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
Nail structure
> Nail plate from nails matrix (root) overlies Nail bed
-pink due to dermal capillaries
Nail bed produces some keratin
Growth in root
-keratinocytes divide and mature (keratinize) into nail plate
Epithelial cuticle protects nail
Blood vessels and the skin
> Blood supply
-deep vessels in subcutaneous layer
-superficial vessels in reticular dermis
»_space;capillaries from this to basement membrane
-Control dilation or contraction to regulate flow
»_space;Controls heat loss
Thermoregulation
> Maintain body temperature ay 37C
-Depends on metabolic and
physical factors
»_space;Evaporation of sweat
»_space;Diversion of blood from
deep to superficial
vessels
Temperature and forensics
> In hot environment:
-Core body temperature
rises
-More sweat released
-Cools the body so that
core returns to 37C
Leave latent fingerprints
-Mixture of natural
secretions from skin
Skin glands
> Exocrine (secrete products into ducts leading to environment NOT to bloodstream)
-Sebaceous
-Sweat
Products keep ski pH between 6-7
-Discourage microbial
growth
Sebaceous glands
> Derived from epidermal cells
Associated with hair follicles
Produce oily sebum (triglycerides, cholesterol, proteins, electrolytes)
-Flows into hair
follicles/travels to skin
surface
-Protects hair and skin from
dehydration/cracking
-Toxic to bacteria to stop
infection
Sensitive to androgens (more active at puberty)
Most in scalp, face, chest, back/non in hairless skin
None on soles or palms
Sweat glands
> Within dermis
Roughly 2.5 million over whole body surface
Coiled tubes that secrete watery substance
Two types
-Eccrine
»_space;Most numerous on palms, soles, forehead, armpits
»_space;Not in mucous membrane
»_space;Under thermal control for thermoregulation
-Apocrine
Composition of sweat
> Mainly water, but other chemicals diet-dependent
Minerals
-Sodium (0.9 g/L)
-Potassium (0.2 g/L)
-Calcium (0.015 g/L)
-Magnesium (0.0013 g/L)
Trace elements (zinc, copper, iron, nickel, lead, chromium)
Urea and lactate
Fatty acids
Some people also secrete amino acids in sweat
Some people do not secrete sweat from palms
-‘Non-secretors’
-They will rarely leave a
latent print on any surface
Sweat analysis
> Take sweat swabs
-Sterile cotton swabs
-Remove water by
sublimation (freeze drying)
-Leave amino acids and
protein matter
-Analyse by
chromatographic methods
Problems of sweat analysis
> Generally no biomarkers
-E.g. lactate also in semen
and saliva
Difficult to locate with alternative light sources as is dilute substances
Heterogeneity of composition between donors and samples from a single donor
-Affected by temperature,
beauty products, smoking,
medication, alcohol, hand
writing, skin pH, age, eating,
gender
Uses of sweat analysis
> Use to detect incidental impurities e.g. drugs and their metabolites
New biomarker found called dermcidin (human antibiotic)
-Identified with antibody
Apocrine glands
> Large sweat glands
Ducts empty into hair follicles
Open into hair follicles in armpits, genitals, eyelids
Active at puberty
Produce sticky, odourless, protein-rich secretion (glycogen, water, electrolytes, and sialomucin)
-Bacteria cause odour over time
