Integumentary System

Organs of the Integumentary System

• Skin and its accessory structures (hair, nails, glands)

• Blood vessels

• Muscles

• Nerves

2 major levels of the skin (cutaneous membrane)

Epidermis and dermis

Epidermis

• Outer layer of the skin made of 4 main cell types

• Composed of keratinized stratified squamous epithelium

• Avascular

Dermis

• Made of connective tissue (2 layers)

• Binds the epidermis to the underlying tissues

• Vascular

Hypodermis (aka subcutaneous layer)

• Deep to the dermis and functions to attach the skin to underlying tissues and organs

• Not part of the skin

• Composed of areolar and adipose (mostly) tissues

• Contains large blood vessels and nerves that feed superficial layers

Thin skin

In most areas of the body the epidermis is made up of 4 distinct strata; Hairy

Thick skin

Epidermis in areas where exposure to friction is the greatest (ex/ fingertips, palms, and soles) contains an extra layer; Hair-less (typically with a thicker stratum corneum due to increased layers)

Keratinocytes

Produces keratin (hard protein) to protect underlying tissues from abrasion, heat, microbes, and chemicals

Melanocytes

Produce melanin pigment (which functions to absorb UV light and prevent DNA damage)

Langerhans cells

Intraepidermal macrophages involved in immunity

Tactile epithelial (Merkel) cells

Special touch receptors that contact sensory neurons

Stratum basale

• Deepest layer of skin

• Single row of keratinocytes (along with melanocytes and Merkel cells)

• Some cells undergo mitosis

• As new cells are formed older cells are “pushed” toward the surface and become keratinized (cytoplasm is replaced with keratin)

Stratum spinosum

• 8-10 layers of keratinocytes

• Contain organelles and produce coarser bundle of keratin

• Rounded in living tissue but appear shrunk and covered in spines when prepared for microscopic examination

Stratum granulosum

• 3-5 layers of flattened keratinocytes that undergoing apoptosis

• Cells contain keratohyalin (dark staining protein granules that help organize keratin filaments)

• Contain lamellar granules that secrete a lipid-filled product that acts as a water repellant

Stratum lucidum

• Present only in thick skin

• 4-6 layers of flattened, clear, dead keratinocytes

• Provides additional toughness to these regions of the skin

Stratum corneum

• Most superficial layer

• 25-30 layers of flattened, dead, keratinocytes

• Membrane enclosed packages of keratin

• No nucleus or organelles

Keratinization

Cells accumulate more keratin as they move from one epidermal layer to the next (cells are moving further away from blood supply and start to die)

As new keratinocytes are formed in the stratum basale (closest to blood supply and receive nutrients)…

Older cells are pushed to the surface and eventually undergo apoptosis and are sloughed off; Entire process takes 7-10 weeks

Dandruff

An excessive number of keratinized cells that shed from the skin of the scalp

Psoriasis

A disorder in which keratinocytes divide and move more quickly than normal & results in flaky, silvery, scales that form on the skins surface

2 regions the dermis contains

Papillary region and reticular region

Papillary region

• Superficial portion of the dermis that lies just below the epidermis

• Composed of lose areolar connective tissue (contains thin collagen and fine elastic fibers)

• Contains dermal papille

Dermal papille

• Ridges that project into the undersurface of the epidermis and anchor the epidermis to the dermis

• Contains blood vessels that feed epidermis

• Contain touch receptors

• More pronounced on fingertips and toes

• Make fingerprined

Reticular region

• Deepest portion of the dermis

• Composed of dense irregular connective tissue (resists stretching because it contains thick collagen fibers and coarse elastic fibers arranged in net-like manner)

• Contain hair follicles, nerves, and skin glands

• Tears or excessive stretching in this region can cause stretch marks (striae)

A combination of genetic, environmental, and physiological factors

Skin color results from

Melanin, hemoglobin, carotene

Certain pigments can impart a variety of colors to skin

Melanin

• A pigment protein produced by melanocytes in the stratum basale

• Everyone has about the same number of melanocytes

Hemoglobin

• Circulation of dermal blood affects skin color

• Well oxygenated gives pinkish color and poor oxygen gives bluish color (cyanosis)

Carotene

• Diet may affect skin color

• Egg yolks and yellow vegetables contain this pigment that may turn skin orange-yellow

Melanocytes

Image

Anatomy of hair

Parts of hair include the shaft, the follicle. and the root

Hair shaft

Above the skin surface

Hair follicle

Epithelium that surrounds the hair root and bulb

Root

Below the skin surface; May be called either the epithelial root sheath or a dermal root sheath

Hair related structures

Arrector pili muscles and hair root plexus

Arrector pili muscles

Smooth muscles in the dermis that contract with cold/fear forming “goose bumps”

Hair root plexus

Bundle of nerve endings that detects hair movement

4 types of skin glands

Sebaceous glands, eccrine sweat glands, apocrine sweat glands, ceruminous glands

Sebaceous glands

• Secrete sebum (oil) into hair follicles

• Found in most areas of the body except for this skin

• Prevents skin/hair from drying out and inhibits some bacterial growth

Eccrine sweat glands

• Secretes watery sweat through ducts that open to pores in the skin

• Found in most regions of the body

• Helps with thermoregulation as sweat evaporates from skin

Apocrine sweat glands

• Secrete a thicker sweat into hair follicles

• Become active after puberty and found mainly in the axilla, groin, and areolae

• Contents include lipids and proteins that bacteria feed off and gives off a musky scent (body odor)

Ceruminous glands

• Secrete cerumin (wax) into the ear anal (only)

• Provides a sticky barrier that impedes the entrance of foreign bodies and insects

• Waterproofs the ear canal and prevents the growth of bacteria and fungi

Nails

Plates of tightly packed, hard, dead, keratinocytes that form a clear, solid covering over the dorsal surface of the distal portions of the digits

Nail body (plate), free edge, and nail root

3 regions of the nail

Nail body (plate)

• Visible part, filled with harder keratin

• Contains lunula

Lunula

A crescent-shaped area of the proximal portion of the nail body that appear white-ish because it is the thicker, most actively growing region of the nail

Free edge

• Part of nail that extends past the distal end of the digit

• Contains hyponychium

Hyponychium

Thickened region of the epidermis that binds the nail to the tip of the digit

Nail root

Part that is not visible and is buried in a fold of the skin (eponychium or cuticle)

Nail matrix

Proximal to the nail root and contains cells that divide mitotically to produce new nail cells

Thickness, strength, flexibility, degree of keratinization, distribution and type of hair, density and types of glands, pigmentation, vascularity, innervation

Variations of skin throughout the entire body (despite the same basic structure)

Thermoregulation, blood reservoir, protection, cutaneous sensations, excretion and absorption, synthesis of vitamin D

Functions of the skin

Skin homeostatic regulation of body temperature

• Releases sweat onto the surface

• Adjusts the flow of blood in the dermis

How does releasing sweat help with thermoregulation

Evaporation of sweat from the skin helps lower body temperature

How does adjusting blood flow in the dermis help with thermoregulation

Blood vessels in the dermis can dilate allowing more blood to flow closer to the surface of the skin which increases the amount of heat loss through the skin OR in the cold vessels can constrict to decrease the amount of blood flow to decrease heat loss

Dermis houses an extensive network of blood vessels

8-10% of the total volume of blood in an adult

Keratin function in skin

Protects underlying tissues from microbes, abrasion, heat, and chemicals

Tightly packed cells of the epidermis act as a water barrier to

Prevent evaporation

What do macrophages in the epidermis do

Guard against bacteria and viruses

Cutaneous sensations

Touch, pressure, vibration, and thermal sensations that rely on special sensory receptors as well as free nerve ending

Papillary layer (superficial) cutaneous sensations

Free nerve endings - heat, pain, cold, tickle, itch
Meissner’s corpuscles - light touch

Reticular and subcutaneous layer(deep) cutaneous sensations

Pacinian corcupscles

Skin plays a small role in excretion

Small amounts of water is lost through sweat and evaporation while most substances are excreted through the digestive and urinary systems

Skin plays a role in absorption

• Water-soluble substances are not absorbed

• Lipid-soluble substances (fat-soluble vitamins, certain drugs, gases, some hormones) can be absorbed

Transdermal drugs

A drug contained in a skin patch is released continuously at a controlled rate, normally used for drugs that are quickly eliminated from the body

Skin synthesis of vitamin-D

Requires the activation of a precursor molecule in the skin by UV rays in sunlight then enzymes in the liver/kidneys modify the activated molecule producing calcitriol

Calcitriol

The most active form of vitamin-D, as a hormone that aids in absorption of calcium from foods in the gastrointestinal tract into the blood

Small amount of sun exposure is required for vitamin D synthesis

People who avoid sun exposure or live in northern climates need to supplement their vitamin D

Vitamin D also plays a large role in what?

Immunity, it helps activate cells that respond to infections (especially respiratory infections)

2 kinds of wound healing processes can occur

Epidermal wound healing or deep wound healing depending on the depth of the injury

Epidermal wound healing

Occurs following a superficial wound that only affects the epidermis then skin returns to normal function

Deep wound healing

Occurs when an injury extends to the dermis and subcutaneous layer which may result in loss of function and/or development of scar tissue

Epidermal wound healing

For abrasions or minor burns, basal cells of the epidermis enlarge and migrate across the would until the would is eventually sealed and the epidermis returns to normal

Phases of deep wound healing (involves bleeding)

1. Inflammatory phase - clot forms to unite the edges

2. Migratory phase - clot becomes a scab, fibroblasts migrate to the area and secrete collagen fibers to help bind the edges together

3. Proleferative phase - wxtensive growth of epithelial cells beneath any scab

4. Maturation phase - scab falls off once the epidermis has returned to normal thickness

Scar formation

Occurs due to extensive collagen build-up from fiberblasts

Skin burns

Tissue damage caused by excessive heat, electricity, radioactivity, or corrosive chemicals that denature the proteins in the skin; graded according to their severity

1st degree burns

Involves only the epidermis; Ex/ sunburn

2nd degree burn

Destroys the epidermis and part of the dermis; Ex/ blister

3rd degree burns

Full thickness burn that destroys the epidermis, dermis, and subcutaneous layer