Tissue Engineering in Skin Flashcards
What is the role of skin?
- Barrier between the internal and external environment: provides protection against physical, chemical and biologic agents
- Plays an important role in thermoregulation
- Small degree of self-regeneration (not for deep injuries and burns)
What is the composition of skin?
1) epidermis
2) dermis
3) subcutaneous layer
What is the epidermis?
- made of closely packed epithelial cells (4 or 5 layers)
- no blood vessels (avascular)
What is the dermis?
dense and irregular connective tissue that houses blood vessels, hair follicles and sweat glands
What is the hypodermis?
composed mainly of loose connective and fatty tissues
What are the different layers that make up the epidermis?
stratum basale
stratum spinosum
stratum granulosum
stratum corneum
What is the name of the cells in all the layers of epidermis (exp stratum basale)?
keratinocytes
What is the role of keratinocytes?
“manufacture” and store keratin
= intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties.
What are cells in the stratum basale known as?
stem cells
What happens to keratinocytes in the stratum corneum?
they are dead and regularly slough away, being replaced by cells from the deeper layers
What is the dermis?
contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands Made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts
What are the two layers of connective tissue in the dermis?
1) papillary layer
2) reticular layer
What is the papillary layer?
loose mesh collagen/elastin fibres
• Fibroblasts, small number of fat cells (adipocytes), phagocytes (fight bacteria or other infections)
• Abundance of small blood vessels
What is the reticular layer?
dense, irregular tight meshwork of collagen and elastin fibres
• Well-vascularized
• Rich nerve supply
What are the skin problems related to loss of barrier?
- Extensive full thickness skin loss-burns
- Chronic non-healing ulcers
- Minor cuts and abrasions
- Genetic blistering diseases
What are examples of inflammatory skin diseases?
- Eczema
- Acne
- Dermatitis-including contact dermatitis
What are skin problems related to melanocytes?
- Vitiligo
* Melanoma
What are the skin problems related to keratinocytes?
• Carcinoma-basal cell and squamous
What are the skin problems related to hair?
- Hair loss
* Hair greying
What are the tissue engineered skin applications?
- Burns
- Chronic wounds
- Reconstructive surgery-using TE skin
- Synthetic scaffolds for tissue engineering
- New Approaches: Artificial Stem Cell Microenvironments
What is a burn?
Damage to the skin or other body parts caused by extreme heat, flame, contact with heated objects, or chemicals
What is a first degree burn?
Superficial Causes local inflammation of the skin (e.g: sunburns)
What is a second degree burn?
Deeper Pain/redness/mild amount of swelling+ Blistering of the skin
What is a third degree burn?
Even deeper (involving all layers of the skin) Nerves and blood vessels are damaged White and leathery and tend to be relatively painless
What does the treatment of burns depend on?
- Depth/area/location
* Material that may be burned onto or into the skin
What is a skin graft?
Sections of the epidermis or dermis that have been separated from one part of the body and reaffixed to a site where the skin has been removed or damaged
What is a split thickness skin graft?
- Involve only the epidermis and a small portion of the dermis (leaving behind enough of the dermis for the donor site to heal by reepithelialisation)
- Can survive in less ideal recipient sites (with less vascularity)
- Thinner grafts are more likely to contract when healing
What is a full thickness skin graft?
- Involve the epidermis and the entire dermis
- Limited to smaller wounds and require well-vascularized wound beds in order to support the grafted skin.
- Maintains more of the normal characteristics of the skin (texture, colour, thickness)
- Less likely to contract as it heals
What is the limitations of skin grafts?
limited availability of healthy donor tissue
How are cultured epithelial autografts (CEA) produced?
- Methodology requires keratinocytes to grow into integrated sheets of cells 2/3 layers thick so they can be detached
- This is achieved using a feeder layer of murine feeder cells and foetal calf serum (mitogen rich) for rapid growth of cells
- Methodology works well-based on 1975 method but contains man, mouse and cow reagents
What are the disadvantages of using CEA?
- Fragile cell sheets
- Difficult to handle
- “Take” less than 50% overall
- Timing of producing sheets (9-12 days) difficult to manage to needs of patients
- Over 50% of CEA cultured were wasted because of timing difficulties
What is myskin?
• Silicone substrate: - Chemically modified dressing - Plasma polymerization used to attach acid functional groups on inert surface - Very thin layer that allow cells to grow on biofuctionalised surface but also to migrate to the wound bed when placed • Silicone substrate • Sub-confluent cell layer • Patient’s own cells • Biopsy • Cell expansion • Cell storage • Weekly application • Removed after 3/4 days • Repeated applications
What is plasma polymerisation?
- Low-pressure plasma
- Monomer (liquid/gas)
- Reliable technology
- Reproducible
- Flexible
- Control
- Scalable
- Cost-effective
How is myskin different?
- Cells do not form a sheet so time window is flexible
- More robust delivery system
- Easier for the clinician to handle
- Cells can be stored for multiple applications
What are chronic wounds?
those that fail to heal despite conventional wound care
What are the characteristics of chronic wounds?
- A poor vasculature supply
- Often infected
- Wound bed matrix often very inflamed, high levels of degradative enzymes, poor granulation tissue
When do chronic wounds generally occur?
- Arterial vasculature problems
- Diabetes
- Bed-sores in elderly or immobile patients
What are the problems identified in clinical use of reconstructed skin?
- Loss of grafts due to delayed vascularisation
* Contraction of grafts post healing
How is skin graft contracture reduced?
Currently patients wear pressure garments that prevent/reduce contracture.
How can tissue engineering grafts be lost in relation to angiogenesis?
- Tissue engineered grafts can be lost because they are slow to vascularise on patient
- Tissue engineered skin unlike a splitthickness skin graft has no residual dermal vasculature to assist in vascularisation
How is reconstructed skin developed?
Need a skin replacement material which is equivalent to a split-thickness skin graft (0.4-0.8 mm thickness)
TE involves the right type of cells and a matrix
What are the problems with reconstructed human skin based on donor dermis?
Major issue is safety
• To reduce risk to patients need to use skin from accredited skin banks
• Even with extensive screening there will always be some level of risk of disease (viral) transmission
• This may reduce clinical uptake of skin prepared using donor dermis except for life threatening conditions
Secondary but significant issue
• Availability of skin from accredited UK tissue Banks is a major problem in practice
How is electrospinning used in grafts?
Electrospun scaffolds as an alternative to human allodermis
• Developing biocompatible biodegradable scaffolds of PLA/PLGA
• Evaluated scaffold breakdown in vitro and in vivo
What is the VERSATILE TOOL (VERSATILE polymeric devices) ?
- Easy to handle
- Easy to control degradability and porosity
- Easy biofunctionalisation
- Medical Grade and FDA Approved Polymers
- 3D Environment for cells to grow
- (“Similar to ECM”)
How does the electrospun scaffold breakdown?
- PLLA scaffold-no significant change in overall size after 12 months
- P(D,L)LA-co-PGA 85:15 degrades away leaving no residual trace after 5/6 months
- P(D,L)LA-co-PGA 75:25 undergoes significant degradation after 3 months
- P(D,L)LA-co-PGA 50:50 exhibits fibre merging after 2 weeks
What is the Tissue and immune system response to implanted scaffolds?
- No gross inflammatory response to scaffolds
- Difficult to find any trace of where fibres were implanted post degradation
- Rapid neovascularisation (confirmed by CD31 staining)
What are stem cell niches?
well-defined environments that keep stem cells undifferentiated, protected and producing progenitors for tissue repair
present chemical, topological and metabolic characteristics
What are the components of stem cell niche?
- Stromal support cells: including cell-cell adhesion molecules and secreted soluble factors, which are found in close proximity to stem cells.
- Extracellular matrix (ECM) proteins: act as a stem cell “anchor” and constitute a mechanical scaffolding unit to transmit stem cell signaling.
- Blood vessels: carry nutritional support and systemic signals to the niche from other organs and also participate in the recruitment of circulating stem cells from and to the niche.
- Neural inputs: favour the mobilization of stem cells out of their niches and integrate signals from different organ systems
Why a “simplified” Artificial Niche?
It would be advantageous to be able to design and manufacture biomaterial devices containing “reservoirs” of stem cells The concept of “Stem Cell Niche” involves a high degree of complexity
1) Mimic aspects the physiological environment
2) Address key challenges of tissue engineering
3) Stem cell therapies
What would constituent a successful artificial microenvironment (niche)?
1) A structure which provides physical protection 2) Epithelial cells that possess stem cell capability 3) Appropriate ECM adhesion proteins 4) Adjacent stromal cells
What does mimicking the stem cell niche mean?
(i) mimicking the biochemical behavior of the stem cell niche
(ii) mimicking the spatial morphology of a niche.
What are the manufacturing approached to mimicking the stem cell niche?
1) Polydimethylsiloxane (PDMS) Stamping/Molding, also known as soft Lithography
2) Thermal Imprinting Methods
3) Compression methods (e.g. compressed collagen)
4) Use of Microfluidic Platforms
5) Combinations of additive manufacturing techniques and electrospinning
What are rete ridges?
Dermal protusions of 50 µm to 200 µm in depth and 50 µm to 400 µm in width They increase the contact area between the epidermis and the dermis: • Improving shear resistance • Enhancing nutrient diffusion • Aiding keratinocyte differentiation (high production of β1 integrins)
How do rete ridges change among the skin?
- Scalp: shallow
* Palms: narrow
How do rete ridges change when aging?
they tend to disappear (losing regenerative capacity)
How are rete ridges mimicked?
Combination of Additive Manufacturing and Electrospinning
What is biofunctionalisation?
Modification of a material to provide/improve biological function and/or stimulus, maintaining its biocompatibility.
What are examples of a biofunctionalization agent?
- Proteins, growth factors
- Antibodies, fluorescent agents
- Functional Groups (acid/amino)
What Biofunctionalisation Strategies?
Covalent bonding Embedded/trapped Simple adsorption Electrostatic
