, 2008). This process also occurs in birds to a limited extent (Fischer and Reh, 2000). The continued production of sensory receptor cells in these epithelia requires a mitotic cell population that can act like the stem cells in nonneural epithelia. In the case of the olfactory epithelium, there are at least two types of mitotic cells: the globose basal cells (GBCs) and the horizontal basal cells (HBCs). The GBCs are mitotically active in the normal, undamaged epithelium and selleck products act as a multipotent progenitor

to generate all of the other types of olfactory cells, including the sensory receptors (Caggiano et al., 1994, Chen et al., 2004 and Huard et al., 1998). The more slowly cycling (or even quiescent) HBCs are more like “stem cells” serving both to replenish the more actively proliferating GBCs (Iwai et al., 2008) and as a reserve pool after more extensive damage to the receptors (Leung et al., 2007). The model of a slow-cycling stem cell (HBC) with a more rapidly cycling, transit-amplifying progenitor cell (GBC) has similarities with nonneural epithelia, like the epidermis (Watt et al., 2006).The situation in the vestibular system of nonmammals is somewhat different, in that there does not appear

learn more to be a committed hair cell progenitor. Rather, it appears that some or all of the support cells remain capable of mitotic division and divide at a low rate to produce both additional hair cells and support cells as the epithelium grows. In the retina, the source of the new rods in the fish is a group of cells

called the rod precursors (Johns and Fernald, 1981), which typically generate only rod photoreceptors under normal conditions and are likely derived from the Müller glia (more on this later). The different progenitors/precursors in these systems also share some common molecular expression patterns that are similar to those expressed during initial development (see Figure 3 and further discussion below). In the olfactory epithelium, for example, at least some of the GBCs express Ascl1, Org 27569 Neurog1, Sox2, and Pax6, genes critical during olfactory epithelial development (Guo et al., 2010 and Manglapus et al., 2004). NeuroD1 is expressed at a slightly later stage, in the cells that will differentiate into the olfactory receptor neurons. In the inner ear, the support cells also express Sox2 (Oesterle et al., 2008), and many of the support cells that are in the S-phase or M-phase of the cell cycle, as well as the newly generated postmitotic daughters, express Atoh1 (Cafaro et al., 2007).