•In a simplified model, think
of the 210 kinds of cells found in the human body as falling in five categories:
•The list is in order of
increasing cell-type specificity and decreasing potency to differentiate into other cell types. Starting at conception and throughout life,
all cells on this list except the
senescent ones will selectively reproduce and possibly differentiate into cells of types further down in the
list.
•In some cell lines there are
actually many more intermediate forms of progenitor cells, but a model of five categories of cells is sufficient
for this discussion.