Skin

Question 1

Whats the integumentary system?

Answer 1

The skin and its accessory structures; hair, nails, glands and sensory receptors.

Question 2

What are the two major skin types?

Answer 2

• Thick skin (hairless) - five layers, found on the palms, palmar surface of fingers and soles.
• Thin skin (hairy) - 4 layers covering all other parts of the body.

Question 3

Whats the skin composed of?

Answer 3

The epidermis, dermis (10 times size of epidermis) and hypodermis.

Question 4

Whats the epidermis?

Answer 4

A keratinised, stratefied squamous epithelium composed of layers/strata of keratinocytes that differ in form and function.

Question 5

What are the 5 layers of the epidermis?

Answer 5

Stratum corneum, stratum lucideum, stratum granulosum, stratum spinosum and stratum basale.

Question 6

Describe the stratum corneum.

Answer 6

Multiple rows of dead, flat, enucleate keratinocytes that comprise mostly of cross linked keratin fibres (squame cells).

Question 7

Describe the stratum lucideum.

Answer 7

Multiple rows of clear, flat, dead, keratin-rich keratinocytes - these are only found in thick skin.

Question 8

Describe the stratum granulosum.

Answer 8

Multiple rows of flattened keratinocytes containing darkly staining keratohyalin granules and lipi rich lamellar bodies - on release these form a hydrophobic impermeable barrier.
The stratum granulosum contains a desmosome connecting it to the stratum spinosum.

Question 9

Describe the stratum spinosum.

Answer 9

Multiple rows of closely packed, multisided keratinocytes tightly linked by desmosomes. The stratum spinosum consists of golgi apparatus, ribosomes, keratin fibres and rough endoplasmic reticulum.
There are desmosomes connecting it to the stratum granulosum above and the stratum basale below.

Question 10

Describe the stratum basale.

Answer 10

A single layer of columnar keratinocytes resting on a basement membrane - the site of production for new keratinocytes. The stratum basale contains mitochondrion, nucleus, ribosomes and scattered keratin fragments.
There are desmosomes connecting it to the stratum spinosum and there are hemidesmosomes connecting it to the basement membrane.

Question 11

Whats keratin?

Answer 11

A family of cytoskeletal filament-forming proteins; there are 54 proteins in two families - type 1 (acidic) and type 2 (neutral or basic).
25-30% of proteins in the epidermis are keratin whilst 80-85% of proteins in the stratum corneum are keratin.

Question 12

Whats the structure and role of keratin in the epidermis?

Answer 12

Keratins form branching rope like bundles in the cell cytoplasm - they associate with junctional (anchoring) complexes at the cell periphery.
Keratins maintain the structural integrity of the epidermis and allow the epidermis to resist and dissipate mechanical forces.

Question 13

How can keratin be found in the epidermis? (structures)

Answer 13

• 1 strand is a monomer.
• 2 strands twisted is a coiled-coil dimer.
• 2 twisted strands staggered -s a staggered tetramer of two coiled-coil dimers.
• 2 of the twisted staggered strands is two tetramers packed together.
• 8 of the twisted staggered strands is tetramers twisted into a rope like filament.

Question 14

Whats the lipid matrix?

Answer 14

The lipid matrix in the stratum corneum acts as the skins main water proofing barrier.

Question 15

What do surface keratinocytes do?

Answer 15

Produce keratin fibres.

Question 16

What are melanocytes?

Answer 16

Cells containing the pigment melanin.

Question 17

What’s the basal lamina?

Answer 17

The basement membrane is an acellular layer between the epidermis and dermis.

Question 18

What are junctional complexes?

Answer 18

They anchor keratinocytes to each other and to the basement membrane. There are two types of junctional complexes: desmosomes (connect keratinocytes to each other) and hemidesmosomes (connect epidermal cells to fibres of the basal lamina).

Question 19

Describe the structure of the desmosome?

Answer 19

The plaque binds to the adjacent plasma membraneswith transmembrane glycoprotein (cadherin) binding in the intracellular space. Also on the plaque is intermediate filament keratin.

Question 20

Describe the structure of the hemidesmosome.

Answer 20

Just one plaque from the epidermal cells binds integrins to the basal lamina. Also on the plaque is the intermediate filament keratin.

Question 21

Langerhans cells are found in the stratum spinosum, what are they?

Answer 21

A highly motile antigen presenting cell. They internalise, process and present foreign antigens to T cells. They may have additional immunomodulatory functions.

Question 22

Melanocytes are found in the stratum basale, what do they do?

Answer 22

Melanocytes synthesize and package the pigment melanin into vesicular melanosomes.

Question 23

Merkel cells are also found in the stratum basale, what are they?

Answer 23

Merkel cells are sensory receptor cells that make synapse like contacts with somatosensory nerve endings and function as highly sensitive touch receptors for the detection of shape curvature and structure.

Question 24

Describe the transfer of melanosomes from melanocytes to keratinocytes in the epidermis.

Answer 24

1 - Melanosomes are produced by the golgi apparatus of the melanocytes.
2 - Melanosomes move into the melanocyte cell processes.
3 - Epithelial cells phagocytize the tips of the melanocyte cell processes.
4 - The melanosomes have been transferred to epithelial cells and are now inside them.

Question 25

What do melanosomes do in keratinocytes?

Answer 25

Migrate to the peri-nuclear region (around the nucleus in the cytoplasm) and form a protective cap, protecting the nucleus from UV radiation to prevent DNA mutation.

Question 26

What determines the colour of a persons hair and skin?

Answer 26

The production of melanin (number of melanocytes is similar in all individuals). There are two forms of melanin produced by melanocytes: eumelanin and pheomelanin - the relative amounts of these two pigments will determine a persons skin colour.

Question 27

What are the 2 regions of the dermis?

Answer 27

Papillary (20%) and reticular (80%).

Question 28

The dermis is separated from the epidermis by the basement membrane/basal lamina. Whats the function of this?

Answer 28

A bridging structure, regulator of proliferation in the stratum basale and regulator of movement of cells and bioactive molecules between the epidermis and dermis.

Question 29

Whats the papillary matrix?

Answer 29

Loose connective tissue composed of fine elastic fibres with interspersed capillary loops and nerve fibres.

Question 30

Whats the reticular matrix?

Answer 30

Dense connective tissue composed of randomly orientated collagen fibres, elastin, proteoglycans and glycosaminoglycans.

Question 31

What cell types are found in the dermis?

Answer 31

Matrix synthesizing fibroblasts, immune cells, vascular smooth muscle cells and sweat gland myoepithelial cells.

Question 32

Where does hair grow from?

Answer 32

Hair follicles located in the dermis - follicles develop from downward projections of the epidermis.
The hair is fed by blood vessels at the base of the follicle giving it nourishment to grow.

Question 33

What is hair composed of?

Answer 33

Columns of dead keratinized epithelial cells - it originates from a germinal matrix or stratum basale located in the hair bulb.
The hair consiste of a medulla, a cortex (the site of cell divisions that produce hair cells) and a cuticle. In the papilla, they are surrounded by a hair follicle wall composed of connective tissue and epithelial tissue.

Question 34

What is horripilation?

Answer 34

The erection of hairs on the skin due to cold, fear or excitement.

Question 35

What are the 4 stages of hair growth?

Answer 35

1 - Anagen - active growth phase
2 - Catagen - transition phase, hair transitions upwards towards skin pore and dermal papilla begins to separate from follicle.
3 - Telogen - Resting phase, dermal papilla fully separates from follicle.
4 - Exogen - hair sheds.

Question 36

What are glands?

Answer 36

Glands are specialised secretory structures formed by epithelial cells. There are two types of glands; endocrine (ductless) and exocrine (with ducts).
All glands of the skin are excretory.

Question 37

Exocrine glands can be further categorised into what?

Answer 37

• Merocrine glands - vesicles containing secretory products that release contents into the duct.
• Apocrine glands - secretory products are stored in the cell, parts of the cell are then pinched off during secretion.
• Holocrine - cells are replaced with replacement cells and hence shed into the duct, the dying cells then release secretory products.

Question 38

What are the 2 main types of gland in the skin?

Answer 38

Sebaceous and sudoriferous.

Question 39

What is the function of sebaceous glands?

Answer 39

Sebaceous glands discharge a waxy, oily substance called sebum into the hair follicles which lubricate the hair shaft and the skin.
Activity increases at puberty.
Absent from plantar and palmar surfaces.

Question 40

The lipid rich sebum produced by the sebaceous glands contain what?

Answer 40

Triglycerides, wax esters, squalene, free fatty acids and cholesterol.

Question 41

What type of secretion do sebaceous glands do?

Answer 41

Holocrine secretion - the entire cell disintegrates to excrete its substance directly into hair follicles or onto skin surface.

Question 42

Whats the function of sudoriferous glands? Name and describe the 2 types.

Answer 42

Sudoriferous glands are sweat glands that can come in two forms:
- Apocrine (25%) - The duct of the apocrine gland opens into the hair follicle and secretes sweat via a merocrine mechanism. They are similar to eccrine glands but more viscous due to presence of lipids and proteins. The activity of apocrine glands is linked to emotional state as secretion may contain pheromones and they activate during puberty.
The apocrine glands have a limited distribution to the armpits, groin and bearded region of face.
- Eccrine (75%) - The duct opens at the epidermal surface and secretes a hypotonic and slightly acidic liquid (containing water, sodium chloride and potassium ions, urea and lactate) via a merocrine mechanism.
Eccrine glands are widely distributed, particularly on the forehead and plantar/palmar surface. Roughly 600ml a day is secreted from these glands at rest.

Question 43

What sensory receptors does the skin contain?

Answer 43

Mechanoreceptors, thermoreceptors and nociceptors which respond to stimuli.

Question 44

Outline the 6 receptors in the skin.

Answer 44

• Free nerve endings respond to pain heat and cold.
• Free nerve endings wrapped around hair respond to touch.
• Merkel disc responds to light touch.
• Meissners corpuscle responds to light touch.
• Pacinian corpuscle responds to strong pressure.
• Ruffini corpuscle responds to continuous pressure.

Question 45

The skin contributes to temperature homeostasis - outline the two ways it does this.

Answer 45

• Heat loss via evaporation of sweat produced by the eccrine glands. Typically 600ml is produced daily but this can increase up to 1600ml.
• Fluctuations in blood flow is another way in which the skin can regulate body temperature.

Question 46

How does blood flow change in both hot and cold conditions?

Answer 46

In cold conditions, warm blood enters from the arteries. superficial blood vessels then constrict to reduce heat loss while deeper blood vessels dilate to allow blood to bypass the colder surface vessels. The blood then leaves in veins, retaining most of its heat.
However, in hot conditions, warm blood enters from the arteries, then superficial blood vessels dilate to allow heat loss and deeper vessels constrict to shunt blood to the skin surface. Blood then leaves in veins having lost some of its heat.

Question 47

Vitamin D3 is a sercosteroid produced by the keratinocytes of the stratum spinosum and stratum basale. How does this happen?

Answer 47

7-dehydrocholesterol is converted to pre-vitamin D3 which then undergoes spontaneous isomerisation to form vitamin D3 upon exposure to the UVB component of sunlight.

Question 48

The skin itself is a target for the actions of the active form of vitamin D3. What role does it have?

Answer 48

keratinocytes express the Vitamin D receptor and the 1-alpha hydroxylase necessary for the conversion of 25(OH)D3 produced in the liver indicating it may participate in the local regulation of their proliferation and differentiation.
It may also have an immunomodulatory role in the skin, augmenting the tissues innate immune response (increased production of antimicrobial peptides, increased phagocytic activity of tissue macrophages)whilst simultaneously depressing the adaptive immune response (suppression of DC function and the production of pro-inflammatory cytokines).

Question 49

Describe the structure of the stratum corneum.

Answer 49

The corneocytes are held in position with intercellular lipids and connected to adjacent corneocytes via corneodesmosomes.

Question 50

The stratum corneum is a permeability barrier that minimises Trans-Epidermal Water Loss (TEWL). What qualities does the stratum corneum have enabling it to do this?

Answer 50

• Filagrin - a filament aggregating protein.
• Lamellar body containing neural lipids.
• Corneocytes have a cornifies envelope composed of insoluble cross linked proteins.

The concentration of water inside the body is much higher then outside and with such a large surface area, we need to make sure that water cant be lost by diffusion accross our skin.

Question 51

The human epidermis is colonised by a wide variety of microorganisms. Outline the different possible relationships of a person with microorganisms.

Answer 51

Commensal - When one partner benefits but the other suffers harm.
Mutualistic - Both partners benefit.
Pathogenic - a microorganism capable of causing a disease.

Question 52

The antimicrobial barrier of the skin is multi factorial and not limited to the stratum corneum. Outline the different mechanisms of antimicrobial defence.

Answer 52

Intact skin provides a physical barrier to invasion
Acidity of the skin surface and stratum corneum is regulated by;
- secretion of sebum and eccrine sweat.
- enzymatic breakdown of phospholipids to produce free fatty acids.
- extrusion of H+ in exchange for Na+ by cells in the stratum granulosum.
- release of acidic metabolic by products by microorganisms.

Production of a wide range of antimicrobial peptides (AMP’s) - these are rapidly produced in response to invading pathogens.

The skin forms a major part of the innate immune response.

Question 53

AMP’s are produced by the skin in response to invading pathogens as a rapid reaction force. What are some of the AMP’s present in the skin?

Answer 53

Cathelicidin, lysozyme, dermicidin and B-defensin.

Question 54

Epidermal keratinocytes and Langerhans cells cooperate in the mobilisation and targeting of cells of the adaptive immune system. How do they do this?

Answer 54

Release of intercellular chemical messengers; cytokines and chemokines.
Dynamic adjustment of tight junctions - disruption of the corneal layer or antigen infiltration allows the Langerhans cell to become active and to sample antigens in the stratum corneum.

Question 55

What is epidermal homeostasis in health?

Answer 55

Loss of cells from the stratum corneum is precisely matched by the proliferation, upward migration and differentiation of cells from the stratum basale. This maintains the barrier function of the skin.

Question 56

Where can epidermis stem cells be found in the skin?

Answer 56

Specialised niches within the inter-follicular epidermis and bulge region of the hair follicle.

Question 57

In healthy skin, homeostasis maintains a functional barrier. In atopic dermatitis or eczema this balance is disrupted. In what way is it different?

Answer 57

Desquamation (shedding of outer layer of stratum corneum cells) accelerates and the barrier is weakened. This allows more facile penetration of irritating chemicals which exacerbates the condition.