Physiology Of The Skin

Question 1

What is the largest organ of the body?

Answer 1

Skin

3.6 Kg
2 m2

Question 2

3 layers of skin

Answer 2

Epidermis

Dermis

Subcutis

Question 3

A waterproof barrier

Answer 3

Tight junctions between cells in stratum granulosum, epidermal lipids and keratin in stratum corneum form both an inside-out and outside-in barrier to water

Prevents transepidermal water loss

Question 4

Functions of epidermis

Answer 4

Waterproofing
Physical barrier
Immune function
Vitamin D synthesis (Endocrine)
UV protection
Thermoregulation

Question 5

Dermis

Answer 5

Thermoregulation
Vitamin D synthesis (Endocrine)
Sensory organ

Question 6

Subcutis

Answer 6

Thermoregulation
Energy reserve
Vitamin D storage
Endocrine organ
Shock absorber

Question 7

Why does skin wrinkle when wet?

Answer 7

Skin on fingers and toes wrinkles if immersed for approx. 5 mins

Mediated by sympathetic nervous system

Due to vasoconstriction in dermis

Improves grip

Question 8

A physical barrier

Answer 8

Structure of skin helps resist trauma

Stratified epithelium helps resist abrasive forces

Fat in subcutis acts as shock absorber

Question 9

Vitamin D synthesis and storage

Answer 9

7-dehydrocholesterol in plasma membranes of epidermal keratinocytes and dermal fibroblasts converted to previtamin D3 by UVB

15-25 mins whole body exposure produces up to 10,000 IU Vitamin D

Serum concentrations peak 24-48 hours after exposure

Lipid soluble – can be stored in subcutis adipocytes

Vit D3 can be found in oily fish/meat

Question 10

Skin is an endocrine organ

Answer 10

Site of hormone action

Androgens act on follicles and sebaceous glands

Thyroid hormones act on keratinocytes, follicles, dermal fibroblasts, sebaceous glands, eccrine glands

Question 11

Site of hormone synthesis

Answer 11

Vitamin D3 – unique site for cholecalciferol synthesis

17β-hydroxysteroid dehydrogenase in sebocytes and 5α-reductase in dermal adipocytes convert dehydroepiandrosterone (DHEA) and androstenedione to 5α-dihydrotestosterone

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) synthesised by dermal fibroblasts

Question 12

A barrier to ultraviolet light

Answer 12

Both UV-A and UV-B damage skin
-Burns
-Suppress action of Langerhans cells
-Photo-aging
-DNA damage (skin cancers)

Question 13

Skin colour depends on

Answer 13

Melanin

Carotenoids

Oxy/deoxyhaemoglobin

Question 14

Melanin

Answer 14

Synthesised in melanosomes within melanocytes from tyrosine

Transported via dendrites to adjacent keratinocytes

Pheomelanin (red/yellow)

Eumelanin (brown/black)

Melanocyte density varies between body sites

Red hair contains more pheomelanin

All skin types contain more eumelanin than pheomelanin.

Photoprotective – scatters/filters UV light

Question 15

Melanin is prone to photodegradation- may generate reactive oxygen species

Answer 15

Pheomelanin increases release of histamine

Lots of melanin = less able to utilize UV light to make vitamin D

Question 16

How do we respond to sunlight?

Answer 16

Immediate pigment darkening

Persistent pigment darkening (tanning)

Delayed tanning

Question 17

Immediate pigment darkening

Answer 17

photooxidation of existing melanin

redistribution of melanosomes

occurs within minutes and lasts hours-days.

Question 18

Persistent pigment darkening (tanning)

Answer 18

UVA&raquo_space; UVB
Ultraviolet A is better at oxidising melanin than UVB

oxidation of melanin

occurs within hours, lasts 3-5 days

Ultraviolet A (UVA) has a longer wavelength. It is associated with skin aging
Ultraviolet B (UVB) has a shorter wavelength- It is associated with skin burning
UVB stronger than UVA

Question 19

A barrier to infection

Answer 19

Skin presents a large surface area to environment

The properties that render the skin a barrier to water also help prevent infection

A range of peptides synthesised by granular layer keratinocytes have antimicrobial properties
- Cathelicidin-related antimicrobial peptide (Cramp – called LL37 in humans)
-β defensins
-S100A7 and S100A8

Question 20

The skin is an immune organ

Answer 20

Innate and acquired immune functions

Epidermis
-Langerhans cells

Dermis
-Regulatory T cells
-Natural killer cells
-Dendritic cells
-Macrophages
Mast cells

Question 21

The skin is an immune organ- epidermis

Answer 21

Keratinocytes secrete cytokines and chemokines that maintain populations of leucocytes in skin

Langerhans cells are antigen-presenting cells and secrete cytokines

Question 22

When skin is challenged

Answer 22

LC migrate to dermis and lymph nodes and activate a T-cell response

Keratinocytes proliferate & secrete cytokines

Leucocytes enter skin from blood

Question 23

The skin is a sensory organ

Answer 23

Merkle cells - basal epidermis (Light touch)

Encapsulated mechanoreceptors in dermis
-Pacinian corpuscles (Pressure/Vibration)
-Meissner corpuscles (Touch)

Myelinated and unmyelinated sensory nerve endings in dermis (Pain, Itch, Temperature)

Question 24

Skin helps regulate body temperature

Answer 24

Slide 27-32

Question 25

Skin as an energy store

Answer 25

Subcutaneous fat acts as an insulator, a shock absorber and as an energy store

White adipose connective tissue