10.1 - Skin Structure Flashcards
what is the skin also known as
the integumentary system
what are the 3 main layers of the skin
epidermis
* outer layer (most superficial)
* approx 3mm
* derived from ectoderm
* has 4 layers called keratinocytes
dermis
* regular, dense, connective tissue + collagen + elastin
* find many structures here, eg sweat glands
* important for flexibility and strength in the ksin
* 0.6mm - 1cm
* derived from ectoderm
hypodermis
* below dermis
* aka ‘superficial fascia’
* majority of adipose tissue is found here
* highly vascularised
* derived from mesoderm
what is the cutis
epidermis and dermis
☞ derived from ectoderm
hypodermis is not in this, as it is derived from mesoderm
the hypodermis
features (functions on different cards)
- innermost layer of skin
- aka superficial fascia
- derived from mesoderm (therefore not considered part of skin)
- majority of adipose tissue found here: storing energy
- thickness varies in different regions + different individuals → depends on amount of fat
- varies considerably between m+f (different card)
- highly vascularised (some neurovascular bundles and lymphatics)
- loose connective tissue - fibroblasts, macrophages and fibres
- thinning of this layer during aging is responsible for wrinkling and hypothermia
functions of hypodermis
(features on diff card)
- energy store - generates heat
- insulator for underlying muscle heat generation
- shock absorber - cushions impact, and acts as protection for underlying structures
- connects skin to underlying muscle and bones
- hormones - eg produces leptin to control eating habits
differences in thickness of hypodermis between males and females
men
* thickest in abdomen and shoulders
* abdomen - white adipose tissue for energy storage
* shoulders - brown adipose tissue
women
* thickest in hips, thighs and buttocks
* this is where white adipose tissue is found
☞ for both, relatively thick on the palms of the hands and feet
features of the dermis
- between the epidermis and hypodermis
- aka corium
- with epidermis collectively = cutis
- variable thickness 0.6-3.0 mm
- contains hair and sweat glands, and blood vessels and nerves
Three layers
☞ papillary dermis: upper, paler part. Less collagen and elastin.
☞ dermal papillae: interdigitating parts of papillary dermis
☞ reticular dermis: lower, darker, part. Dense collagen and elastin.
what are the functions of the dermis
- contains hair and sweat glands for thermoregulation
- contains sensory structures - special senses - touch
- gives structure to skin and therefore body shape
features of the epidermis
- outermost layer made of epithelial cells (keratinocytes)
- four layers of cells (thin skin) or five (thick skin) ☞ on different card
- held together laterally by adherens junctions
- held together basal-apical by desmosomes
- some terminal nerve endings
- no blood vessels - all nutrients come through dermis
- rete ridges sometimes present on keratin layer (forms fingerprints)
functions fo the epidermis
- prevents water loss (layer of lipid)
- prevents entry to bacteria and parasites
- special cells that present pathogens to immune cells
- synthesis of keratin (keratin layer on surface that can be lost)
- prevents underlying tissue loss due to abrasion
what are the four/five layers of the epidermis
stratum corneum/corny/horny layer
* outermost layer made of squames
* thick on palms and soles of feet (prone to injury)
* continously shed
stratum lucidium/clear layer
* only found on palms and soles of feet
stratum granulosum/granular layer
* stratified squamous epithelium
* very darkly stained
* lamellar granules (filament associated proteins that assemble keratin fibrils and release it)
* tonofibrils (bundles of keratin filaments and keratohyalin granules made by lamellar bodies)
* golgi apparatus appears increased in this layer
stratum spinosum/spinous/prickle cell
* cuboidal epithelium in 3 layers held together by desmosomes
* producers of lamellar bodies (keratohyalin factories and lipid production)
stratum basale/basal cell layer
* tall columnar epithelial
* constantly renew keratinocytes by cell division
* as daughter cells differentiate they move away from epidermis-dermis junction, become stratum spinosum
* these make keratin filaments (tonofilaments) - lose ability to divide
* melanocytes (produce melanin) found here
keratinocytes
- synthesise keratins (fibrous protiens)
- keratins contribute to strength of epidermis
- keratins are main constituents of hair and nail
- normal transit time of a keratinocyte from basal layer to stratum corneum in the epidermis is 28-40 days
- this transit time is massively reduced in psoriasis
- hyperkeratosis: too much keratin being put on the surface of the skin
keratin synthesis pathway
- starts at basal cells, already making keratin
- here, they are called tonofilaments (intermediate keratin filaments)
- there are different types of keratin made in different cells in the body, and not all cells can
- as the intermediate keratin filaments divide, they push cells upward ☞ daughter cell become spinous cell and then granular cell
- its at this point, lamellar bodies are produced (contain a lot of lipid) and keratin forming factories (where tonofilaments coming together to form tonofibrils)
- tonofibrils are released and act as water barrier between dead and live skin cells
- as cells are pushed further up, they make keratohyalin granules (can see dark on microscope)
- cells undergo apoptosis (lose all organelles)
- cells pushed through water barrier mechanically
- cells lose ability to maintain pH so become more acidic
- cells lose water and pushed down, and keratin in cells becomes more compacted (cells get smaller)
- filaggrin proteins from desmosomes (holding these cells together) are broken down by pH
- this allows upper (dead) cells ‘squames’ to fall off (desquamation of keratin layer)
melanocytes
- found in the epidermis
- produces melanin
- combination of different types determines their hair and skin colour
- the melanin is moved into sacs called melanosomes
- melanosomes are transferred to neighbouring keratinocytes by pigment donation (phagocytosis of tips of melanocyte
- melanin protects nucleus against UV light, therefore preventing DNA damage
- these are not increased in number in dark skin, just produces more melanosomes
- histology: look similar to langerhan’s cells, but melanocytes are seen at regular intervals on basal layer of epidermis
langerhans cell
- most commonly found in stratum spinosum of epidermis
- come from the bone marrow and are part of immune response
- difficult to see histologically without special stains but looks a lot like melanocytes (but different layer)
- highly specialised capacity to present antigens to T-lymphocytes to mediate immune reactions (eg contact dermatitis)
what are merkel cells
- mechanoreceptor cells associated with sensory nerve endings
- difficult to see histologically without special stains
- have a ‘halo’ due to schwann cells surrounding the nerve cells - the schwann cells are lipid so not stained
two main types of skin
with exceptions
hairy skin tends to be thin skin
non-hairy tends to be thick skin
exceptions to the rules include the lips, back of pinna, some areas of external genitalia
what are the features and functions of non-hairy (thick) skin
location
* palmar surface of hand
* plantar surface of foot
* area between fingers and toes (webs)
structure
same as rest of body except….
* no hair, no arrector pili muscles, no sebaceous glands
* thicker strata corneum
* thinner dermis
* increased density of mechanoreceptors
functions
* prevents tissue loss to abrasion
* increased friction between skin and surfaces
* increased sensation
what are the features and functions of non-hairy (thick) skin
location
* palmar surface of hand
* plantar surface of foot
* area between fingers and toes (webs)
structure
same as rest of body except….
* no hair, no arrector pili muscles, no sebaceous glands
* thicker strata corneum
* thinner dermis
* increased density of mechanoreceptors
functions
* prevents tissue loss to abrasion
* increased friction between skin and surfaces
* increased sensation
thick (non-hairy) vs thin (hairy) skin
thick/non hairy
* no hair follicles
* no sebaceous glands
* no arrector pili muscles
* pronounced ridges and furrows on surface
* regular shaped dermal papillae
* stratum lucidium often visible
thin/hairy
* presence of hair follicles
* sebaceous glands present
* arrector pili muscles present
* smaller riges and furrows
* irregular dermal papulae
* stratum lucidium absent
what are skin appendages and examples
skin associated structures that serve a particular function
* hairs
* arrector pili
* sebaceous glands
* sweat glands
* nails
all developed from ‘down growths’ of epidermal epithelium during 3rd month of fetal development
what are the three types of hair
lanugo
* covers developing fetus
* sometimes seen at birth
* people with albanism don’t have this
* thin and long
vellus
* replaces lanugo
* short, thin, light coloured and soft
* not connected to sebeceous gland
terminal
* present on head (scalp and eyebrows, nasal passage), axillae, external genital region
* long, wide, dark coloured and coarse
* produced by the actions of testosterone
what are the funtions of hair
thermoregulation
* hair lays flat when body temp is within normal range or elevated
* hair stands erect when low temp (due to arrector pili muscles contracting, stimulated by sympathetic fibres from autonomic NS)
* also acts as partial barrier to UV light
sexual attraction
* apocrine glands secrete oils (pheromones)
* these attract opposite sex
sensation
* hairs have sensory nerve endings within bulb
* detect air movement, physical interactions
* vibrations transmitted to bulb, which is transmitted to CNS
* provides sensory awareness eg bug landing on hair
protection
* eyelashes and nasal hair prevent dust and pathogens from entering body
* eyebrows reduce the amount of light and sweat entering the eyes
* axilla hair acts as conduit to conduct sweat away from body
arrector pili muscle
- consist of smooth muscle fibres attached to papillary region of dermis (origin) and hair bulb (insertion)
- fight or flight response, or the cold causes involuntary contraction
- ‘goose bumps’
- muscle contracts when stimulated by sympathetic fibres from the autonomic NS
- squeezes sebacous gland at same time, releasing sebum release
sebaceous glands
and functions
- holocrine glands that produce sebum (oily/waxy substance)
- found mostly in the face and scalp
- also eyelids, penis, labia, nipples and lip
- part of the pilosebaceous unit
functions
* lubricate skin and hair
* reduce water loss from skin and hair
* aid skin flexibility
* protect skin and hair from too much moisture + UV damage
* reduce epidermal damage from friction
* facilitate the cooling down of skin in hot conditions (in conjuctions with sweat glands)
sweat glands in the skin
- can be apocrine or eccrine
- apocrine glands secrete a thick, odorous fluid into hair follicles
- eccrine sweat glands secrete a water fluid onto the surface of the skin
- the sweat glands are found
- both are fomed as downgrowths of the epidermis
mechanoreceptors of the skin
- end bulbs thermoreceptor
- free nerve endings have nocicreceptors to register pain
-
tactile discs attaches to basal layer keratinocyte. senses touch, pressure and texture
☞ merkel in non-hairy
☞ meissner in hairy - meissner corpuscle tapping and flicker movements
- pacinian corpuscle for vibration and pressure, particularly high frequency
- ruffinis corpuscle for join movements and tissue stretch
- root hair plexus vibrations in hair shaft
what are the frequencies detected by meissners corpuscle and pacinian corpuscle
pacinian corpuscle
100 → 1000 Hz
but optimum is about 450 Hz
meissners corpuscle
10 → 50 Hz
but optimum is about 35 Hz
nails
location
on the distal surfaces of phalanges (fingers and toes)
structure
* mainly α keratin
* made up of nail plate, matrix, and nail bed below it, and the grooves surrounding it
functions
* protection of distal phalanx and surrounding soft tissues
* enhances precise delicate movements
* enhances sensitivity of fingertip
* ‘extended precision grip’ eg picking up needle
note that growth of nails is correlated with length of phalanx
functions of skin
summary
- protection and repair eg uv protection by melanocytes, repair of epidermis by keratinocytes, wound healing of papillary dermis by fibroblasts
- temperature regulation and excretion of waste products eg via sweat glands, excretion of urea, NaCl, water and CO2
- lubrication sebaceous glands release oil onto surface and act as water barrier
- storage white adipose tissue provide lipids for energy
- vit D synthesis covered in MEH
- absorbtion small amounts of water and O@ and some drugs
- aesthetics as a mode of communication eg tattoos
- sensation specialised strucutres (mechanoreceptors) for pressure, touch, vibrations etc
note: first few mm of skin can respire via gas exchange of O2 and CO2
