Answer
In the processes of growth and maturation, cells differentiate to perform their special functions. At different times, mature cells may become damaged, or worn out, and need to be replaced by cell division, growth, and maturation or differentiation. New cells of a tissue may come from original parenchyma (functional cells ), or from existing stroma (cells of the supporting tissue ).
When tissue is damaged, there may be different types of repair responses. If parenchyma responds adequately, stem cells from protected locations ( for example in the liver,skin, and GI tract), divide, move into the damaged area, differentiate, and replace damaged tissue. In such a case, original function of the tissue is preserved. If, on the other hand, stromal (connective) tissue alone tries to effect repair of the tissue damage, fibrosis takes place , a scar is formed, and the normal function of the tissue is not fully recovered . Full tissue repair requires parenchymal and stromal regeneration, and the acquisition of new blood vessels to nourish the regenerated tissue.
Work Step by Step
Regarding regeneration and repair, tissues may be divided into three groups;
1. Tissues that are always capable of division, and divide continuously. Examples of this type of are
to be found in epidermis, and in the epithelial linings of GI tract, vagina, and oral cavity.
2. Tissues whose mature cells can recover their capacity for cell division if challenged by tissue
damage.This kind of so-called stable cells is found in parenchyma of liver and kidneys.
3. Cells that have permanently lost their capacity to carry out mitosis at full maturity : these cells
replace damaged tissue with collagen fibers through the process of fibrosis which results in scar
formation. This is the result of connective tissue repair.
In general, parenchyma replenishes satisfactorily through the activity of stem cells. However,
all the kinds of muscle tissues repair slowly.
Among connective tissues , cartilage regenerates poorly, but bone usually repairs readily because
of its better blood supply. Finally, under normal circumstances , nervous tissue regenerates very,
very poorly or not at all.
