New neurons from old beliefs in the adult piriform cortex? A Commentary on: "Occurrence of new neurons in the piriform cortex"

In a recent mini-review (Yuan et al., 2015), support is given to the idea that neurons are generated during adulthood in the mammalian piriform cortex (PC), their periventricular origin being also discussed. It is known since long time that a subpopulation of cortical layer II cells in the adult PC of rodents express immature neuronal markers such as polysialylated NCAM (PSA-NCAM; Seki and Arai, 1991; Bonfanti et al., 1992) and doublecortin (DCX; Nacher et al., 20)2). These immature neurons have been found in most mammals studied so far, their occurrence being restricted to the paleocortex in rodents (Seki and Arai, 19) 1; Bonfanti el al., 1992; Nacher et 2002), and extended to neocortical areas in species with increasing brain size and gyrencephaly, e.g., rabbits, cats, and primates (Bonfanti and Nacher 2012). The fact that these cortical cells express markers of immaturity which are usually present in the young neurons produced within adult neurogenic sites (Bonfanti and Theodosis, 1)91; Brown et al., 2003), suggested the possibility that they might be newly generated. Across the years, different laboratories undertook 5-bromo-2-deoxyuridine (BrdU) pulse-chase experiments to test this hypothesis. A first group of studies reported that some of the cells in the PC layer II have been generated during adulthood (Bernier et al., 2002 Pekcec et 2006; Shapiro et al., 2007a,b). However, the number of recently generated neurons was extremely low, they appeared to have a transient existence, and their precise location in the PC was not properly indicated. In parallel, many other studies did not find evidence for the incorporation of new neurons in the PC of adult rats (Nacher et al., 2002; Gomex-Climent et al., 2008), rabbits (Luzzati et al., 20)9), and cats (Varea et al., 2011). These latter studies by no means exclude the possibility of adult neurogenesis in this area; yet, for their relevance, they should be reported in any review article dealing with such controversial issue. By contrast, in their mini-review Yuan et al. deliberately chose to ignore these articles, together with those that have provided solid evidence for the embryonic origin of most of these cells ((Gomez-Climent et al., 2008), suggesting that they represent a "reservoir" of non-newly generated, immature neurons which maintain features of structural plasticity throughout life (Bonfanti and Nacher, 2012).