The further development of the intestine involves (1) the differentiation of the Paneth cells and the progeny of intestinal stem cells, which was discussed in Chapter 5; and (2) interactions between the gut epithelium and symbiotic bacteria to complete the differentiation of the cell types, which will be discussed in Chapter 24. Interestingly, in at least parts of the endoderm, differentiated cells can revert to becoming stem cells. This may be due to their being exposed to the outside environment as we eat and breathe. When existing stem cells are removed from the stomach, a type of secretory cell (called the chief cell) loses its differentiated properties and becomes a stem cell (Stang et al. 2013). Similarly, the differentiated cells of the trachea can divide to generate lung stem cells when the original stem cells are removed (Tata et al. 2013). Even the Paneth cell precursor of the intestine, which is usually committed to maturing into differentiated Paneth cells and enteroendocrine cells, can return to a stem cell state if the intestine is injured (Buczaki et al. 2013). So, the endoderm cells appear to be remarkable, if not unique, in having a high degree of plasticity between differentiated and stem states.
Lung stem cells
The stem cells of the airway passages are fascinating in many ways. As mentioned earlier, the endoderm can replace its stem cells by reverting differentiated cells to a stem cell condition. In the lungs, one group of differentiated cells, the Clara cells, can divide to produce stem cells when the original stem cells are removed. In addition, the parent stem cells provide niches for their daughter cells to develop (Pardo-Saganta et al. 2015). The basal stem cells of the trachea produce progenitor cells that will become the secretory and ciliated cells of the airways. The progenitor cells, however, need the Notch signal from their parent cells in order to remain progenitor cells. Without Notch, they undergo terminal differentiation into ciliated cells. Thus, the parent stem cell here is still “taking care” of its daughters and maintaining their proliferative capacities.