The Integumentary System Flashcards

Functional Anatomy of the Skin

Integumentary System Overview

• The integumentary system includes the skin and its accessory structures.
• It is the most accessible organ system.
• It can be referred to as skin or integument.
• It constitutes 16% of total body weight, covering 1.5–2 m2 in surface area.
• It serves as the body’s first line of defense against the environment.
• It has two major components:
  • Cutaneous membrane
  • Accessory structures

Integumentary System Components

• Cutaneous membrane
  • Epidermis:
    • Composed of stratified squamous epithelium.
  • Dermis:
    • Papillary layer:
      • Areolar connective tissue.
    • Reticular layer:
      • 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)

Subcutaneous Layer (Hypodermis)

• Not part of the integument.
• Separates the integument from deep fascia.

Functions of the Integumentary System

• Protect underlying tissues and organs against impact, abrasion, fluid loss, and chemical attack.
• Excrete salts, water, and organic wastes via integumentary glands.
• Maintain normal body temperature through insulation or evaporative cooling.
• Produce melanin, which protects underlying tissues from ultraviolet (UV) radiation.
• Produce keratin, which protects against abrasion and serves as a water repellent.
• Synthesize vitamin D3 (cholecalciferol), a steroid that is subsequently converted to calcitriol, a hormone important for normal calcium metabolism.
• Store lipids in adipocytes within the dermis and adipose tissue in the subcutaneous layer.
• Detect touch, pressure, pain, and temperature stimuli, relaying this information to the nervous system.

Epidermis

• Composed of multiple layers of cells (strata).
• The primary cell type in the epidermis is the keratinocyte.
• Deeper layers of the epidermis form epidermal ridges which are adjacent to dermal papillae.
• These epidermal ridges increase surface area for better attachment.

Thin Skin

• Covers most of the body surface.
• Contains four strata (layers).

Thick Skin

• Found on the palms of the hands and soles of the feet.
• Contains five strata (layers).

Fingerprints

• Unique pattern of epidermal ridges on the surface of fingertips.
• Pattern remains unchanged during lifetime.
• Prints of these patterns are used to identify individuals.

Epidermal Layers Overview

• The entire epidermis lacks blood vessels.
• Cells obtain 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
  • Attached to the basement membrane by hemidesmosomes.
  • Most cells here are basal cells, stem cells that divide to replace more superficial keratinocytes.
  • Merkel cells that respond to touch are also found here.
• Stratum spinosum (“spiny layer”)
  • Composed of 8–10 layers of keratinocytes bound together by desmosomes.
  • Contains dendritic (Langerhans) cells, which function as 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 stratum corneum from underlying layers.
  • Composed of flattened, densely packed dead cells filled with keratin and keratohyalin.
• Stratum corneum (cornu, horn)
  • Outermost, protective region with 15–30 layers of keratinized cells (filled with keratin).
  • Dead cells remain tightly connected by desmosomes.
  • Water-resistant, but not waterproof.
  • Water loss occurs through insensible perspiration (unable to see or feel) and sensible perspiration (sweat).

Factors Influencing Skin Color

• Presence of pigments in the skin, including carotene and melanin (absence of which leads to albinism).
• 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.
• Melanin:
  • Brown, yellow-brown, or black pigment.
  • Produced by melanocytes in the stratum basale
    • Differences in skin pigmentation arise from the amount of melanin produced, not from the number of melanocytes.
  • Packaged into melanosomes (vesicles)
    • Melanosomes are transferred to keratinocytes.
    • The size of melanosomes and point of transfer vary with skin color.

Effects of Blood Supply on Skin Color

• Hemoglobin is the red pigment found in red blood cells.
• Blood flows to the dermis through the subpapillary plexus.
  • More blood flow to a region results in a redder color.
  • Less blood flow to a region initially results in a pale color.
    • Sustained reduction of blood flow decreases available oxygen; the skin then has a bluish color (cyanosis).
    • Cyanosis is most apparent in very thin skin (lips, beneath nails).

Skin Cancer

• Basal Cell Carcinoma:
  • Most common form of skin cancer.
  • Originates in the stratum basale due to mutations caused by overexposure to UV radiation.
  • Has virtually no metastasis, and most people survive.
• Malignant Melanoma:
  • 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 Overview

• Located between the epidermis and hypodermis.
• Contains two fiber types:
  • Collagen fibers
  • Elastic fibers

Dermis Layers

• Papillary Layer:
  • Named for the dermal papillae present in this region.
  • Composed of areolar tissue.
  • Contains capillaries, lymphatic vessels, and sensory neurons.
• Reticular Layer:
  • Interwoven meshwork of dense, irregular connective tissue containing collagen and elastic fibers.
  • Contains blood vessels, lymphatic vessels, nerve fibers, and accessory organs (hair follicles, sweat glands).

Subcutaneous Layer (Not Part of Skin)

• Separates the skin from deeper structures.
• Dominated by adipose tissue, which serves as an important energy storage site.

Adipose Accumulation Pattern

• Accumulation in men:
  • Neck, arms, lower back, buttocks.
• Accumulation in women:
  • Breasts, buttocks, hips, thighs.
• For both adult sexes:
  • Few cells on the back of hands and surfaces of feet.
  • More abundant in the abdominal region (“potbelly”).

Sensory Receptors in the Epidermis

• Free Nerve Endings:
  • Sensitive to touch and pressure.
• Tactile Corpuscles:
  • Detect texture and steady pressure.

Sensory Receptors in the Dermis

• Meissner Corpuscles:
  • Detect light touch, pressure, and vibration.
• Pacinian Corpuscles:
  • Detect deep pressure and vibration.
• Ruffini Corpuscles:
  • Sensitive to pressure and stretching of the skin.

Tension (Cleavage) Lines

• Formed by the arrangement of collagen and elastic fibers in the skin.
• Clinically significant for surgery and wound healing.
  • Cuts parallel to cleavage lines heal better with less scarring.
  • Cuts perpendicular to cleavage lines tend to pull open, resulting in more scarring.

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 essential functions.
  • Dehydration and electrolyte imbalance can lead to kidney impairment and circulatory shock.
• Severity depends on depth of penetration and total area affected.

Partial-Thickness Burns

• First-Degree Burn:
  • Only the surface of the epidermis is affected.
  • Example: most sunburns
    • Skin redness (erythema) results from inflammation.
• Second-Degree Burn:
  • The entire epidermis and possibly 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.

Full-Thickness Burns

• Third-Degree Burns:
  • Destroys epidermis, dermis, and extends into the subcutaneous layer.
  • Less painful than second-degree burns due to nerve damage.
  • Extensive burns of this type cannot repair themselves; skin grafting is usually necessary.

Effects of Burns on Skin Functions

• Fluid and Electrolyte Balance
• Thermoregulation
• Protection from Infection

Evaluating Burns in a Clinical Setting

• Depth of burns is assessed with a pin; absence of reaction indicates a third-degree burn (loss of sensation).
• Percentage of skin that has been burned is estimated using the Rule of Nines, a method of estimating the percentage of surface area affected by burns (modified for children due to different body proportions).

Emergency Treatment of Burns

• Replacing lost fluids and electrolytes.
• Providing sufficient nutrients due to increased metabolic demands for thermoregulation and healing.
• Preventing infection by cleaning and covering the burn, and administering antibiotics.
• Assisting tissue repair with skin grafts, where areas of intact skin are transplanted to cover the burn site.

Skin Grafts

• Split-Thickness Graft:
  • Transfer of epidermis and superficial portions of the dermis.
• Full-Thickness Graft:
  • Transfer of epidermis and both layers of the 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.

Burn Recovery

• Young patients with burns over 80% of the body have a 50% chance of recovery, assuming access to medical treatment, including grafting.
• Advances in cell culturing are improving survival rates; new epidermis can be grown in the laboratory and transplanted to cover burns.

Accessory Structures of the Skin

Accessory Structures of the Integument

• Called epidermal derivatives, originating from the epidermis during embryological development.
• Hair follicles and exocrine glands develop from epithelial columns.
  • Cords of epidermal cells growing into the dermis.
• Nails form from thickened epidermal cells nestled into the dermis, forming a nail field.
• Located in the dermis but project through the epidermis to the surface.

Accessory Structures of the Skin include:

• Hair follicles, which produce hairs that protect the skull and provide delicate touch sensations.
• Exocrine glands
  • Sweat glands, which assist in thermoregulation and excrete wastes.
  • Sebaceous glands, which lubricate the epidermis.
• Nails which protect and support tips of fingers and toes.

Hair

• Found almost everywhere on the body except:
  • Palms of hands, sides and soles of feet, sides of fingers and toes, lips, parts of external genitalia.
• Each hair is produced by a hair follicle, a 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 the 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.
• Sebaceous Gland:
  • Produces secretions to coat the hair and skin surface.

Hair Formation

• Begins at the hair bulb, an expanded base of the hair follicle.
• The bulb surrounds the hair papilla, a peg of connective tissue filled with blood vessels and nerves.
• The hair matrix consists of actively dividing basal cells in contact with the hair papilla.

Hair Structure

• 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 surface of the hair.

Keratin in Hair

• Medulla (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.

Internal Hair Follicle Structure

• Internal Root Sheath:
  • Surrounds hair root and deeper portion of the shaft.
  • Produced from the hair matrix.
• External Root Sheath:
  • Extends from the skin surface to the hair matrix.
• Glassy Membrane:
  • Thickened, clear basement membrane.
• Connective Tissue Sheath:
  • Surrounds the epithelial cells of the hair follicle.

Hair Growth

• Hairs grow and shed in a hair growth cycle that has two phases:
  • Active phase
    • Lasts 2-5 years.
    • Hair grows are a rate of $0.33$ mm/day.
  • Resting Phase
    • Hair loses attachment to the follicles and becomes club hair.
    • Club hairs shed when the follicle is reactivated and new hair formation begins.

Sebaceous Glands

• Holocrine glands that discharge an oily lipid secretion called sebum.
• Secretion is released when the arrector pili muscle contracts, lubricating hair and skin and also having an antimicrobial function.

Sweat Glands

• Produce watery secretion by merocrine secretion.
• Myoepithelial cells (myo-, muscle) squeeze the gland to discharge secretion to wash the epidermal surface.
• There are two types:
  • Apocrine sweat glands.
  • Merocrine sweat glands.

Apocrine Sweat Glands

• Found in axillae, around nipples, and in the pubic region.
• Produce sticky, cloudy, odorous secretion with complex composition that is strongly influenced by hormones.
• Include ceruminous glands and mammary glands.

Merocrine Sweat Glands

• Secrete directly onto the surface of the skin.
• Highest number found on palms (~ $500$ glands/cm$^2$ [~$3000$/in$^2$]) and soles.
• Produce watery secretions with electrolytes and are important in thermoregulation and excretion.

Nails

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

Nail Structure

• Nail Body:
  • Visible portion of nail, bordered by lateral nail grooves (depressions) and lateral nail folds (ridges).
    • Covers the nail bed (underlying epidermis).
    • Lunula: pale crescent on proximal part of the nail body.
    • Free edge: distal part of the nail body.
• Nail Root:
  • Epidermal fold where nail production occurs.
• Eponychium:
  • Portion of stratum corneum of nail root extending over exposed nail (also known as the cuticle).
• Hyponychium:
  • Area of thickened stratum corneum under free edge.

Nail Appearance

• Can be used for diagnosis, the cells producing nails can be affected by conditions that alter body metabolism.
  • Examples:
    • Nails pitted and distorted: psoriasis.
    • Concave nails: some blood disorders.

Clinical Modules

Age-Related Changes Affecting the Integument

• Fewer Melanocytes:
  • In light-skinned people, skin becomes very pale.
  • There is also an increased sensitivity to sun exposure, making sunburn more likely.
• Drier Epidermis:
  • Decreased sebaceous gland activity.
• Thinning Epidermis:
  • Declining basal cell activity.
  • Connections between epidermis and dermis weaken, making them both 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 (to about half of levels at age 21) increase the chance of skin damage and infection.
• Thinning Dermis:
  • Fewer elastic fibers result in sagging and wrinkling skin.
• Decreased Perspiration:
  • Sweat glands are less active, increasing the risk of overheating.
• Reduced Blood Supply:
  • Cools skin and stimulates thermoreceptors, making a person feel cold even in a warm room and decreasing the ability to lose heat.
• Slower Skin Repair:
  • For example, blister repair that takes 3–4 weeks in a young adult takes 6–8 weeks in a 65- to 75-year-old.
• Fewer Active Follicles:
  • Thinner, finer hairs are produced (gray or white from decreased melanocyte activity).
• Altered Hair and Fat Distribution:
  • Decreased sex hormone levels.

Endocrine and integumentary system interactions

• Allow communication between skin and the rest of the body.
• Steroid hormones (glucocorticoids)
  • Loosen intercellular connections and reduce epidermis effectiveness as a barrier to infection.
• Thyroid hormones
  • Maintain normal blood flow to subpapillary plexus.
• Sex hormones
  • Increase epidermal thickness
  • Accelerate wound healing
  • Increase number of dendritic cells protecting against cancer cells and pathogens.
• Growth factors
  • Stimulate cell growth and cell division
  • Epidermal growth factor (EGF)
    • Peptide that has widespread effects on epithelia
    • Produced by salivary glands and glands in duodenum
    • Functions
      • Promotes basal cell division in stratum basale and stratum spinosum
      • Accelerates production of keratin
      • Stimulates epidermal development and repair
      • Stimulates synthesis and secretion in glands
• Growth hormone (GH)
  • Stimulates fibroblast activity and collagen production
  • Stimulates basal cell division
    • Thickens epidermis
    • Promotes wound repair

Vitamin D3 Production

• Sunlight
  • UV radiation causes epidermal cells of the stratum spinosum and stratum basale to convert a steroid to cholecalciferol (vitamin D3).
  • The Liver creates an intermediate product, and kidneys convert it to calcitriol.
    • Calcitriol allows calcium and phosphate absorption in the small intestine.
• Sources
  • Diet
    • Naturally from fish, fish oils, and shellfish
    • From fortified food products
• Inadequate supply of calcitriol leads to impaired bone growth and maintenance.
• In children, this leads to rickets.
  • Flexible, poorly mineralized bones result from not enough sunlight or dietary cholecalciferol (vitamin D3).
  • The Bone matrix has insufficient calcium and phosphate.
  • This is uncommon in the United States because vitamin D3 is added to milk.
  • Decreased bone density in elderly due to insufficient dietary intake. Additionally, skin production of cholecalciferol decreases by 75%, increasing the risk for fractures and slowing the healing process.

Integumentary System Repair

• The Integument can repair itself, even after extensive damage given its 4 phases:

Four Phases in Skin Regeneration After Injury

• Inflammation Phase:
  • Initial injury causes bleeding and mast cell activation.
  • Mast cells stimulate inflammation, producing swelling, redness, heat, and pain.
• Migration Phase (after several hours):
  • A blood clot (scab) forms at the surface.
  • Cells of the stratum basale divide and migrate along the wound edges.
  • Macrophages remove debris and pathogens.
  • If damage extends 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):
  • The Scab is undermined by migrating epidermal cells.
  • Phagocytic activity is almost complete.
  • The Blood clot is disintegrating.
  • Fibroblasts have formed collagen fibers and ground substance.
• Scarring Phase (several weeks after injury):
  • The Scab is shed, and the epidermis is complete.
  • A Shallow depression marks the injury site.
  • Fibroblasts continue to create scar tissue, an inflexible, fibrous, noncellular material.
• Keloids
  • Raised, thickened mass of scar tissue that begins at the injury site and grows into the surrounding dermis.
  • They are covered by a shiny, smooth epidermal surface.
  • Most common on the upper back, shoulders, anterior chest, and earlobes.
  • A form of body decoration in some cultures.