Integumentary System

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Introduction

integument = skin (epidermis + dermis)

epidermis - protection layer

stratified squamous epithelia cells

pain receptors

avascular

melanocytes - melanin

keratin

immune cells

function is protection

dermis - functional layer

two layers of connective tissue:

papillary (loose)

reticular (dense irregular)

collagen

elastin

fibroblasts

vascular (contains blood vessels)

sensory (touch) receptors

nerves

lymphatics

hypodermis - connection layer

fatty layer, not technically part of this system

two types of connective tissue:

adipose

areolar

connects integument to organs and muscles

integumentary system also includes nails, glands (sweat and sebaceous), and hair

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Levels of Organization

molecular level - keratin, melanin, vitamin D

microscopic level - stem cells and skin cells

tissue level - epithelial and connective tissue

organ level - skin, consisting of epidermis, dermis, hypodermis, as well as hair, nails, and glands

Molecular Level

keratin - group of fibrous proteins that give hair, skin, and nails their tough, water-resistant properties

secondary structure - alpha-helix

Two alpha-helix structures wind around each other to form a coiled coil structure

protofilament - pair of coiled coils

filament - four right-hand twisted protofibrils

melanin - photopigment with molecular structure that allows it to absorb UV radiation

transforms radiation into heat

prevents DNA damage from sun

gives skin, hair, and eyes their color

produced by melanocytes inside their special vesicles called melanosomes

albinism - inability of melanocytes to produce melanin

vitamin D

epidermis synthesizes precursor to vitamin D when exposed to UV radiation

cholesterol in skin produces cholecalciferol, an isomer of vitamin D3

liver converts cholecalciferol into calcidiol

kidneys convert calcidiol into calcitriol (active form of vitamin D)

vitamin D is actually a hormone - essential for absorption of calcium and phosphorus

essential for immunity against bacterial, viral, and fungal infections

link between insufficient vitamin D and cancer

insufficient vitamin D can lead to rickets in children (osteomalacia in adults)

Microscopic Level

epidermis (epithelial layer)

4-5 layers of stratified squamous epithelia

keratinocytes

top layers are made up of keratinocytes (cells containing keratin)

most superficial layer is composed of dead keratinocytes and periodically sloughs away

as keratinocytes move superficially, they lose cytoplasm and become flattened

basal cells - tissue-specific stem cells (can turn into a variety of cell types found in the tissue)

daughter basal cells most commonly replace lost keratinocytes

desmosomes (projections from epithelial cells in bottom layer of epidermis) and dermal papillae (projections from cells in the dermis) interlock with each other (like Velcro) to strengthen the bond between the epidermis and the dermis

Merkel cells - sensory receptors that detect light touch; form synaptic connections with sensory nerves (found in abundance on hands and feet)

melanocytes - found in bottom layer of epidermis and produce melanin; cellular extensions of the melanocytes reach up in between keratinocytes

dendritic (Langerhans) cells - tissue macrophages that engulf foreign organisms and signal the immune system

eczema - allergic reaction caused by overactive immune cells in skin

cancer - result of DNA damage that accumulates in a cell over time

UV rays can lead to mutations in genomes of various skin cells

an accumulation of mutations can trigger cells to grow out of control

normal signals that control cell division is lost and overgrowth of cells form a tumor

benign tumors stay in one place

metastasize - some tumors can produce cells that dislodge and establish tumors in other areas of the body

types of skin cancer

basal cell carcinoma - caused by mutations in basal cells

most common form of cancer

starts as an uneven path, bump, growth, or scar on the skin surface

responds best to treatment (complete surgical excision of the lesion) when caught early

squamous cell carcinoma - affects keratinocytes

lesions are usually scaly and red and are most common on the scalp, ears, and hands

can metastasize if not removed

surgery and radiation are used to cure it

melanoma - affects melanocytes

most fatal of skin cancers because it is highly metastatic and difficult to detect before it has spread

usually asymmetrical brown and black patches with uneven borders and a raised surface

treatment is surgical excision and immunotherapy

dermis - composed of connective tissue

collagen and elastin provide structure and elasticity

Pigmentation

skin has different types of melanin, including:

pheomelanin - reddish

eumelanin - dark brown

sun exposure stimulates keratinocytes to secrete a peptide hormone that stimulates melanocytes into producing melanin -> melanocytes produce and transfer melanin to keratinocytes

increased melanin protects skin's DNA from UV ray damage

melanosomes - cellular organelles that contain melanin are temporary structures that are eventually destroyed by fusion with lysosomes (this is why tanning is impermanent)

freckles - caused by uneven distribution of melanocytes in the skin

Organ Level

epidermis has 4 - 5 distinct layers (depending on region of the body):

stratum basale (stratum germinativum)

contains primarily a single layer of basal cells, but also contains melanocytes, Langerhans cells, and Merkel cells (upper levels contain mostly--or completely--keratinocytes)

primary function is to divide and replicate cells that are lost from the surface

basal cells form desmosomes (cell-to-cell junctions) between themselves and the underlying dermis

fingerprints are the result of basal cells dividing at different rates during embryonic development (forming deep ridges that get transmitted to the upper layers of skin)

stratum spinosum - name is due to spiny appearance of polyhedral-shaped cells and desmosomes

8-10 layers of keratinocytes

Langerhans cells interspersed throughout to help with immunity

stratum granulosum - named for grainy appearance due to changes in keratinocytes

3-5 layers of keratinocytes

become flatter and membranes thicken

these keratinocytes generate large amounts of keratin and keratohyalin proteins

these proteins, along with other lipids and enzymes, form vesicles called lamellar granules, which may be secreted by exocytosis

these secretions retard water loss and entry of foreign materials

the proteins eventually make up the entire mass of the keratinocytes (nuclei and other cell organelles disintegrate) -- this marks the transition between metabolically active strata below and dead strata above

stratum lucidum - unique to palms and soles where skin is thicker

appears lucid (clear)

smooth, clear, thin layer

flat and densely packed layer

stratum corneum - most superficial layer of epidermis; gets name from increased keratinization (also called "cornification") of cells in this layer

15-30 layers of dead cells

dry, dead layer prevents growth of microbes

resistant to penetration by water

protects inner layers from environmental damage

dead cells are shed periodically (approx. every two weeks)

thick skin vs. thin skin

thick skin has substantially more layers of cells than thin skin (more than 300 versus 15 in the stratum corneum)

thick skin found on palms of hands, soles of feet, and lips and is adapted to specialized activities that have more wear and tear

thick skin doesn't have hair and has few glands

calluses - occur because basal stem cells are triggered to divide more often (especially when skin is subject to constant mechanical stress)

corns - specialized form of calluses due to abrasion on skin in an elliptical-type motion

Dermis

two distinct layers of connective tissue

papillary layer - areolar (loose) connective tissue

fibroblasts dispersed within collagen and elastin fibers forms a loose mat containing an abundance of small blood vessels

phagocytes

lymph vessels

sensory receptors

reticular layer - dense irregular connective tissue; "reticulated" means net-like

mesh of collagen fibers and elastin fibers give it a reticulated appearance

fibrocytes form the bundles of collagen that extend into the papillary layer and hypodermis

collagen binds with water which keeps the skin hydrated

turgor - elasticity and resiliency of the skin

dehydration causes loss of turgor (skin remains domed if pinched instead of immediately flattening out)

collagen injections (introduces collagen back into the skin) and Retin-A creams (stimulate blood flow and repair) help restore skin turgor

vasculated and innervated

fibroblasts - produce collagen and elastin fibers in the ECM

mast cells - involved in allergic reactions

Hypodermis (subcutis or subcutaneous layer)

made up of areolar tissue, collagen, and elastic fibers but also adipose tissue

adipose tissue accumulates fat, serving as an energy reserve and insulation to prevent heat loss

vascular

Accessory Structures

Hair

hair follicle - downward extension of epithelial cells into the dermis

hair forms in similar process as skin (division and differentiation of basal cells into keratinocytes that get pushed up and become flattened, dead, keratinized cells)

hair shaft - part of hair exposed above surface of skin

hair root - part of hair held within the skin

hair bulb - bulge at base of hair root

hair matrix - layer of basal cells that makes up the hair bulb

melanocytes in the hair matrix produce melanin

melanin production decreases with age and hair tends to lose its color

hair papilla - made up of connective tissue, blood capillaries, and nerve endings; surrounded by the hair bulb

basal cells in center of hair bulb form layers of keratinocytes:

medulla

cortex

cuticle

external hair shaft composed entirely of keratin

hair follicle has three layers:

internal root sheath - derived from basal cells in the hair matrix

external root sheath - made up of basal cells at the base of the hair root

glassy membrane - thick, clear connective tissue sheath covering hair root and connecting it to dermis tissue

arrector pili - smooth muscle attached to hair root that contract in response to stimuli (make hair stand up)

phases of hair growth:

anagen - hair grows during this phase

catagen - follicle becomes inactive

telogen - follicle becomes smaller and becomes detached from dermal papilla at the base

return to anagen - basal cells produce a new hair follicle

alopecia - hair thinning or hair loss

often experienced in advanced age

most often caused by genetic sensitivity of hair follicles to the androgen hormone dihydrotestosterone (DHT)

this causes hair follicle to receive less blood flow so hair follicles begin to atrophy and hair produced is thin

male pattern baldness - results from mutation on the X chromosome

Nails

nail body - main portion of the nail; is formed on the nail bed

composed of keratin-rich, densely packed dead keratinocytes

nail bed is rich in blood vessels (making it appear pink), except at the base where it has a crescent-shaped region called the lunula

nail root - where the nail body forms

nail folds - overlap the nail on each side and help anchor the nail body

nail cuticle (eponychium) - forms on the superficial surface of the nail body on the nail fold

free edge - part of nail not attached to skin

hyponychium - area beneath free edge of nail

Skin Glands and Secretions

Sweat glands

Two types:

eccrine (merocrine) sweat glands - present throughout skin, especially palms of hands, soles of feet, and forehead

derived from epidermis

coiled structure located in dermis

duct opens to a pore on the skin surface, where sweat is released

some may open into hair follicles, like sebaceous glands

composed mainly of water, but also some salt, antibodies, metabolic waste, and dermcidin (microbe-killing compound)

main function is to help regulate body temp through evaporation

controlled by sympathetic (fight or flight) division of autonomic nervous system

apocrine sweat glands - usually associated with hair follicles in densely hairy areas (armpits and genitals)

larger than merocrine glands and lie deeper in the dermis (sometimes even the hypodermis)

release a thicker fluid with higher concentration of fatty acids (sometimes leading to a whitish color)

fats are decomposed by bacteria on the skin, which can result in body odor

don't function until puberty

stimulated during emotional stress and sexual excitement

Sebaceous glands - oil glands

found all over body

most are associated with hair follicles

sebum - mixture of lipids generated and excreted by sebaceous glands onto hair and skin surface

provide lubrication to dry, dead layer of keratinized cells

has antibacterial properties

prevents water loss

stimulated by hormones, many of which don't become active until puberty

Acne - skin disturbance typically on areas of skin rich in sebaceous glands (face and back)

hormones (such as androgen) stimulate release of sebum

overproduction of sebum can lead glands to get blocked, which can result in formation of blackheads

blackheads - result of hyperkeratinization, leading to formation of a keratin plug and blockage of hair follicles in the area

prone to infection by acne-causing bacteria, which leads to redness and swelling

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Homeostasis

psoriasis - autoimmune disorder where too many skin cells are produced

skin rapidly accumulates and looks silvery-white in appearance and forms plaques

chronic recurring condition

wound repair and scarring

first, a blood clot is formed that scabs over time

fibroblasts and mesenchymal cells mobilize and divide rapidly to repair damaged tissue

this forms a loose, highly vascular tissue called granulation tissue

granulation tissue helps deliver oxygen and nutrients necessary for further repair

immune cells roam the area and engulf any foreign matter (reducing chance of infection and clearing tissue debris)

scars - occur when skin damage is repaired but the healing process prevents regeneration of the original skin structure

fibroblasts often generate greater density of collagen fibers than were in the original tissue

the dense fibrous structure of this connective tissue has few cells and doesn't allow for regeneration of accessory structures

keloid - raised or hypertrophic scar formed when scarring does not stop when wound is healed

atrophic scars - scars from acne and chickenpox that have sunken appearance

burns - wounds caused by exposure to intense heat, radiation, electrical current, or caustic chemicals

causes skin cells to die, resulting in disruption of epithelial barrier that prevents fluid loss

can result in dehydration, electrolyte imbalance, renal or circulatory failure

treated with intravenous fluids to offset dehydration and intravenous nutrients that enable body to repair tissues and replenish lost proteins

high possibility of infection due to loss of protective barrier

classified by severity:

first-degree burns - only epidermis is damaged

heal on their own in a few days

also includes the typical sunburn

second-degree burns - damage to epidermis and dermis

results in blistering of skin

area must be kept clean and sterile to prevent infection

burns heal on their own within a few weeks

third-degree burns - penetrate the full thickness of the skin, including the subcutaneous layer

skin may appear white, red, or black

fourth-degree burns - most sever, affecting underlying muscle and bone as well

third- and fourth-degree burns are usually not as painful because the nerve endings are damaged

but these levels of burn result in rapid water loss and infection due to exposure of underlying tissue and require emergency trauma care

these types of burns cannot be repaired locally because the local repair machinery is damaged

require excision of affected tissue (or amputation in severe cases) followed by skin grafting from an unaffected part of the body or from skin grown in a tissue culture for grafting purposes

aging

basal cells divide less reliably and aging skin becomes thinner

dermis weakens as elastin fibers become cross-linked and don't stretch

wounds repair more slowly

hypodermis loses structure due to reduction and redistribution of fat (due to changes in hormonal levels)

accessory structures have lowered activity

thinner nails and hair

reduced amounts of sebum and sweat

reduced sweating leads to intolerance of extreme heat

less active melanocytes - leads to paler appearance

less active dendritic cells - lowered immunity

wrinkling of skin due to breakdown of collagen and elastin, weakening of muscles under skin, and inability of skin to retain adequate moisture

thermoregulation

sensible perspiration - sweat glands excrete water, salt, and other substances to cool the body

blood vessels dilate (leads to redness of skin) and sweat evaporates

hypodermis adipose tissue helps insulate the body

arrector pili muscles help insulate body by trapping air on surface of skin

dermal blood vessels constrict to decrease blood flow

sense of touch

skin has sensory receptors that communicate with central nervous system (CNS)

Merkel discs - more concentrated in fingertips and lips (areas most sensitive to touch)

tactile (Meissner's) corpuscles - respond to pressure

lamellated (Pacinian) corpuscles - respond to vibration

nerve endings sensitive to temperature and pain

vitiligo - melanocytes in certain areas lose ability to produce melanin (possibly due to autoimmune reaction)

jaundice - yellowed skin caused by liver disease or liver cancer -> accumulation of bile and the yellow pigment bilirubin

Addison's disease - excessive corticotropin-releasing hormone (CRH) -> stimulates excessive levels of melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) -> leads to deep bronzing of skin

sudden drop in oxygenation can turn skin ashen

cyanosis - blue-colored skin resulting from prolonged reduction in oxygen levels (most obvious where capillaries are close to surface--lips and nail beds)

elevated blood levels of carbon monoxide can causes lips to appear bright cherry red