Do brain neurons have a hidden clock?

In the mammalian brain, excitatory neurons are born at different times and settle into specific layers. But what happens if they are in the same layer? Specifically, in the mouse primary visual cortex (V1), neurons connecting to lateral visual areas are evenly distributed throughout layer 2/3, while those heading to medial areas prefer more superficial sublayers. Scientists used advanced techniques to discover that neurons bound for lateral areas are preferentially born at embryonic day 15.5 (E15.5) compared to day 16.5 (E16.5). Meanwhile, those heading to medial areas show no significant preference. This suggests that the birth timing of neurons influences their final destination, even within the same layer. A true biological clock guiding the brain!

The study combined EdU birthdating with retrograde viral tracing to map neuron connections. EdU (5-ethynyl-2'-deoxyuridine) is a thymidine analog that incorporates into DNA during synthesis, allowing cells born at a specific time to be labeled. Retrograde viral tracing, on the other hand, uses modified viruses to follow neuron pathways. These viruses are injected into target areas and travel back along neuronal connections, allowing the origin of neurons to be identified. The study revealed that neurons born at E15.5 are more likely to project to lateral visual areas, while those born at E16.5 show no clear preference. This suggests that neurogenic timing may play a crucial role in specifying neuron projection identities, even within the same cortical layer. Further research could explore whether this mechanism is conserved in other species and whether it can be influenced by environmental or genetic factors.