Vascularisation in tissue engineering

Question 1

What components make up the vascular system?

Answer 1

Macrovessels: arteries and veins (1mm-1.5cm diam)

microvessels: arterioles and venules (7un - 1mm)
cappillaries: facilitate nutrient distribution (5-10um)

Question 2

What is angiogenesis?

Answer 2

• Growth factor released by hypoxic cells
• Metalloproteinases released by endothelial cells
• budding and sprouting
• example from tumour angiogenesis (similar process)

Question 3

What are the problems with vascularisation around the implant?

Answer 3

• TE constructs can be made in perfusion bioreactor
• once implanted large implants regions over 200um become hypoxic
• wound healing around implant
• response is vascularisation of implant (angiogenic factors)
• vascularisation is too slow

Question 4

What are the four strategies of vascularisation?

Answer 4

• scaffold design
• inclusion of angiogenic factors
• in vitro prevascularisation
• in vivo prevascularisation

Question 5

What is scaffold design?

Answer 5

Pore size is >250um for vascular ingrowth

Random pores vs ordered pores

Question 6

What is the difference between random and ordered pores in scaffold design?

Answer 6

RANDOM

• gas foaming
• phase seperation
• freeze drying
• particle loading
• contorted pathways
• interconnectivity issues

ORDERED

• additive manufacturing
• controlled sized and shape pathways
• interconnectivity is excellent and tuneable

Question 7

What is angiogenic factor delivery?

Answer 7

• vascular endothelial GF and basic fibroblast GF: recruitment of endothelial cells
• delivery of these GF: leaky and disorganised (dependent on dosage)
• vessels need to be stabilised by SMCs and PCs and ECM production
  (platelet derived GF: recruitment or pericytes and SMCs)
  (transforming GF-beta : production of ECM)
  (angioprotein 1)

Question 8

What are indirect approaches to angiogenic factor delivery?

Answer 8

Sonic hedgehog analog
Hypoxia inducing factor 1
Bone morphogenic protein (2,4,6)

Question 9

What are the benefits of indirect approaches to angiogenic factor delivery?

Answer 9

Induce cells around the implant to produce angiogenic factors

• cells produce the correct concentrations
• GF gradients help capillary
• cocktails of angiogenic factors (VEGF, Ang-1 and -2) is released by cells to regulated vessel formation and maturation

Question 10

What are examples of avascular tissue?

Answer 10

Cartilage, skin epidermis, cornea and len

Question 11

What is the role of platelet derived GF?

Answer 11

Recruitment of pericytes and SMCs

Question 12

What is the role of Transforming GF-beta

Answer 12

Production of ECM, correct interaction in between mural and endothelial cells

Question 13

What are the different approached to angiogenic factor delivery?

Answer 13

recombinant protein and gene delivery via biomaterial
• Genetically engineered cells to overexpress particular GFs
• Release can be linked to diffusion of Biomaterial can be biodegradable. Is not ideal, and difficult to tune

Question 14

What is in vivo prevascularisation?

Answer 14

1) In vitro construct manufacture,
2) in vivo implantation near artery or (wrapped in) axially vascularised tissue
3) vascularisation period (several weeks)
4) harvest with feeding artery
5) re-implantation with anastomosis of vascular axis

Question 15

What is in vitro prevascularisation?

Answer 15

• Endothelial cells form prevascular structures when cultured correctly in vitro
• Upon implantation the vascular network can anastomose spontaneously on ingrowing vasculature
• Skin, skeletal and cardiac muscle and bone have been tried

Question 16

What are the advancements that need to be made in in vitro prevascularisation?

Answer 16

• Need to find culture conditions for both cell types: Use of growth factors VEGF could negatively affect other cells (e.g. MSCs differentiate to fibroblasts instead of bone)
• Endothelial cells organise without using angiogenic factors
• Has shown effective for skin and muscle but not bone
• Source of endothelial cells is crucial (HUVECs, HDMECs or Endothelial Progenitor Cells)

Question 17

What is an alternative approach to in vitro prevascularisation?

Answer 17

Use a vascularised tissue e.g. rats intestine, decellullerise it and use it as a vascular network.
• Decellullarised tissue doesn’t invoke the immune response

Question 18

What are the advantages and disadvantages to scaffold design?

Answer 18

+ versatile strategy
+ easy to develop
+ easy to translate to multiple tissues

• still relies on vessel ingrowth from host
• limites result to be expected if used as only strategy
• can introduce, for instance, cell seeding problems

Question 19

What are the advantages and disadvantages to in vitro prevascularisation?

Answer 19

+ does not rely on ingrowth of host vasculature into entire contruct
+ no extra surgery necessary

• complex strategy, varying from tissue to tissue
• vessel maturation in vitro needs attention
• anastomosis not as fast as with in vivo prevascularisation

Question 20

What are the advantages and disadvantages to in vivo prevascularisation?

Answer 20

+ direct perfusion after microsurgery
+ mature, organised vasculature

• extra implantation/surgery necessary
• need of finding a proper location with vascular axis for prevascularisation
• scaffold might be filled with fibrous tissue during intial implantation