Ch 5 study guide: Integumentary System: Skin Structure and Function

• Skin, or the cutaneous membrane, is the largest organ and integral for homeostasis
• Cutaneous membrane has two main components: the epidermis and the dermis
• Accessory structures include sweat glands, sebaceous glands, hair and nails

Epidermis

• The epidermis is the superficial layer made of keratinized stratified squamous epithelium
• The dermis is deep to the epidermis and consists of dense irregular connective tissues
• The avascular epidermis relies on diffusion of oxygen and nutrients from blood vessels in the dermis
• 50% of epidermal cells are too far from blood supply, thus superficial layers contain only dead cells

Hypodermis

• Also known as the superficial fascia or subcutaneous fat, lies deep to the dermis
• It anchors skin to deeper structures, made of loose connective and adipose tissues with abundant blood supply

Functions of the Integumentary System

• Protection from mechanical trauma, pathogens, and the environment
• Keratinized stratified squamous epithelium acts as a barrier
• Glands create a slightly acidic pH inhibiting pathogen growth
• Skin protects from UV light and repels ionic and polar covalent molecules for fluid and electrolyte homeostasis
• Sensation is enabled by the nervous system to perceive internal or external changes
• Sensory receptors detect changes like heat, cold, and pain
• Thermoregulation relies on negative feedback loops for stable internal temperature determined by muscle activity and metabolism

Temperature Regulation

• Sensory thermoreceptors detect temperature increase in skin and body fluids
• The hypothalamus acts as a thermostat, processing thermoreceptor input and responding
• Sweat glands are stimulated to release sweat, using water's heat-carrying capacity for effective cooling
• Cutaneous vasodilation widens blood vessels in the dermis, increasing heat radiation
• When body temperature returns, thermoreceptors stop signaling the hypothalamus, ending control center responses
• Thermoreceptors detect temperature drops and relay info to the hypothalamus
• Blood vessels in the dermis constrict decreasing blood flow, limiting heat loss
• Vasoconstriction redirects blood flow to deeper tissues to conserve heat
• When body temperature returns, thermoreceptors stop signaling the hypothalamus, ending heat conservation responses

Other functions

• Excretion eliminates waste and toxins, with skin and accessory structures making a small contribution
• Skin is vital for vitamin D synthesis, converting cholecalciferol to calcitriol for calcium ion absorption
• Calcium ions are critical for nerve function, muscle contraction, bone maintenance, and other functions

Keratinocytes

• Keratinocytes make up 95% of the epidermis
• These cells manufacture keratin, a tough fibrous protein making the epidermis resistant to mechanical trauma
• They are linked by desmosomes making the epidermis stronger

Layers of the Epidermis

• Organized into five structurally distinct strata: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum
• The stratum basale (germinativum) is a single layer of stem cells on the basement membrane
• These are metabolically and mitotically active cells involved in vitamin D synthesis and dead keratinocyte replacement
• The stratum spinosum is the thickest layer on top of the stratum basale and remains metabolically and mitotically active
• The stratum granulosum has three to five layers of cells with cytoplasmic granules filled with keratin bundles or a lipid-based substance
• Lipids secreted by exocytosis provide waterproofing for maintaining internal fluid and electrolyte homeostasis
• The stratum lucidum is a narrow layer of clear dead keratinocytes found only in thick skin
• The stratum corneum is the outermost layer with several layers of dead flattened keratinocytes filled with keratin bundles
• Stratum corneum cells are sloughed off as desmosomes are lost

Keratinocyte Life Cycle

• The epidermis must maintain integrity due to physical and environmental stresses
• The stratum corneum continuously sheds dead cells which are replaced from deeper layers
• The migration from deepest to superficial layers takes 40–50 days

Other Cells of the Epidermis

• Dendritic (Langerhans) cells located in the stratum spinosum are phagocytes that protect from pathogens
• Merkel cells are scattered throughout the stratum basale and detect light touch and discriminate shapes/textures
• Melanocytes are located in the stratum basale, produce melanin, a protein skin pigment

Thick and Thin Skin

• Thick skin is on the palm of the hand and sole of the foot
• Thick skin has all five epidermal layers and a very thick stratum corneum but no hair follicles, but many sweat glands
• Thin skin which covers the rest of the body, only has four layers (lacks stratum lucidum) has numerous hairs, sweat glands, and sebaceous glands

The Dermis

• Vascular layer deep to the epidermis
• it provides the blood supply for the epidermis, contains sensory receptors, and anchors the epidermis in place
• The dermis is composed of two distinct layers made up of two types of connective tissue

Papillary Layer

• Thinner most superficial of the 2 layers
• Made of loose connective tissue
• Collagen fibers at the dermis-epidermal junction anchor the epidermis
• Dermal papillae are tiny projections where the papillary layer contacts the epidermis
• Loops in the dermal pupillae allow oxygen and nutrients to diffuse to the avascular epidermis
• Tactile (Meissner) corpuscles are sensory receptors responding to light touch stimuli in areas like fingertips, lips, face, and external genitalia

Reticular Layer

• the deep thicker layer that separates the dermis from the hypodermis
• Mostly of dense irregular connective tissue that consists largely of irregularly arranged collagen bundles as well as elastic fibers, proteoglycans, and lamellated (Pacinian) corpuscles
• Collagen bundles strengthen the dermis and prevent traumatic injuries from damaging deeper tissues
• Elastic fibers allow the dermis to return to its shape after stretching
• Proteoglycans draw water to keep the skin firm and hydrated
• Lamellated corpuscles respond to changes in pressure and vibration
• Blood vessels, sweat/sebaceous glands, hairs, and adipose tissue are found embedded in the reticular layer

Skin Markings

• Visible lines created by the dermis and epidermis interaction
• Visible in the thick skin of the palms/fingers as well as the feet/toes
• Dermal ridges are found where dermal papillae are more prominent due to collagen bundles
• Dermal ridges indent the overlying epidermis creating epidermal ridges for gripping
• The reticular layer is also responsible for skin markings

Skin Pigmentation

• Mostly determined by melanin in the stratum basale of the epidermis
• Melanin protects keratinocyte DNA from mutations caused by UV radiation
• Melanocytes form contacts with keratinocytes in the stratum basale and spinosum

Melanin

• Melanin synthesis increases with exposure to UV radiation leading to tanning
• Immediate response to UV radiation is the oxidation of melanin causing it to darken
• The delayed effect of UV radiation is DNA damage in melanocytes, stimulating melanin production
• Secondary function of melanin is to reduce vitamin D synthesis in response to UV radiation
• Skin color depends on the number of melanocytes
• A freckle is small area of increased pigmentation due to a local increase in melanin production
• A mole/nevus is increased pigmentation due to local proliferation of melanocytes

Other Pigments Affecting Skin Color

• Carotene, a yellow-orange pigment in food items, accumulates in the stratum corneum
• Hemoglobin, an iron-containing protein in red blood cells, binds to and transports oxygen
• Oxygen binds to the iron found in hemoglobin in an oxidation reaction
• The oxidation reaction is the same reaction that causes iron to rust forming a bright orange-red which give blood its color
• Hemoglobin's affect on skin color depends on blood flow in the dermis

Skin Color as a Diagnostic Tool

• Color changes from blood flow in the dermis can be useful in the diagnosis of disease
• Erythema occurs when blood flow in the dermis increases, causing a reddish color change
• Erythema can occur in response to exercise, trauma, fever, or infection
• Pallor occurs when blood flow in the dermis decreases
• Pallor can occur during heat conservation, fight or flight response, fear, or a severe infection
• Cyanosis occurs from a loss of hemoglobin and less oxidized iron
• Cyanosis needs immediate attention can occur when someone has difficulty breathing, has low levels of hemoglobin or when hemoglobin is unable to bind to oxygen

Accessory Structures of the Integument

• Hair, nails, and glands are derived from epithelium and assist in the overall function of the integumentary system

Hair

• Also known as pili are filamentous structures which protrude from the skins surface except the thick skin, lips, and external genatalia.
• Hair provides protection by protecting substances and organisms from entering the eyes and nose
• Hair on the head protects from UV radiation and mechanical trauma
• Associated with sensory neurons, playing a role in detecting environmental changes

Hair Structure

• Composed of the shaft and the root, both made of keratinized stratified squamous epithelial cells
• The shaft projects from the skin's surface, made of dead keratinized epithelial cells
• Root embedded in the dermis and is surrounded by small sensory neurons. The root is indented at its base projecting blood vessels from the dermis called a hair papilla
• The root and the hair papilla are the hair bulb
• Many epithelial cells in the root are alive and not completed keratinization. A small number of keratinocytes are the matrix found at the base of the root

More Hair Structure

• The root is embedded in the hair follicle which is an infolding of the epidermis called the epithelial root sheath which extends into the dermis/hypodermis
• The epithelial root sheath has an outer component that anchors the follicle to the dermis and an inner component that is anchored tightly to the hair root
• A strand of hair has three regions as seen in a transverse section: the inner medulla, the middle cortex, and the outermost cuticle
• Small bands of smooth muscle called arrector pili muscles attach to the dermal root sheath on one end and the dermal papillary layer on the other.
• The contraction of arrector pili muscles causes the hair to stand up, a process called piloerection, which gives the skin a dimpled appearance

Hair Growth

• Averages 1-1.5 cm per month varies between individuals. Growth occurs in a cycle with the growth stage and the resting stage
• In the growth stage, mitosis occurs in the matrix which divide and push old cells away from the blood supply where keratinization and death occur. This stage varies from a month to as long as six years
• In resting stage, mitosis in the matrix ends and the follicle shortens pushing the hair to the surface remaining dormant for months
• The hair either falls out or is pushed out by a new hair

Types of Hair

• Lanugo is a thin, nonpigmented hair found covering a fetus.
• Lanugo typically falls around birth and are replaced with terminal/vellus hairs
• Terminal hairs are thick, coarse, and pigmented found surrounding the eyes and scalp
• Vellus hairs are thinner, nonpigmented hairs
• Most of the vellus hair is replaced after puberty determined by gender
• Hair color is largely determined by the melanin produced in the matrix by melanocytes
• Less melanin is produced while aging causing hair to be gray/white

Nails

• Hard accessory structures on the end of digits with stratified squamous epithelium filled with hard keratin
• The nail plate is the most visible, and it sits on an underlying epidermal nail bed
• The nail plate is divided into the nail body and the nail root at the nail matrix
• Nail growth occurs at the nail matrix where the active cells push keratinocytes distally
• Fingernails grow .5mm per week while toenails grow more slowly
• Nails do not contain melanin so are translucent except at the lunula: a half-moon shaped region of the proximal nail plate that accumulates keratin
• Functionally, nails protect the underlying tissue, distal of the fingers and toes, from trauma and enabling more precise gripping

Glands

• Both are derived from epithelial cells in the epidermis but are located in the dermis
• The two basic types of glands are sweat (sudoriferous) glands that produce sweat and sebaceous glands that produce oily sebum
• Four types of sweat glands that differ structurally and in the products they secrete
• Each gland secretes products from secretory glands by exocytosis (merocrine secretion)

Sweat Glands

• Eccrine sweat glands are the most prevalent type and coiled tubular glands found in the dermis.
• Sweat which contains mostly water, waste products, exits through the duct through a sweat pore onto the epidermal surface.
• The primary function is thermoregulation
• Apocrine sweat glands are specific to the axillae, anal area, and areola
• These are large glands which release a protein-rich secretion into a hair follicle (secretions become odiferous once skin bacteria metabolize their contents and are influenced by sex hormones and become active after puberty)
• Ceruminous glands are modified apocrine glands that release cerumen (ear wax) into hair follicles found in the ears.
• Cerumen traps incoming particles in the tube leading to the tympanic membrane and lubricates it
• Mammary glands are highly specialized sweat glands that produce a modified sweat product, milk

Sebaceous Glands

• Sebaceous glands empty into a hair follicle or a small pore where they secrete sebum
• Sebaceous glands are located everywhere except the palms and soles with the greatest number found on the face and scalp.
• Sebum is a waxy, oily mixture of mostly lipids, is released by holocrine secretion where secretory cells accumulate sebum until the cell ruptures
• Sebum contains cellular fragments, debris, and lipids which coat hair and provide it with a hydrophobic barrier to inhibit water loss
• Sebum inhibits/kills bacteria