Introduction to Skin Flashcards
The skin is the largest organ in the body. What % of total body weight does it make up?
12-15%
How many cells are in the skin?
1.6 trillion
How often to people visit the GP due to skin problems?
Most common reason for visiting a GP (42%)
Give some examples of skin diseases. (4)
Malignant melanoma (cancer) Psoriasis (inflammation) Diabetic ulcer (trauma) Epidermolysis bullosa (genetic)
What is the function of the skin? (4)
Protection
Insulation
Provides sensory information
Vitamin D synthesis
What does the skin protect from? (5)
Physical trauma Infection Penetration of drugs and chemicals UV radiation Water loss
What are the layers of the skin?
Epidermis
Dermis
(May classify adipose tissue as a third layer = hypodermis)
What type of epithelium is the epidermis?
Stratified squamous
What is the major cell type of the epidermis?
Keratinocyte
What are the four main layers of the epidermis?
Stratum basale (basal layer) Stratum spinosum (spinous layer) Stratum granulosum (granular layer) Stratum corneum (cornified layer)
What does the epidermis also include?
Hair and sebaceous gland
Describe the appearance of the basal cells of the epidermis.
Basal cells are compact and have dark nuclei
Describe the appearance of the stratum spinosum cells of the epidermis.
Spiny processes. Separate keratinocytes. Larger cells.
Describe the appearance of the granular layer cells of the epidermis.
Contains purple keratinohyaline granules. Start to lose nuclei.
Describe the appearance of the stratum corneum cells of the epidermis.
The cells are flattened and have no nuclei.
What cell type are keratinocytes? What do they produce?
Epithelial cell
Keratins (to make dense network of cytoskeleton to provide strength and integrity)
Keratins are _______ filaments. They are the most abundant proteins in _______, ____ and _____.
Intermediate
Stratum corneum, hair and nails
How many subtypes of keratins are there?
54
What are the two main types of keratins? What defines the difference in the two?
Soft (alpha type) and hard (beta type)
The difference is defined by the secondary structure and S-S bonding – soft keratins are formed from alpha helix/coils and hard keratins are formed from beta sheets.
What is the molecular weight of keratins?
40-50,0000 kD
Keratins always expressed as a pair – what are these pairs?
Acidic and basic pairs (to maintain neutral charge)
What is the expression pattern of keratins specific to?
Epidermal layer
Where does proliferation occur?
In basal layer
The cells once committed to differentiate, being upward migration and terminal differentiation. They are moved up and eventually shed from the surface.
What maintains the epidermis? Describe these.
Population of adult stem cells
Long lived, slow cycling, capable of self-renewal and terminal differentiation
What do basal cells adhere to?
ECM rich basement membrane
What does the basement membrane consist of?
Laminin 332, Collagen IV, and Collagen VII
What do hemidesmosomes do?
They are integrin receptors that link keratin cytoskeleton to the basement membrane
What is the role of the basement membrane? (3)
Essential for cell polarity, regulating basal cell function, and anchoring epidermis to dermis
Describe the stratum spinosum.
3-4 cell layers thick Numerous desmosomes (structures holding cell together) give cells a spiny appearance
Describe the stratum granulosum.
2-3 cell layers
Characterised by large granules of keratohyalin
Give three examples of keratohyalin proteins.
Filaggrin, involucrin, loricrin
Describe the stratum corneum.
A thick cornified envelope, cross-linked by enzymes, that acts as the main protective barrier of the skin. These cells are dead and contain large amounts of intracellular lipids (ceramides) to maintain moisture/prevent dehydration.
Name four types of cell-cell adhesions present in skin.
Adherens junctions, desmosomes, tight junctions, gap junctions
What is an adherens junction? How does this differ from a desmosome?
Cadherin receptor linked to actin cytoskeleton. It links cells to one another and is linked with cytoskeleton to provide mechanical integrity and strength.
A desmosome links to KERATIN cytoskeleton, rather than actin.
What are tight junctions?
Claudin and occludin seal intercellular space (space between cells) so nothing can pass between them
What are gap junctions?
Intercellular pores (within the cell membrane) made up of connexins. They link cells to each other so they can communicate.
What are the three layers of the dermis?
Papillary, reticular, adipose
What is the dermis made up of? What is its role?
Complex mix of macromolecules (mainly ECM), with a few cells interspersed. It is vascularised and innervated.
Provides strength and elasticity to skin
What is the main cell type in the dermis?
Fibroblast
What is the developmental origin of fibroblasts? What do they do?
Mesenchymal origin
Synthesise collagen, elastin and proteoglycans (i.e. they produce ECM)
Where is the papillary layer?
Directly beneath epidermis and basement membrane
What does the papillary layer contain?
Many blood capillaries
Fine, randomly oriented collagen (type III), and elastin – this makes it elastic and deformable
Describe the reticular layer.
It has large and densely packed collagen fibres, so has more strength and rigidity.
It provides much of the mechanical strength of the skin.
Name skin glands and appendages. (3)
Pilosebaceous unit (hair follicle, sebaceous gland)
Eccrine sweat gland
Apocrine gland
Where are hair follicles found? What are they part of?
Over entire body. They are part of the epidermis.
What are the two types of hair follicle?
Vellus (body hair)
Terminal (scalp and secondary sexual hair)
What cells produce hair?
What cells control hair growth?
Matrix keratinocytes
Dermal papilla fibroblasts
Where are hair follicle stem cells localised to? What do they express?
Bulge region
Keratin 15
What do HF stem cells do?
They are slow cycling cells that give rise to hair cells (under normal conditions). They contribute to SG and epidermis upon wounding.
Hair growth is cyclical. What are the three main phases?
Anagen (active), catagen (regressive), and telogen (resting)
What type of gland are sebaceous glands? What do they respond to, and so when do they enlarge?
Exocrine
Androgen-sensitive, enlarge at puberty
What do mature sebocytes contain? How is it released?
Sebum – the cell ruptures and sebum is release into sebaceous duct and onto skin
What does infection of sebaceous glands cause?
Acne
What are eccrine sweat glands for? What are the two main components?
Thermoregulation
Excretory duct and secretory coil
What is a apocrine gland? What do they release?
Sweat gland associated with hair follicles e.g. in axilla and pubic region
Their secretion is odourless. It is broken down on skin by bacteria. Release volatile fatty acids.
What type of cells are melanocytes (and also Langerhan cells)? Where do they sit?
Dendritic cells
Sits in epidermis on basement membrane and hair matrix.
What do melanocytes produce?
Melanin in melanosomes (eumelanin and pheomelanin). Melanosomes are injected into keratinocytes to give skin pigmentation and protect against UV radiation.
Where are Langerhan cells found? What do they do?
Basal and spinous layers
Antigen presenting cell/first line of defence – detects antigens and presents them to T lymphocytes
Where are Merkel cells found? What are they for?
Stratum basale Sensory perception (light touch)
Where are Mast cells found? What do they do?
Dermis
Secretes histamine in response to allergen
What are the three types of wounds?
Superficial – epidermis
Partial thickness – epidermis and dermis
Full thickness – epidermis, dermis, hypodermis
What are the three phases of wound healing?
Inflammation
Proliferation
Maturation
Describe the inflammation phase of wound healing.
- A fibrin blood clot forms and immune cells arrive.
- It can last from minutes to a few days.
- Action of platelets, endothelial cells and immune components. Attach pathogens and seal off the wound. -Regulated by wide range of growth factors and cytokines e.g. IL-1.
Describe the proliferation phase of wound healing.
- Proliferation of keratinocytes (migrate over the wound to restore barrier function)
- Infiltration of fibroblasts to produce ECM
- Done in haphazard way
- Endothelial cells help repair damaged blood vessels
- Regulated by wide range of growth factors and cytokines AND anabolic growth factors (e.g. TGF-beta)
Describe the maturation phase of wound healing.
- Quickly deposited tissue is broken down and remodelled into normal skin structure
- Regulated by some of the same growth factors and cytokines as the other phases
Name three potential complications of wounds/healing.
- Infection
- Chronic wounds (not healed >6 wks) - e.g. venous, pressure, diabetic ulcer
- Scarring (e.g. keloid scar)
