Neurogenesis in the Hippocampus of an Adult Marsupial

In the adult eutherian brain, stem cells in the dentate gyrus continually divide throughout adult life and into old age producing new granule cells. However, it was not known whether this is also the case for marsupials. Previously, in fact, it was thought that marsupials did not have continued neurogenesis in the mature brain. Here we examined neurogenesis in the adult brain of a small marsupial, the fat-tailed dunnart, using 3H-thymidine to label newly generated cells. We showed that neurogenesis takes place in the adult dentate gyrus along its inner margin, as seen in eutherian mammals. Control animals had similar numbers of labeled cells 24 h and 1 month after 3H-thymidine injection. An enriched environment resulted in similar numbers of cells being generated as controls. However, there was a significant decline in the number of labeled cells one month later. Stress and old age resulted in significantly lower numbers of new cells being generated. In immunohistochemically treated control brains, 3H-thymidine-labeled cells at the early stage were sometimes GFAP positive, were not calbindin positive at either stage examined and at the later stage were PSA-NCAM positive. We hypothesize that, as seen in eutherian mammals, the new cells progressed from being GFAP positive at stem cell stage to PSA-NCAM positive during outgrowth of mossy fibers 1 month later, to calbindin positive when mature. It is possible that maturity of these cells was not reached by 1 month as marsupials have a slower metabolic rate and this species also undergoes daily periods of torpor.