The study investigates the decline of hippocampal neurogenesis in humans from fetal to adult stages. Key findings include: 1. **Fetal and Early Postnatal Development**: In the fetal brain, a continuous field of Ki67+ SOX1+ and SOX2+ cells is observed between the dentate neuroepithelium (dNE) and the proximal blade of the dentate gyrus (DG). By 22 weeks post-conception, these cells are distributed throughout the hilus and granule cell layer (GCL). 2. **First Year of Life**: The number of Ki67+ SOX1+ and SOX2+ cells decreases significantly in the first year of life, but they do not form a discrete layer beneath the GCL. 3. **Adolescence and Adulthood**: By 7 years of age, young neurons (DCX+ PSA-NCAM+) are rare in the GCL, and by 13 years of age, they are almost undetectable. In adult normal and epileptic patients, young neurons were not found in the DG. 4. **Monkey Model**: In macaques, a proliferative subgranular zone (SGZ) is present in early postnatal life but diminishes during juvenile development, with a sharp decline in neurogenesis by 7 years of age. 5. **Ultrastructural Analysis**: Ultrastructural analysis shows that DCX+ cells in the DG at birth have a distinct morphology, while those at 7 years of age have more mature neuronal characteristics. 6. **Gene Expression Profiling**: Gene expression data from macaque and human hippocampus show a sharp decline in DCX, TUJ1, and Ki67 expression by 7 years of age. The study concludes that human hippocampal neurogenesis declines rapidly during the first years of life and is extremely rare or absent in adults, raising questions about the plasticity and function of the dentate gyrus in humans compared to other species.The study investigates the decline of hippocampal neurogenesis in humans from fetal to adult stages. Key findings include: 1. **Fetal and Early Postnatal Development**: In the fetal brain, a continuous field of Ki67+ SOX1+ and SOX2+ cells is observed between the dentate neuroepithelium (dNE) and the proximal blade of the dentate gyrus (DG). By 22 weeks post-conception, these cells are distributed throughout the hilus and granule cell layer (GCL). 2. **First Year of Life**: The number of Ki67+ SOX1+ and SOX2+ cells decreases significantly in the first year of life, but they do not form a discrete layer beneath the GCL. 3. **Adolescence and Adulthood**: By 7 years of age, young neurons (DCX+ PSA-NCAM+) are rare in the GCL, and by 13 years of age, they are almost undetectable. In adult normal and epileptic patients, young neurons were not found in the DG. 4. **Monkey Model**: In macaques, a proliferative subgranular zone (SGZ) is present in early postnatal life but diminishes during juvenile development, with a sharp decline in neurogenesis by 7 years of age. 5. **Ultrastructural Analysis**: Ultrastructural analysis shows that DCX+ cells in the DG at birth have a distinct morphology, while those at 7 years of age have more mature neuronal characteristics. 6. **Gene Expression Profiling**: Gene expression data from macaque and human hippocampus show a sharp decline in DCX, TUJ1, and Ki67 expression by 7 years of age. The study concludes that human hippocampal neurogenesis declines rapidly during the first years of life and is extremely rare or absent in adults, raising questions about the plasticity and function of the dentate gyrus in humans compared to other species.