2. Adult Hippocampal Neurogenesis
Neural stem cells were first identified in the adult brain of rodents more than 50 years ago [6] and are found across a variety of species including humans [7]. Interestingly, the production of new neurons is typically limited to two regions: the subventricular zone, which lines the lateral ventricles and sends new neurons to the olfactory bulb via the rostral migratory stream, and the subgranular zone of the hippocampus [8].
It is important to note that neurogenesis, as defined here, requires the proliferation, survival, and differentiation of newly generated cells into neurons. Any number of internal and/or external factors may independently affect the proliferation of progenitor cells, their differentiation into neurons, or their survival rates [9–14]. Thus merely identifying cells as having been produced in the adult brain, without further demonstrating that they survive and become neurons, is insufficient to conclude that neurogenesis has occurred.
Although new neurons can be labelled and observed, the exact role of these neurons in the function of the hippocampus remains to be fully elucidated. New neurons produced in the subgranular zone have been associated with a number of functions [15, 16]. There is solid evidence that new hippocampal neurons are selectively and imperatively involved in spatial learning and memory [17–20]. New neurons in the hippocampus, perhaps more ambiguously, have also been associated with the etiology and treatment of depression [21–23]. Interestingly, the hippocampus is functionally dissociated along the dorsal-ventral axis, wherein the dorsal region plays a larger role in cognitive faculties, while the ventral region is more involved in emotionality [24, 25]. For the purpose this review, however, we will concentrate on the functions of the hippocampus associated with emotionality and stress and refer the reader to reviews that cover the hippocampus' cognitive associations (See: [26, 27]).