Application of Molecular Imaging in Transgenic Animals

In 1983, Schofield indicated that the stem cell was defined as that cell in a tissue which, under normal circumstances, maintained its own population, undiminished in function and size, and provided new functional cells for the tissue of origin. The daughter cells may or may not have to undergo further differentiation and/or maturation in order to achieve their functional stage. The fundamental characteristic of a stem cell, therefore, is self-regeneration[1]. Functionally, stem cells are unspecialized cells that have two defining properties: the ability to differentiate into other cells and the ability to self-regenerate. Stem cells are the undifferentiated cells capable of dividing to maintain a reserve of undifferentiated cells. The ability to differentiate is the potential to develop into other cell types. Stem cells can be divided into four types based on their ability to differentiate. The first is a totipotent stem cell that can develop into all cell types including embryonic membranes, such as the fertilized egg. The second is a pleuripotent stem cell that can develop into cells from all three germinal layers, such as embryonic stem (ES) cells that are obtained from the inner cell mass and they have the ability to differentiate into more than 200 types of cells derived from the body, except for the cells of embryonic membranes[2]. The third is the multipotent stem cell that can expand its potential beyond the tissue from which it is derived, and it has the ability to differentiate into other cells which cannot develop into an entire body.