Cell and Tissue Engineering

Tissue engineering refers to the application of the principles of engineering to cell culture for the construction of functional anatomical units- tissues/organs. The aim of tissue engineering is nothing but to supply the various body parts for the repair or replacement of damaged tissues or organs. It is now possible to grow skin cells, blood cells cardiac cells etc. by using the ability of stem cells to proliferate and differentiate.

During the last decade, the tissue culture work in animals demonstrated that virtually any human tissue or organ can be grown in culture. This became possible only after it became known that the ability of cultured cells to undergo differentiation can be restored. ‘Skin’ was the first organ to be cultured in artificial media and could be successfully used for transplantation following serious skin burns. For past few years some of the biotech companies like ATS (Advanced Tissue Science, USA), Biosurface Technology (BTI, Cambridge) and Organogenesis, are developing artificial skins to the stage of clinical trials.

In the field of tissue replacement, focus of attention is the Artificial cartilage. As it is not vascularized, it is not rejected due to immunogenic response. This will have lots of implications in the treatment of sport related injuries and diseases like arthritis.

Design and engineering of tissues

The design and tissue engineering should essentially cause minimal discomfort to the patient. The damaged tissues should be easily fixed with the desired functions quickly restored. Another important factor controlling the designing of tissue culture is the source of donor cells. The cells from the patient himself are always preferred as it considerably reduces the immunological complications. However, under certain situations, allogeneic cells (cells taken from a person other than the patient) are also used. Other important factors are the support material, its degradation products, and cell adhesion characteristics.

It was demonstrated in 1975 that human keratinocytes could be grown in the laboratory in a form suitable for grafting. A continuous sheet of epithelial cells can now be grown; however, it is still difficult to grow tissue-engineered (TE) skin with the dermal layer including blood capillaries, nerves, sweat glands, and other accessory organs.

Some of the implantable skin substitutes (tissue-engineered skin constructs with a limited shelf life of about 5 days) are:

a) Integra™ – A bioartificial material composed of collagen-glycosaminoglycan and is mainly used to carry the seeded cells.

b) Dermagraft™ – Composed of polyglycolic acid polymer mesh seeded with human dermal fibroblasts from neonatal foreskins.

c) Apligraf™ – Constructed by seeding human dermal fibroblasts into collagen gel, with a layer of human keratinocytes placed on the upper surface.

These tissue constructs integrate into the surrounding normal tissue and form a good skin cover with minimal immunological complications.

The urothelial cells and smooth muscle cells from the bladder are now being cultured, and attempts are ongoing to construct TE urothelium. Some progress has also been made in the repair of injured peripheral nerves using tissue-engineered peripheral nerve implants. The regeneration of the injured nerve occurs from the proximal stump to rejoin at the distal stump.

The regeneration process requires substances like:

a) Conduct Material: Composed of collagen-glycosaminoglycans, PLGA (poly lactic-co-glycolic acid), hyaluronan, and fibronectin. It forms the outer layer.

b) Filling Material: Contains collagen, fibrin, fibronectin, and agarose. This supports the neural cells for regeneration.

c) Additives: A large number of other factors are also added, such as growth factors and neurotrophic factors including fibroblast growth factor (FGF) and nerve growth factor (NGF).

Other important applications of tissue engineering include: gene therapy, pseudo-organs, and model cell systems for developing new therapeutic approaches to human diseases.

Attempts are ongoing to create tissue models in the form of artificial organs using tissue engineering. The artificial liver is being developed using hepatocytes cultured as spheroids and held suspended in artificial support systems such as porous gelatin sponges, agarose, or collagen.

Some progress has been made in creating an artificial pancreas using spheroids of insulin-secreting cells derived from mouse insulinoma beta cells.

Three-dimensional brain cell cultures have been used to study neural myelination, neuronal regeneration, and neurotoxicity of lead. Aggregated brain cells are also being used to study Alzheimer’s disease and Parkinson’s disease. Thyroid cell spheroids are being used to study cell adhesion, motility, and thyroid follicle biogenesis.

DOWNSTREAM PROCESSING

Downstream processing or downstreaming is the extraction and purification of the desired end products of fermentation processes. Such products might include cells, solvents or solutes. Various processes are available for the separation of cells from the fermentation broth in which they are grown, including flocculation, filtration, centrifugation, sedimentation or flotation. The procedure adopted depends on whether it is the cells, or the solution surrounding them, that contains the desired end products.