During development, an undifferentiated cell will become part of a particular type of tissue—this is referred to as cell fate. Cell fate determination occurs as the embryo develops. The timing of this determination varies with the organism, but it is typically quite early. One way the timing can be revealed is to transplant cells from one embryo to a different region of a recipient embryo (Figure 19.2). Do the transplanted cells adopt the differentiation pattern of their new surroundings, or do they continue on their own path, with their fate already sealed?

Experiments on amphibian embryos indicate that determination happens early in development. If the donor tissue is from an early-stage embryo (blastula), it adopts the fate of the new surroundings. In this case, cell fate has not been determined and is influenced by the extracellular environment. But if the donor tissue is from an older embryo (gastrula), it continues on its original developmental path. In this case, cell fate has already been determined and is no longer influenced by the extracellular environment.

Cell fate determination is influenced by changes in gene expression as well as the extracellular environment. You can’t see cell fate determination by looking at an embryo under the microscope—cells do not change their appearance when they become determined. Rather, the changes that mark determination are within the cell’s molecular makeup. Determination is followed by differentiation—the actual changes in structure and function that result in different cell types. Determination is a commitment; the final realization of that commitment is differentiation.

During animal development, cell fate becomes progressively more restricted. This can be thought of in terms of cell potency, which is a cell’s potential to differentiate into other cell types: