Adult neurogenesis has been questioned for many years. In the early 1900s, a dogma was established that denied new neuron formation in the adult brain. In the last century however, new discoveries have demonstrated the real existence of proliferation in the adult brain, and in the last decade, these studies led to the identification of neural stem cells in mammals. Adult neural stem cells are undifferentiated cells that are present in the adult brain and are capable of dividing and differentiating into glia and new neurons. Newly formed neurons terminally differentiate into mature neurons in the olfactory bulb and the dentate gyrus of the hippocampus. Since then, a number of new research lines have emerged whose common objective is the phenotypical and molecular characterization of brain stem cells. As a result, new therapies are successfully being applied to animal models for certain neurodegenerative diseases or stroke. At present, and in years to come, this finding extends to the adult human brain, and gives reason and hope to all the previous studies.
The concept of adult neurogenesis is relatively modern. It has surprised the scientific community and has ruled out the established idea that we are born with a set number of neurons. This discovery has come about progressively throughout the last century. In this work, the authors review some of the methods of research in adult neurogenesis placing emphasis on electron microscopy, a technique in which they are well practiced. Electron microscopy has been essential in the description of the cytoarchitecture of the different cell types populating the subventricular zone (SVZ), an area that underlies the lateral wall of the lateral ventricle, and where new neurons are generated during adulthood. Time-dependent studies have elucidated the temporal profile and lineage progression of SVZ cells from the generation of new cells to the integration into their target tissue, the olfactory bulb. Similarly, adult neurogenesis has been described in the dentate gyrus of the hippocampus, and is being investigated in other regions of the cerebrum and the spinal cord. Neurogenesis was first demonstrated in mice, but has been subsequently observed in other mammals such as rabbits, cows, monkeys, and humans. The meaning of neurogenesis in humans remains unknown, and it is essential to understand it for potential application in the therapy of neurological diseases.
