Microscopic image of CA3 pyramidal neurons. To take the picture, they were fixed and then colored in order to be able to completely reconstruct their shapes. © Jose Guzman / ISTA Jonas Group
Pyramidal neurons are lined up next to each other in the hippocampus from birth. This is where memories are stored and sensory impressions are linked. An Austrian research team has now found that this nerve network is a dense network with random connections at the beginning of life and is thinned out and optimized over the course of development.
The hippocampus is a seahorse-shaped structure of the brain where fleeting perceptions are processed into lasting memories. The image shows eight CA3 pyramidal neurons examined in a mouse brain slice. In the hippocampus, the cell nuclei of these neurons form an arch, which is indicated by the dashed line. In a new study, a working group led by Peter Jonas at the Institute of Science and Technology Austria shows how the nerve network in this brain region develops after birth.
Until now, it was unclear whether the neuronal network in the hippocampus is already established in the organism at birth or whether the network only emerges piece by piece. As an analogy, the researchers use the image of a page: is existing information supplemented, organized and deleted over the course of life (tabula plena) or is the initially completely blank page written on over the course of life (tabula rasa)? Is everything created from the beginning, or does experience only shape who we are?
The CA3 pyramidal neurons, which store memories in the hippocampus of mammals, were examined. Electrical signals from these neurons were measured in mice using the so-called patch clamp technique. In addition, sophisticated microscopes were used to observe the development of the neuronal network. The mice were examined at three different developmental stages: shortly after birth (days 7-8), adolescence (days 18-25) and adulthood (days 45-50).
The result amazed the researchers: Over the course of life, the distances between individual neurons become larger and the thread-like extensions of the cells become less and less connected. “Intuitively, you would expect a network to grow and become denser over time. But here we see exactly the opposite. It is a Pruning model“In the beginning it is full, later it is refined and optimized,” says Peter Jonas. This type of development probably has an advantage: a network that is already widely branched at the beginning may allow nerve cells to connect with each other quickly and efficiently. If all the connections had to be formed again, the cells would initially be too far apart and efficient communication would hardly be possible.