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A.  Define stem cells and differentiation, and describe the different types and the extent to which they are present during the life of an individual (include the fertilized egg as well as the adult that eventually results).
B.   Which of the four tissue types contain populations of stem cells of known function, and what is that function? Which types of tissue have stem cells of unknown function? Do any types of tissue lack stem cells?
C.   How do fully differentiated cells differ from stem cells? How can stem cells be used in medical treatment? Give examples.
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12 years ago
A.   Stem cells are cells that are mitotic and not fully differentiated. Differentiation is the process cells undergo as they become more and more specialized in structure and function as is typical of the specific tissues they compose. The fertilized egg and the cells resulting from the first few cleavage divisions after fertilization are totipotent, which means they can ultimately produce all the types of cells in an adult. Within the first week after conception, the cells begin differentiation and become capable of producing many types of cells but not all. These cells are pluripotent. By adulthood, stem cells can be described as multipotent, which are stem cells that can produce many of the cell types in a specific tissue, and committed stem cells, which can only become one specific cell type within that tissue. 

B.   Epithelial and connective tissues have active populations of multipotent stems cells, which replace cells lost to damage or normal turnover. Muscle and nervous tissues contain stem cells but appear to be unable to replace lost cells. They were once thought to lack stem cells altogether. 

C.   Once cells are fully differentiated, they can no longer divide to produce more cells. Disease conditions or injuries that result in cell loss may be remedied by treatment with appropriate stem cells. Examples are neurological injuries and diseases marked by irreversible damage to cells that cannot (yet) be replaced by new cells.
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