Which cells are characterized as undifferentiated?

Stem cells are characterized as undifferentiated because they have not yet developed into specific cell types with specialized functions. These cells possess the unique ability to divide and differentiate into various cell types, which can then form different tissues and organs in the body. Stem cells can remain in this undifferentiated state for long periods and can respond to certain signals that prompt them to differentiate into specialized cell types when needed. This characteristic is fundamental to their role in growth, repair, and maintenance of the body's systems. The other types of cells listed have already undergone differentiation, meaning they have developed specific functions and characteristics, unlike stem cells.