Figure 1. Stages in the differentiation of a stem cell. Embryonic stem cells are nearly totipotent, whereas a stem cell in an adult tissue may be pluripotent (for example, hematopoietic stem cell) or unipotent (epithelial cell, for example). An adult tissue may contain the stem cells and precursor cells separated from full differentiation by one or several steps. The periodontal ligament has been shown to contain precursor cells or progenitors to cementoblasts (Pitaru et al., 1994). The differentiated cell may also consist of subpopulations of the same cell type (McCulloch and Bordin, 1991; Fries et al., 1994).