The essential function of nervous tissue is communication. Nervehave the capacity to react to various physical and chemical stimuli: this is called nervous response. Furthermore, they can conduct these impulses along their cell body and eventually.transmit them to other nerve or to muscle fibres. Nerve can be very long, sometimes more than one metre, such as the nerve cell stretching from the spinal cord to the big toe. Because nervous tissue is so specialized, connective tissue cells are needed for support, nourishment and insulation. Under a microscope this connective tissue looks white, whereas nerve cells look grey.
The brain’s outer layer is made up of cortical tissue called grey matter. Here the cell bodies of the numerous nerves can be found. The interconnections between the various parts of the cortex lie more deeply in the brain and are called white matter. In the spinal cord the cell bodies of the nerve cells lie in the middle, with the grey matter surrounded by white matter. Although nervous tissue is the most highly specialized tissue in our body, it has a disadvantage: unlike other tissue types, it does not have the ability to replace nerve cells that are damaged.
We all have a unique tissue type. Although extremely similar, even the tissue types of identical twins are different. Put another way, our bodies recognize the difference between our own tissue and that of anyone else. It is for this reason that transplant operations involving donor organs do not have a higher rate of success.
The barrier to transplantation is the process of rejection: the body destroys foreign tissue in much the same way as it attacks an invading virus. Although not all the complexities of the process are understood, scientists have made a great deal of headway in pinpointing the mechanisms involved. All normal cells have complex proteins on their surface called histocompatibility antigens. If the body does not recognize these antigens as being its own, the immune system swings into action to produce antibodies to expel the intruder. To prevent organ rejection, therefore, it is necessary to somehow trick the body into accepting a tissue graft as ‘self. A number of drugs are administered which have the ability to suppress the body’s natural immune response, although it is not known for certain how they work. Of these, Cyclosporin, discovered by accident in the late 1970s, is the most commonly used.
Another approach to organ transplants has been made possible by new techniques that allow a person’s tissue type to be reliable determined. As stated previously, no two people share exactly the same set of antigens. They may, however, have one or more in common. The greater number of antigens that are shared by a potential donor and the recipient, the greater the chance that the organ will not be rejected. Different laboratories throughout the world have therefore standardized their system for tissue typing.