Integumentary System

Picture of Skin

The Integumentary System

• Largest organ system

• Includes: skin, hair, nails, sweat glands, and, sebaceous glands

• Function: protective barrier between internal and external enviornment

Functions of the Skin: Protection

• Act as a physical barrier against mechanical injury

• Prevents excessive water loss and dehydration

• Protects against chemical exposure and microbial invasion

• Shields underlying tissues from ultraviolet (UV) radiation

Functions of the Skin: Sensation

Contains specialized sensor receptors that detect:

• Touch and pressure (mechanoreceptors)

• Temperature (thermoreceptors)

• Pain (nociceptors)

Functions of the Skin: Thermoregulation

Regulates body temperature through:

• Sweat production (evaporative cooling)

• Vasodilation and vasconstriction of dermal blood vessels

Function of the Skin: Excretion

Eliminates small amounts of metabolic wastes such as urea and salts through sweat

Functions of the Skin: Vitamin D Synthesis

UV radiation triggers the conversion of precursor molecules in the skin into vitamin D, which is essential for calcium absorption and bone health

Functions of the Skin: Immune Defense

Contains immune cells such as Langerhans cells, which help protect against pathogens

Three Distinct Layers

1. Epidermis

2. Dermis

3. Hypodermis (subcutaneous layer)

Epidermis

• Outermost layer of the skin

• Consists of keratinized stratified squamous epithelium

• Avascular and depends on diffusion from dermis for nurtient

Keratinocytes (Epidermal Cell)

Most abundant; produce keratin for protection

Melanocytes (Epidermal Cell)

produce melanin pigment

Langerhans (dendritic) cells (Epidermal Cell)

Involved in immune response

Merkel Cells (Epidermal Cell)

function in touch sensation

Layers of the Epidermis (Superficial to Deep)

1. Stratum Corneum

2. Stratum Lucidum

3. Stratum Granulosum

4. Stratum Spinosum

5. Stratum Basale (germinativum)

Pigmentation and UV Protection

Melanocytes synthesize melanin, which accumulates over the nuclei of epidermal cells, forming a protective “umbrella” that shields DNA from UV damage

Melanocytes

• Spider-shaped epithelial cells found in the bottom 2 layers of epidermis (stratum basale & stratum spinosum)

• Produces pigment called melanin

  • Absorbed by nearby epidermal cells

Difference in dark and light skintones

• Both have the same number of melanocytes

• Difference is the amount of melanin produced

Variations in skin color results from

Differences in melanin production and distribution rather than the number of melanocytes

Excess UV exposure

• Potentially leading skin cancers

• Basal cell carcinoma (stratum basale)

• Squamous cell carcinoma (stratum spinosum)

• Melanoma (melanocytes; most dangerous)

Basal cell carcinoma

uncontrolled division of cells in the stratum basale layer

Squamous Cell Carcinoma

uncontrolled division of cells in the stratum spinosum layer

Melanoma

uncontrolled division of melanocytes

Dermis

• Lie beneath the epidermis

• Provides structural support, nourishment, and sensory input

• Composed of dense irregular connective tissue

  • Rich in collagen and elastic fibers

Components of the Dermis

• Blood Vessels

• Lymphatic Vessels

• Nerve ending and sensory receptors

• Hair Follicles

• Sweat and sebaceous glands

Dermal Papillae

Uneven junction between epidermis and dermis forms dermal papillae:

• increase surface area for diffusion

• create fingerprints (epidermal ridges)

• Improve grip and tactile sensitivity

Layer of the Dermis

1. Papillary Layer

2. Reticular Layer

Papillary Layer (Dermis Layer)

• Made of areolar connective tissue

• Contains capillaries and sensory receptors

Reticular Layer

• Thicker and deeper

• Composed of dense collage fiber bundles

• Forms lines of cleavage (tension lines)

Clinical Relevance

Incisions made parallel to cleavage lines heal faster and produce less scarring

Vasodilation

During physical activity or heat exposure, vasodilation of the dermal blood vessels increases heat loss

Vasconstriction

During cold exposure = vasoconstriction reduces heat loss

Decubitus Ulcers

Prolonged pressure that restricts blood flow can lead to decubitus ulcers (pressure sores)

Hypodermis

• Subcutaneous layer

• Lies beneath the dermis

• Consists of adipose and areolar connective tissue

  • Insulates and stores nutrients

• Technically not part of skin

Functions of the Hypodermis

• Anchors skin to underlying tissues

• Acts as an insulating layer

• Stores energy as fat

• Absorbs shock

Sweat (Sudoriferous) Glands

Skin contains millions of sweat glands that contribute to temperature regulation and waste removal

Eccrine glands (merocrine)

• Widely distributed

• Produce watery sweat for thermoregulation

Eccrine glands Sweat

• Long tubes that open into pores on skin surface

• Sweat is 99% water

  • Salts, vitamins, wastes, and an antimicrobial peptide called dermcidin

• Sweat is generally acidic

Apocrine glands (merocrine)

• Found in axillary and genital regions

• Produce thicker secretions containing proteins and lipids

• Become active at puberty

Apocrine glands: Sweat

• Contains all sweat components PLUS fatty substances and proteins

• Bacertia begin to break down fat & proteins causing body odor

Sebaceous Glands

Secretes sebum, an oily substance that:

• Function: Lubricates skin and hair, kills bacteria

• Prevents excessive water loss

• Inhibits bacterial growth

• Sebum production is hormonally regulated and often increases during adolescence

  • Also based on inheritance

Sebum (oil) Secretion

• Usually secreted onto hair → more oil glands on scalp and face

• None on palms or soles of feet

Hair

Composed of dead, keratinized cells and varies in thickness and distribution (hard keratin)

Functions of Hair

• Protection from UV radiation

• Thermal insulation

• Sensory detection

• Protection of sensitive structures

Hair Growth and Structure

Hair grows from the hair bulb within the follicle (root). Growth cycles include:

• Anagen (growth)

• Catagen (transition)

• Telogen (resting)

Shaft of Hair

• Part of the hair that sticks out the skin

• Protected by the outermost layer called a cuticle

  • Conditioner smooths down the rough surface of the cutricle

Hair Bulb

• Hair cells divide within the follicle in this region

• Cells are filled with keratin and pigments

Arrector Pili

• Dead cells are continuously pushed out as new cells are formed

• Tiny muscles attached to the hair shaft to make hair “stand on end”

Texture and Color of hair

• Texture is based on the shape of hair follicle opening (curly, straight, wavy)

• Hair color determined by melanin present at base of hair follicle

Nails

Nails are modified epidermal structures composed of hair keratin

Structure of a Nail

• Protective and useful tools

  • Example: picking up things, scratching

• 4 Basic Parts:

  • Free edge & Body (visible)

  • Root & Nail Bed (not visible)

How are nails formed?

• Nail matrix produces heavily keratinized cells, becoming the nail body

  • Nail body is pink due to the presence of capillaries

• Nail is protected on three sides by nail folds

• Lunula - “little moon”; whiter due to thickness of nail

Eponychium

• The cuticle

• Provides protection seal for the nail matrix

• Provides a seal so foreign objects and bacteria can’t enter the nail bed