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Regenerative capabilities of Tissues

Following an injury, tissues may heal or regenerate. This involves the restitution of a tissue identical to that lost by injury. Healing is a fibro proliferative response that �patches’ tissue rather than restores a tissue. However some tissues are capable of complete regeneration following an injury for example bone and skin epithelial tissue. For tissues incapable of regeneration, repair is accomplished by ECM deposition, producing a scar for example cardiac muscle and nervous tissue of the CNS.

i. Tissues that regenerate poorly or not at all - These include nervous tissue (neurons) in the CNS, cardiac muscle tissue and cartilage. Mainly replaced by scar tissue, fibrosis in the cardiac muscles and cartilage, and gliosis in nervous tissue.

ii. Tissues that regenerate effectively - These include epithelial tissue, bone, adipose tissue, smooth and skeletal muscle (to some extent), tendon

i. Tissues that regenerate poorly (with description of repair process)

• Nervous tissue :

In the peripheral nervous system вЂ" After damage to an axon (when it is cut or sectioned) in the PNS, the nerve cell undergoes a process known as chromatolysis. The cell swells and its Nissl granules disappear. The myelin degenerates and gets broken up into segments called ovoids. The Schwann cells in the vicinity engulf the ovoids and break them down into small vacuoles, which are eventually removed by macrophages.

In addition to their phagocytc activity the Schwann cells proliferate and those from the segment distal to the cut join with those proliferating from the proximal stump. Thus continuity is established across the gap by the Schwann cells. Now from the portion of the axon still connected to the cell body neurofibrils sprout out, and grow at the rate of about 1mm per day. Some of these fibrils may reach the end-organ and form the replacement axon.

The final stage is the reformation of myelin sheath as the axon regenerates and increased in diameter.

The effective end result of the healing process depends on various factors; If the nerve trunk is not severed in a neuron with a damaged axon an excellent regeneration can be expected. Functional recovery is more complete when a pure motor or sensory nerve is cut, whereas mixed nerves like the median nerve may show poor recovery.

In the central nervous system вЂ" The role of the Schwann cells is replaced by the oligodendroglia. The damaged neural tissue shows chromatolysis followed by necrosis and is often replaced by neuroglia to form glial scar tissue. But, there is considerable evidence that some amount of regeneration is possible and is sometimes associated with the presence of neural stem cells.

• Cardiac