Lecture 8: Skin structure and function Flashcards
Skin layers
> 3 layers
-Epidermis (stratified
squamous epithelial layer)
>Made up of keratinocytes
-Dermis (supportive
connective tissue matrix)
>Made up of fibroblasts
and immune cells
-Hypodermis (adipose or
fat layer)
>Made up of adipocytes
Functions
> Prevents mechanical abrasion of underlying tissues and coordinated wound healing
Prevents desiccation or water absorption
Prevents injury by chemicals/radiation
Barrier to pathogens
Mechanism of sensation
Some metabolic functions
Mechanism of thermoregulation
The hypodermis
> Loose connective tissue and fat
Gives stability to structures above
Reduces heat loss from body (insulation)
Absorbs below to the body
Energy reserve
The epidermis
> Made up of:
-Keratinocyes (>90%)
-Melonocytes (5–10%)
-Merkel cells (infrequent)
Rate ridges highly developed in areas exposed to stress e.g. hands/feet
-Increase contact with
dermis to prevent
epidermis falling off
(blister formation)
Why is the epidermis stratified
Stratification has advantages over monolayer:
> Stem cell population protection deep inside
> Upper cells provide protection
> Monolayer is more difficult to replace if abraded (barrier function would be lost)
-Dividing population and
protective population are
the same
Stem cells in the epidermis
> Stem cells at tips of rete ridges
-Unlimited division possible
Transit amplifying cells make up rest of basal layer
-1-2 divisions possible
Basal layer
> Columnar cells anchored to basement membrane
-Henidesmosomes
Proliferative
-Divide every 200-400
hours
Undifferentiated
Spinous layer
> Cells larger and flattened
-8-10 cells thick
Differentiation starts
-biochemical markers change –> keratin
Desmosomes
Langerhans’ cells –> allergic reaction response
Granular layer
> Keratohyalin granules containing profilaggrin
Lipid-filled lammellar granules
Transition zone
> Between granular (living) and cornified (dead) layers
Enzymatic activity and cellular restructuring
Profilaggrin cleared to filaggrin
-causes keratin filaments to be reconstructed
Apoptotis –> controlled cell death –> recycling
Organelles are destroyed by protease, DNase, RNase, acid hydrolases and plasminogen activator
Lamellar granules fuse with the plasma membrane and release lipid into extracellular space
-lipids form sheets for ‘waterproofing’
Cornified envelope forms
How is damage from radiation prevented?
> Melanocytes synthesise melanin
Melanin
-absorb UV radiation
-scavenges free radicals
-production increased by
sun exposure
-2 types
»_space;Eumelamin is brown-
black
»_space;Phaemelanin is yellow-
red
Melanocyte function
> Melanosomes are vesicles containing melanin
-Move along arm-like structures called dendrites
-Passed to keratinocytes
Form cap over keratinocyte nucleus for protection
How is infection prevented?
> Langerhans’ cells migrate from bone marrow to spinous layer in epidermis
-Take up and process
microbial antigens
-become antigen-
presenting cells
-present antigens to T-cells
(lymphocytes)
The dermis
> Directly below epidermis
Thickness from 0.6mm on eyelids to 3mm on palms and soles
Contains:
-Most cells (immune cells
involved in inflammation,
allergy and wound healing)
-dermal appendages (hair
follicles, sweat glands,
blood vessels)
-fibroblasts
Functions of fibroblasts
> Produce chemicals/stimulants which diffuse to epidermis to control protiferation
Produce collagen for tensile strength for resistance to longitudinal stress
Produce elastin for stretch and recoil for flexibility and movement
Produce structural proteoglycans/ glycosaminoglycans for hydration and burgor
Make extracellular matrix (cells embedded within being in direct contact)
The dermis
2 layers
> superficial papillary layer containing sensory nerves and loose conective tissue
> underlying reticular layer contains collagen fibres
> anchors papillary layer to subcutaneous layer
Mechanism of sensation
> Nerves give perception of sensation
-most common where
sensation important
Sensors in epidermis and dermis
-Merkel cells in basal layer
of epidermis
-Touch
»_space;Meissner’s corpuscle- in
dermal papillary of
hands/feet
-pressure/vibration
»_space;Paccinian corp
Metabolic functions
> Synthesis of vitamin D3 in dermis
Subcutaneous layer
-lipid metabolism and
storage
-storage of lipid-soluble
vitamins
Hair
> Roughly 5 million hairs on the body
-98% on the skin surface
-no hair on parts of genitals or palms/soles
-Dense on scalp
Emerge from follicles
-from deep dermis/subcutis to surface
-2 parts called the root and shaft
Hair structure
> Dermal papilla formed from connective tissue
-Holds capillaries and nerves
Hair bulb surrounds papilla with epithelial cells
-proliferative cells
3 layers
-inner medulla contains flexible keratin
-outer cortex contains hard keratin
-hair fibre coated in keratin called cuticle
2 layers of epithelial wall
-internal root sheath surrounds the root
-external root sheath covers entire length of follicle
Hair colour
> Melanocytes at base of hair produce melanin pigment
Overtime, less pigment is made
-Grey colour
Hair types
> Lanugo hairs
-form at 20 weeks gestation, usually shed before birth
-fine and long
Vellus hairs
-commonest hair type over body surface
-short, fine and light colour
Terminal hairs
-thick and long
-10,000 on scalp, also on eyebrows, bears, pubic areas
Hair production
> Growth is cyclical at rate of approximately 0.4mm/day
Growth
-hair root firmly attached to matrix
Growth stops
-follice inactive, shrinks so attachment broken down
Growth restarts
-old hair pushed upwards and is shed
Some old drugs can halt hair growth
-cytotoxins, herparin, warfarin, carbimazole, colchicine, vitamin A, poisons e.g. Thallium
Nail
> Dense plate of cells
Packed with hardened keratin (0.3-0.5mm thick)
Protects finger tip and help grasping
Fingernails grow at 0.1mm/day
-Toenails more slowly
