblasts have formed collagen fibers and ground substance
  • Scarring phase (several weeks after injury)
    • Scab is shed; epidermis is complete
    • A shallow depression marks the injury site
    • Fibroblasts continue to create scar tissue – inflexible, fibrous, noncellular material