, 2002 and Manglapus et al., 2004): Mash1+

GBCs are destroyed by the MeBr, but they reappear in increased numbers SCH 900776 cost two days after the MeBr. Ngn1/NeuroD+ cells are also lost with MeBr damage, but are present 3 days after the damage ( Guo et al., 2010) and precede production of new receptor neurons, which appear by 4 days postlesion. Hes1 is expressed by the sustentacular cells in the normal epithelium, but after MeBr, GBCs also express Hes1, and some of these go on to differentiate into sustentacular cells. Since the olfactory epithelium displays such robust regeneration it begs the question as to why we lose olfactory sensation as we age. The loss of sensory perception can result from changes in the sensory epithelia or, alternatively, from changes in the brain critical for processing the sensory information. There is evidence, however, that the number of receptor cells declines with age in humans. Moreover, in rats and mice, the density A-1210477 of proliferating (BrdU+) cells in the epithelium declines as the size of the epithelium grows (Weiler and Farbman, 1997); thus, while the overall number of proliferating cells does not decline by very much, the turnover of the receptor neurons, as indicated by the number of BrdU/OMP+ cells, declines with age (Kondo et al., 2010). This decline is also seen in the vomeronasal organ of mice, where Brann and Firestein (2010) reported that

the number of proliferating cells declines with age. Taken together, these studies suggest that the production of new receptor cells may not be able to keep pace with the increased loss of these cells that accompanies

increasing age. Some of the first evidence for regeneration of hair cells came from studies of the lateral line organs in fish and amphibia. The lateral line organs of fish and amphibia consist of mechanosensory neuromasts distributed along the body surface. In urodeles, after amputation of the tip of the tail, new neuromasts are generated in the lateral line organ at the stump and migrate to form new organs as the tail regenerates (Stone, 1937). Studies by Jones and because Corwin demonstrated that a low level of ongoing hair cell production is dramatically upregulated after hair cells in the lateral line are destroyed with a laser (Jones and Corwin, 1993 and Jones and Corwin, 1996). Direct time-lapse recordings demonstrated that the regenerated hair cells arose from support cells (Jones and Corwin, 1993). A similar increase in mitotic proliferation in the support cells occurs in zebrafish after various types of ototoxic damage (Hernández et al., 2007, Ma et al., 2008 and Williams and Holder, 2000), and the proliferating support cells go on to replace the hair cells within 48 hr of the insult. Hair cell regeneration has also been extensively studied in both auditory and vestibular sensory organs.