THE INTEGUMENTARY SYSTEM

Skin accounts

 10−15% of an individual’s total body weight making it largest organ in body

cutaneous membrane has two…

Epidermis, Dermis

Epidermis 

superficial layer that consists of keratinized stratified squamous epithelium resting on a basement membrane

Dermis

deep to epidermis and basement membrane; consists of loose connective tissue and dense irregular connective tissue

Accessory structures of skin

 embedded in cutaneous membrane: sweat glands, sebaceous glands, hair, and nails

Skin contains

sensory receptors and arrector pili muscles

Epidermis is avascular:

Must rely on diffusion of oxygen and nutrients from blood vessels in deeper dermis; limits epidermal thickness

entirely

About 50% of cells in epidermis are too far from adequate blood supply to sustain life; superficial layers are made up … of dead cells

Hypodermis

also known as superficial fascia or subcutaneous fat, is deep to dermis

Although not part of skin, hypodermis does anchor skin to deeper structures like muscle and bone

Made of loose connective and
adipose tissues; has an
abundant blood supply

intengumentary, Protection

from mechanical trauma, pathogens, and environment is most obvious function:

Stratified squamous, keratinized epithelium provides a durable but flexible surface; protects body from mechanical trauma like stretching, pressure, or abrasions

Glands

secrete a variety of antimicrobial substances; sebaceous gland secretions give surface of skin a slightly acidic pH (called acid mantle); inhibits growth of many pathogens

Provides protection from a number of environmental hazards including absorption of ultraviolet light (UV) before it damages deeper tissues

Skin secretes

 hydrophobic lipid-based chemicals; repel ionic and polar covalent molecules like salt and water; critical for maintaining water and electrolyte homeostasis in a wide range of weather conditions

Sensation

 process that enables nervous system to perceive changes in the body’s internal or external surroundings; critical to homeostasis:

Skin has numerous sensory receptors or cellular structures that detect changes in internal and/or external environment

Receptors allow us to detect potentially harmful stimuli such as heat, cold, and pain; could lead to tissue damage

Thermoregulation

Process that relies on negative feedback loops for maintenance of a stable internal temperature 

Internal body temperature is determined mostly by muscle activity and many chemical reactions involved in metabolism

Sensory receptors (thermoreceptors)

in skin detect an increase in temperature in both skin itself and internal body fluids

Control center

 in hypothalamus of brain acts as a thermostat

 thermoregulatory center

 receives input from thermoreceptors; processes and then responds to sensory inputs

Control center stimulates sweating

 sweat glands are stimulated to release a watery fluid called sweat; water carries a great deal of heat with it when it evaporates; provides for an effective cooling mechanism

Control center stimulates cutaneous vasodilation

response triggered by hypothalamus; causes blood vessels in dermis to widen (dilate); increased blood flow through dilated vessels increases amount of heat radiated away from body into environment; cools body 

Body temperature returns to normal range and cooling mechanisms decline by negative feedback…

when thermoreceptors no longer sense body temperatures above normal range they stop sending signals to hypothalamus; ends control center responses; sweating and vasodilation ends

Thermoreceptors

detect body temperature drop below normal range; relay information to thermoregulatory center in hypothalamus

Vasoconstriction

redirects blood flow to deeper tissues

Excretion 

 process where waste products and toxins are eliminated from body; most occurs at other organs like kidneys; skin and its accessory structures make a small but significant contribution

 vitamin D synthesis

cells found deep in epidermis convert vitamin D from an inactive form (precursor) to active form

Precursor

 modified cholesterol molecule; converted to cholecalciferol when epidermis is exposed to UV radiation

liver, kidneys

Cholecalciferol is released into blood; modified first by … then by … to form calcitriol 

Epidermis

most superficial layer; composed of several cell types; most numerous are keratinocytes

Manufacture keratin

 tough fibrous protein that makes epidermis more resistant to mechanical trauma Linked to each other by desmosomes; makes epidermis stronger

Stratum basale – (stratum germinativum) 

single layer of stem cells resting on basement membrane; closest cells to blood supply in dermis; therefore most metabolically and mitotically active cells in epidermis; involved in vitamin D synthesis and replacement of dead keratinocytes (lost from more superficial layers)

Stratum spinosum

thickest layer, sits on top of stratum basale so still close to blood supply; also metabolically and mitotically active

1. Stratum granulosum 

1. Three to five layers of cells with prominent cytoplasmic granules; filled with keratin bundles or a lipid-based substance; both secreted by exocytosis

2. Hydrophobic nature of lipids provides waterproofing; critical for maintaining internal fluid and electrolyte homeostasis; also leads to isolation and death of cells in this layer and in more superficial layers

Stratum lucidum 

narrow layer of clear, dead keratinocytes; found only in thick skin

Stratum corneum

outermost layer of epidermis; consists of several layers of dead flattened keratinocytes with thickened plasma membranes; filled mostly with keratin bundles and little else; sloughed off or exfoliated mechanically as desmosomes holding neighboring cells together are lost

Keratinocyte life cycle

 location and functions of epidermis subjects it to both physical and environmental stress; stratum corneum is continuously shedding dead cells that must be replaced to maintain integrity of epidermis:

Dead keratinocytes are replaced by mitosis of cells in stratum basale and spinosum where blood supply is available for such activities

Dendritic (Langerhans) cells 

located in stratum spinosum; phagocytes of immune system; protect skin and deeper tissues from pathogens

Merkel cells

 oval cells scattered throughout stratum basale; sensory receptors associated with small neurons in dermis:

Detect light touch and discriminate shapes and textures

Found in large numbers in regions that are specialized for touch; fingertips, lips, and at base of hairs

Melanocytes 

 located in stratum basale; produce melanin; protein skin pigment ranging from orange-red to brown-black

Thick skin

about as thick as a paper towel, has all five epidermal layers and a very thick stratum corneum; does not have hair follicles but contains many sweat glands

 thin skin

Areas of body not subjected to as much mechanical stress are covered with…

Callus

additional layers of stratum corneum; form in either thick or thin skin in response to repetitive pressure

Dermis

 highly vascular layer deep to epidermis

Functions:

Provides blood supply for epidermis

Contains sensory receptors

Anchors epidermis in place

Dermis

 Composed of two distinct layers made up of two types of connective tissue

Papillary layer

Reticular layer

Papillary layer 

 thinner most superficial of two layers; composed of loose connective tissue:

Special collagen fibers are found in this layer at dermis-epidermal junction; extends into epidermal basement membrane to anchor epidermis to dermis

Dermal papillae

tiny projections found at surface of papillary layer where it comes into contact with epidermis:

Contain tiny blood vessels called capillaries arranged in loops; extend up into most superficial part of dermal papillae

Tactile (Meissner) corpuscles 

 also found in dermal papillae; sensory receptors that respond to light touch stimuli; more numerous in regions of body where sensation is a primary function; skin of fingertips, lips, face, and external genitalia

Reticular layer

 deep thicker layer that separates dermis from hypodermis; mostly dense irregular connective tissue that consists largely of irregularly arranged collagen bundles: 

Collagen bundles strengthen dermis and prevent traumatic injuries from damaging deeper tissues

Elastic fibers allow dermis to return to its original shape and size after stretching

Lamellated (Pacinian) corpuscles 

 found embedded within reticular layer; sensory receptors that respond mainly to changes in pressure and vibration associated with skin

Blood vessels, sweat glands, hairs, sebaceous glands, and adipose tissue are found in reticular layer

Skin markings 

small visible lines in epidermis created by interaction between dermis and epidermis; best seen in thick skin of palmar surfaces of hands and fingers and plantar surface of feet and toes

Dermal ridges

found in areas where dermal papillae are more prominent due to presence of thick collagen bundles

 epidermal ridges

Dermal ridges indent overlying epidermis to create…

fingerprint

Sweat pores open along these ridges and leave a thin film or … on things touched with fingers

tension or cleavage lines

Gaps found between collagen bundles in dermis create indentations in epidermis called…

flexure lines

In areas of body, such as surrounding joints, reticular layer is tightly anchored to deeper structures that create deep creases called …

melanin

Skin color – mostly determined by various amounts of orange-red to black protein pigment …

melanocytes

Melanin is produced by … in stratum basale of epidermis

tyrosine

Melanin is composed of two molecules of amino acid…

tyrosinase

chemically bonded by a series of reactions catalyzed by enzyme …

 melanosome

reactions occur in a stepwise fashion within a special vesicle called a ….

sunburns

People of all skin pigmentations can develop … and are at risk for skin cancers

released by exocytosis

Melanosomes migrate to ends of these arms where … absorbed or taken into cytoplasm of surrounding keratinocytes

excess

Individuals living in regions exposed to high amounts of UV radiation (such as Africa) may have developed darker skin to prevent … vitamin D production

enough

People in areas with less UV radiation (such as northern Europe) developed lighter skin so they could synthesize … vitamin D

Freckle

small area of increased pigmentation; resulting from increased melanin production in local spot

Mole or nevus

area of increased pigmentation; due to a local proliferation of melanocytes, not an increase in melanin production

Albinism

melanocytes fail to manufacture tyrosinase; results in lack of skin pigmentation and greatly increased risk of keratinocyte DNA damage from UV radiation

Carotene

 yellow-orange pigment found in food items such as egg yolks and orange vegetables

Lipid-soluble molecule that accumulates in stratum corneum

Imparts a slight yellow-orange color that is particularly visible in stratum corneum of thick skin

Hemoglobin

found in red blood cells, is an iron-containing protein that binds to and transports oxygen throughout body:

Oxygen binds to iron in hemoglobin in an oxidation reaction; (same reaction that causes iron to rust); oxidized hemoglobin changes color to a bright orange-red; gives blood its characteristic color

indirect

Hemoglobin’s effect on skin color is an … result of blood flow in dermis; color of blood in deeper dermis is visible through epidermis

Erythema

 occurs when blood flow in dermis increases causing a color change that makes skin more reddish

cause erythema include:

 trauma, fever, and infection

Pallor

occurs when blood flow in dermis decreases; results in loss of normal pinkish hue; most visible in pale-skinned individuals; epidermis may take on whitish color of collagen in dermis

Can also occur when nervous and endocrine systems alter blood flow to dermis as part of a flight or fight response

Cyanosis

 sign that someone needs immediate attention; occurs when hemoglobin has very low levels of bound oxygen; blood turns reddish purple; skin takes on a faint bluish hue; can occur when 

Someone has difficulty breathing

Hemoglobin or red cell levels are low in blood

Hemoglobin is unable to bind to oxygen

Accessory structures or appendages

 of integument include hair, nails, and glands; derived from epithelium only; assist in overall function of integumentary system

Hair (pili)

small filamentous structures that protrude from surface of skin over entire body except in regions with thick skin, lips, and parts of external genitalia

Hair

Does provide protection by preventing substances and organisms from external environment from entering eyes and nose

On head, protects underlying skin of scalp from UV radiation and mechanical trauma

Hairs are associated with a small sensory neuron; plays a role in detecting changes in environment

Hair

composed of two main parts; shaft and root; both made up of stratified squamous keratinized epithelial cells in various stages of development

Shaft

Portion of hair that projects from skin’s surface

Made up of columns of dead keratinized epithelial cells that have completed keratinization process

Root 

Segment of hair embedded in dermis; surrounded by a small sensory neuron

hair papilla

Root is indented at its base by a projection of blood vessels from dermis called a …

hair bulb

Root and hair papilla are collectively known as …

Matrix 

small number of keratinocytes found at base of root; actively divide

hair follicle

Root is embedded in …

epithelial root sheath

an infolding of epidermis called …

Strand of hair has three visible regions in a transverse section:

Inner medulla, Middle cortex , Outermost cuticle 

Inner medulla

soft core only found in thick hair (like on head); composed of a soft keratin

Middle cortex

(hair) highly structured and organized with several layers of keratinocytes
containing hard keratin; provides strength to strand

Outermost cuticle

consists of a single layer of overlapping keratinocytes containing hard keratin; provides mechanical strength

dermal root sheath

Surrounding epithelial root is a …; consists of connective tissue; supports follicle and keeps it separate from dermis

arrector pili muscles

Small bands of smooth muscle called … attach to dermal
root sheath on one end and dermal papillary layer on the other

Contraction of these tiny muscles causes hair to stand up (piloerection); gives skin
a dimpled appearance, commonly called “goosebumps”

Hair growth

averages between 1−1.5 cm per month; varies between individuals; growth is not continuous but occurs in a cycle with following two main phases growth stage, resting stage

growth stage

 mitosis occurs in matrix:

Cells divide and push cells above them farther away from blood supply where they keratinize and die

Stage varies in duration from a month to as long as six years; depends on location of hair

resting stage

mitosis in matrix ends as cells die:

Follicle shortens and hair is pushed toward surface where it remains dormant for a month or two

Falls out on its own or is pushed out by a new hair in growth stage

Nails

hard accessory structures that are located at ends of digits; composed of stratified squamous epithelium filled with hard keratin

Nail plate

most visible component of nail, sits on top of an underlying epidermal nail bed; divided into: Nail body, Nail root, nail matrix

Nail body

 visible portion
of nail plate

Nail root 

 portion of plate
that lies under skin

nail matrix

containing
actively dividing cells is
found

Proximal nail fold

on proximal edge covering nail root

eponychium (cuticle)

distal edge of this fold is called the …consists of only stratum corneum

Medial and lateral nail folds

on medial and lateral edges of nail plate respectively