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, is 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. The other important factors are –the support material, it’s degradation products, cell adhesion characteristics etc. 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 be grown now however there is still difficult to grow TE skin with the dermal layer with all the blood capillaries, nerves, sweat glands, and other accessory organs.

Some of the implantable skin substitutes which are tissue engineering skin constructs with a limited shelf life of about 5 days are:
a) Integra TM – A bioartificial material composed of collagen-glycosaminoglycan and is mainly used to carry the seeded cells.
b) DermagraftTM- This is composed of poly glycolic acid polymer mesh seeded with human dermal fibroblasts from neonatal foreskins.
c) ApligrafTM- It is constructed by seeding human dermal fibroblasts into collagen gel with the placement of a layer of human keratinocytes on the upper surface.

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

The urothelial cells and smooth muscle cells from bladder are now being cultured and attempts are on 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 distal stump.

The regeneration process requires substances like-
(a) Conduct material- The conduct material is composed of collagen- glycosaminoglycans, PLGA (poly lactic- co- glycolicacid), hyaluronan and fibronectin and forms the outer layer.
(b) Filling material- The filling material contains collagen, fibrin, fibronectin and agarose. This supports the neural cells for regeneration. and
(c) Additives- A large number of other factors are also added e.g. growth factors, neurotrophic factors such as fibroblast growth factor (FGF), nerve growth factor (NGF).

 The other important applications of tissue engineering are in gene therapy, pseudo-organs and as model cell systems for developing new therapeutic approaches to human diseases.The attempts are on to create tissue models in the form of artificial organs using tissue engineering. The artificial liver is being created using hepatocytes cultured as spheroids and held suspended in artificial support system such as porous gelatin sponges, agarose or collagen. Some progress has been made in the area of creating the artificial pancreas using spheroids of insulin secreting cells which have been developed from mouse insulinoma beta cells.

Three dimensional brain cell cultures have been used for the study of neural myelination, neuronal regeneration, and neurotoxicity of lead. The 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. (Table 8.2 page 155, gupta)

Table depicting the technological goals and areas of research in tissue engineering

Growth of cells in three- dimensional systems

Delivery systems for protein therapeutics

Cell cultivation methods for culturing ‘recalcitrant cells’

Expression of transgenic proteins in transplantable cells

To develop vehicles for delivering transplantable cells

Development of markers for tracking transplanted cells

Avoiding immunogenicity in transplantable cells

Development of in vivo and ex vivo biosensors for monitoring cell behaviour during tissue production

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.

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