Key Concepts of Section 21.3

• Planaria contain pluripotent stem cells termed cNeoblasts that are important for regeneration of body parts removed by amputation.

• Most stem cells in animals are multipotent, except for germ-line stem cells that are unipotent.

• Stem cells are undifferentiated; they can undergo symmetric or asymmetric self-renewal divisions such that their number remains constant or increases over the organism’s lifetime (see Figure 21-11).

• Stem cells are formed in niches that provide signals to maintain a population of undifferentiated stem cells. The niche must maintain stem cells without allowing their excess proliferation and must block differentiation.

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• Stem cells are prevented from differentiating by specific controls that operate in the niche. A high level of β-catenin, a component of the Wnt signaling pathway, has been implicated in preserving stem cells in the germ line and intestine by directing cells toward self-renewal division rather than differentiation states.

• In the Drosophila germarium, a few cells form the germ stem-cell niche, sending signals directly to the adjacent stem cells. Daughter cells that are displaced from the niche cells undergo proliferation and differentiation into germ cells (see Figure 21-12).

• Populations of stem cells associated with the intestinal epithelium and many other tissues regenerate differentiated tissue cells that are damaged, sloughed, or aged (see Figure 21-13).

• Intestinal stem cells reside in the bases of intestinal crypts, adjacent to Paneth cells, which form part of the niche, and are marked by expression of the Lgr5 receptor (see Figure 21-13).

• In the blood-cell lineage, different precursor types form and proliferate under the control of distinct cytokines (see Figure 21-17). This system allows the body to specifically induce the replenishment of some or all of the necessary blood-cell types.

• Hematopoietic stem cells can be detected and quantified by bone marrow transplant experiments (see Figure 21-18) and their niche cells detected using a combination of marker surface proteins (see Figure 21-19).

• Plant stem cells persist for the life of the plant in the meristem. Meristem cells can give rise to a broad spectrum of cell types and structures (see Figure 21-20).

• A negative feedback loop involving the WUS transcription factor maintains the size of the shoot apical stem-cell population.