In single-celled organisms, both daughter cells usually (though not always) resemble the parent cell. Similarly, in multicellular organisms, when many types of cells divide, the daughter cells look a lot like the parent cell—liver cells, for instance, divide to generate liver cells with the same characteristics and functions as their parent, as do insulin-producing cells in the pancreas. In contrast, stem cells and certain other undifferentiated cells can generate multiple types of differentiated descendant cells; these cells often divide in such a way that the two daughter cells are different. Such asymmetric cell division is characteristic of stem cells and is critical to the generation of different cell types in the body (see Chapter 21). Often one daughter cell resembles its parent in that it remains undifferentiated and retains its ability to give rise to multiple types of differentiated cells. The other daughter cell divides many times, and each of its daughter cells differentiates into a specific type of cell.
The planarian Schmidtea mediterranea is best known for its capacity to regenerate complete individuals—with a normal head—from minuscule body parts formed by dissection (see Figure 1-22e). Planaria contain stem cells that replace cells lost to normal turnover. In portions of a dissected animal, they will, after several cell divisions, generate any cell type needed during regeneration. These stem cells have served as a potent experimental system to discover how heads and tails, each built of many types of cells, are formed (see Chapters 16 and 21). The hormones that instruct stem cells in different parts of the body to generate specific types of cells are similar to those used in mammals, including humans, in development (see Chapter 16), and thus future studies on planarian regeneration may inform scientists how to regenerate human body parts such as a hand or an eye.