Tokyo Medical University discovers previously unknown population of pioneer cells present during the formation of the hippocampus

Lecturer Hiroshi M. Shinohara and Professor Tokiharu Takahashi from the Department of Histology and Neuroanatomy at Tokyo Medical University have made the world's first discovery of a previously unknown population of pioneer neural progenitor cells during the formation of the hippocampus, the brain's center for memory and learning. This was revealed by tracking cells labeled at the very early developmental stage of embryonic day 12.5 using "in utero electroporation," a technique for directly introducing genes into the hippocampus of mouse embryos that Shinohara established through years of trial and error. The hippocampus is an essential brain region for memory and learning, and understanding its formation mechanism could help uncover the causes of psychiatric and neurological disorders such as developmental disabilities, dementia, schizophrenia, and epilepsy. The findings were published in Cellular and Molecular Neurobiology.

The hippocampus is the first region affected during Alzheimer's disease, and morphological and functional abnormalities have also been reported in depression and PTSD. Abnormal hippocampal formation during the fetal period is known to be associated with epilepsy, schizophrenia, and autism spectrum disorder. However, many mysteries remain around hippocampus formation.

Previous research has primarily relied on methods using genetically modified mice to track only cells expressing known genes, but this approach misses cells that do not express known genes. Additionally, because the hippocampus is located deep within the brain and has a complex three-dimensional structure, accurately labeling target cells with conventional gene transfer techniques has been extremely difficult.

After more than ten years of trial and error, Shinohara optimized in utero electroporation for hippocampal research, a technique for directly introducing DNA into specific regions of the fetal brain. Specifically, he fine-tuned the electrode angle through one-degree increments and tested hundreds of combinations of voltage, pulse width, and pulse count. After assessing a vast number of conditions, he finally established a reproducible method.

This technique enables labeling of hippocampal cells based solely on location and timing, without relying on specific gene expression, making it possible to discover cell populations that have previously been overlooked. Furthermore, by combining this with genetically modified mice expressing fluorescent proteins, the researchers established a dual-visualization system that allows simultaneous visualization of two different cell populations within a single mouse.

This revealed two distinct groups: pioneer cells that migrate from the outer to inner regions of the hippocampus while rapidly maturing into neurons to form the foundational structure of the dentate gyrus, and neural stem cells that differentiate slowly, largely remaining in an undifferentiated state, and form a stem cell pool that continues neurogenesis after birth.

The pioneer neural progenitor cells discovered in this research are the cells that first begin drawing the blueprint of hippocampal formation, and as such may become new therapeutic targets for developmental disorders and psychiatric diseases. In particular, understanding how environmental factors during the fetal period, such as maternal stress, infection, and nutritional status, affect these early-differentiating cells is expected to lead to the development of preventive medical approaches. Additionally, further understanding of the molecular mechanisms that control this cell population may contribute to regenerative medicine for repairing a damaged hippocampus and developing treatments to manage age-related memory decline.

Further improvement of the in utero electroporation method that has been established for the dentate gyrus will enable more efficient and stable cell labeling. The research group is currently advancing the development of next-generation technology that can precisely target specific regions and developmental stages of the hippocampus, with revolutionary advances to hippocampal research expected in the near future.

Journal Information
Publication: Cellular and Molecular Neurobiology
Title: In Utero Electroporation Uncovers an Early-Differentiating Subset of Dentate Gyrus Progenitors
DOI: 10.1007/s10571-025-01616-3