Final answer:
Cell differentiation is the process where unspecialized cells become specialized to perform distinct functions in a multicellular organism, with the role of stem cells being crucial in the early stages before cells commit to specific lineages.
Step-by-step explanation:
When a cell in a multicellular organism becomes committed to differentiating into a specific cell type, the terminal fate is normally maintained through all subsequent generations. During this differentiation process, the previously unspecialized cells undergo significant changes in size, shape, metabolic activity, and overall function to take on a specialized role such as that of nerve cells, muscle cells, or epithelial cells. Stem cells are key in this process as they have the potential to differentiate into any cell type in the body. However, soon after development begins, the totipotency of cells is lost, and they become committed to specific lineages, a commitment that is maintained throughout the organism's life through unique genetic expressions of each cell that dictate a cell's morphological and physiological characteristics.