10.1 - Skin Structure

Question 1

what is the skin also known as

Answer 1

the integumentary system

Question 2

what are the 3 main layers of the skin

Answer 2

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

Question 3

what is the cutis

Answer 3

epidermis and dermis
☞ derived from ectoderm

hypodermis is not in this, as it is derived from mesoderm

Question 4

the hypodermis

features (functions on different cards)

Answer 4

• 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

Question 5

functions of hypodermis

(features on diff card)

Answer 5

• 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

Question 6

differences in thickness of hypodermis between males and females

Answer 6

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

Question 7

features of the dermis

Answer 7

• 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.

Question 8

what are the functions of the dermis

Answer 8

• contains hair and sweat glands for thermoregulation
• contains sensory structures - special senses - touch
• gives structure to skin and therefore body shape

Question 9

features of the epidermis

Answer 9

• 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)

Question 10

functions fo the epidermis

Answer 10

• 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

Question 11

what are the four/five layers of the epidermis

Answer 11

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

Question 12

keratinocytes

Answer 12

• 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

Question 13

keratin synthesis pathway

Answer 13

• 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)

Question 14

melanocytes

Answer 14

• 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

Question 15

langerhans cell

Answer 15

• 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)

Question 16

what are merkel cells

Answer 16

• 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

Question 17

two main types of skin

with exceptions

Answer 17

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

Question 18

what are the features and functions of non-hairy (thick) skin

Answer 18

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

Question 18

what are the features and functions of non-hairy (thick) skin

Answer 18

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

Question 19

thick (non-hairy) vs thin (hairy) skin

Answer 19

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

Question 20

what are skin appendages and examples

Answer 20

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

Question 21

what are the three types of hair

Answer 21

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

Question 22

what are the funtions of hair

Answer 22

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

Question 23

arrector pili muscle

Answer 23

• 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

Question 24

sebaceous glands | and functions

Answer 24

* 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)

Question 25

sweat glands in the skin

Answer 25

* 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

Question 26

mechanoreceptors of the skin

Answer 26

* **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

Question 27

what are the frequencies detected by meissners corpuscle and pacinian corpuscle

Answer 27

**pacinian corpuscle** 100 → 1000 Hz but optimum is about 450 Hz **meissners corpuscle** 10 → 50 Hz but optimum is about 35 Hz

Question 28

nails

Answer 28

**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 ## Footnote note that growth of nails is correlated with length of phalanx

Question 29

functions of skin | summary

Answer 29

* **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 ## Footnote note: first few mm of skin can respire via gas exchange of O2 and CO2