ABSTRACT
The eukaryotic cell is a very complex structure that can perform a large variety of specialized functions. There are more than 200 different types of eukaryotic cells in vertebrate animals. These range from the pluripotential hematopoietic stem cell to the highly specialized nerve cell. These different types of cells are grouped into integrated units that make up the various tissues and organs. For example, the gastrointestinal tract in humans consists of different types of absorptive and secretory epithelial cells, muscle cells, immune cells, nerve cells, endothelial cells, connective tissue cells, and endocrine cells. Many of these cell types are short lived. In order to function properly, every organism must be able to replace lost cells. This regenerative capacity is highly variable from tissue to tissue, is tightly regulated in most circumstances in order to allow the new cells to assume their proper differentiated functions, but is altogether nonexistent in certain cell types in the adult organism.
