Lecture 6- Bio-Engineering Models (Skin Bio-engineering)

Why are organ-specific in vitro tissue models needed?

Because tissues in the body are highly organized, complex, and multicellular, so regenerative medicine usually needs a model of a specific organ system.

What common structural feature do skin, trachea, lung, oesophagus, stomach, and intestine share?

They are organized into distinct layers, each with a specific function.

What are the two main compartments in many tissue models?

• Epithelial compartment

• Stromal compartment

What is the function of the epithelial compartment?

It performs the tissue’s main activity, such as absorption, secretion, or protection.

What is the function of the stromal compartment?

It provides:

• physical support

• ECM

• chemical and mechanical signals

• nutrient/oxygen support and waste remova

What are the main layers of human skin?

• Epidermis

• Dermis

• Hypodermis

What are the main function of skin?

Barrier protection

Which cell types are found in skin?

• Keratinocytes

• Melanocytes

• Fibroblasts

• Immune cells

• Sensory neurones

What are the three main types of in vitro skin equivalent shown in the lecture?

• Epidermal model

• Dermal model

• Full-thickness model

Why is there a need for bioengineered human skin equivalents?

Because cosmetic testing on animals was banned in Europe, increasing the need for animal-free in vitro skin models.

What is a major limitation of traditional 2D cell culture?

Cells grow as flat sheets on plastic, which is artificial and not representative of in vivo tissue structure.

Why do 2D cultures often give non-physiological responses?

Because cells become flattened, have altered polarity and reduced cell-cell/cell-matrix interactions, so function changes.

Why are 3D culture environments preferred?

Because they better mimic the in vivo microenvironment and produce more realistic cell structure and function.

What criteria was Alvetex designed to meet?

It had to be:

• inert

• porous

• stable for long-term culture

• adaptable to existing culture formats

• scalable for mass production

Why was polystyrene used for Alvetex?

Because it is inert, consistent, familiar in cell culture, and avoids adding extra biological cues.

What are the key structural properties of Alvetex membranes?

• >90% porosity

• about 200 µm thick

• provide the third dimension for cell growth

Why is the 200 µm thickness important in Alvetex?

Because tissue diffusion limits are around 200 µm, helping prevent tissue death from poor nutrient or gas supply.

How is the Alvetex material made?

Using a high internal phase emulsion (HIPE), followed by polymerization and removal of the aqueous phase to leave a porous structure.

What monomers are used in the Alvetex material?

• Styrene

• Divinylbenzene (DVB) as crosslinker

• 2-ethylhexyl acrylate (EHA) for flexibility

What are the three Alvetex formats described?

• Alvetex Scaffold

• Alvetex Strata

• Alvetex Polaris
  

How do the three Alvetex formats differ?

Mainly by void size and pore size, which affects whether cells can fully enter, partially enter, or not enter

Why are 3D culture systems significant?

Because structure relates to function — 3D systems often show enhanced properties and are more representative of in vivo tissues.

What benefit did 3D culture show for hepatocytes?

Hepatocytes in 3D showed better viability and more in vivo-like structure than in 2D culture.

What culture sequence is used to generate a full-thickness skin model?

• Fibroblast pre-culture

• Submerged culture

• Air-liquid interface

What does immunofluorescence show in the epidermal/full-thickness skin equivalents?

It shows:

• sequential keratinocyte differentiation

• basement membrane formation

• intercellular junctions

• endogenous ECM deposition

What does TEM show in the full-thickness skin equivalent?

It shows:

• desmosomes

• hemidesmosomes

• corneodesmosomes

• terminal differentiation in the stratum corneum

How can skin model complexity be increased?

By adding more cell types

What is the main role of melanocytes in skin?

They produce melanin, which is transferred to keratinocytes to protect nuclear DNA from UV damage

What happens in pigmented skin equivalents after chronic UV exposure?

They show:

• increased melanin deposition

• darker skin tone

• increased melanocyte density and dendricity

What did the sunscreen proof-of-concept study show?

Without sunscreen, UV caused visible epidermal damage; with SPF50+, the epidermis remained organized and intact.