Thus, we identified species-specific calcium dynamics as a mechanism to set tempo, by tuning axon outgrowth duration and with it axon tract morphologies, thereby demonstrating a causal link between the slowing of tempo and the elaborated morphology of human neurons. (12/12)

Increasing calcium influx in human organoids, by stimulating L-Type VGCCs, led to a shorter axon growth duration and shorter axon tract lengths, like mouse. This was phenocopied by elevating cAMP, a downstream second messenger, whereas blocking calcium influx led to the opposite effect. (11/n)

This revealed that calcium influx during different neural activity modes was consistently higher in mouse compared to human, even at stage-matched timepoints. This pointed to calcium dynamics not only correlating with differences in tempo, but also as a strong candidate to drive tempo. (10/n)

Calcium imaging using the GCAMP7f reporter showed the presence of both spontaneous transients and burst activity at distinct developmental stages in each species. (9/n)

We performed a scRNA-seq experiment covering a range of timepoints in both human and mouse, which pointed to neural activity related calcium channels with an expression profile aligning with neuron development, including axon outgrowth. (8/n)

Combining these with micropatterned devices to capture outgrowing axon tracts, we observed that species-specific differences in axon tract length consistently correlate with the duration of the axon tract outgrowth state, irrespective of tissue environment. (7/n)

Studying axon tract formation is challenging, precisely because of these excessive morphologies. To tackle this, we used human and mouse brain organoid slice cultures that develop long-range axon tracts which recapitulate species-specific differences in axon tract length. (6/n)

Each of these evolutionary changes correlate with a prolonged duration during which these structures are established. We hypothesised that slowing of tempo could be a mechanism driving these changes in magnitude ( recently reviewed in doi.org/10.1038/s415... & (doi.org/10.1016/j.ce...) (4/n)

Excited to reconnect with old science friends and meet new ones here on Bluesky! I study human neuron development using brain organoids, other in vitro models, and lots of microscopy 🧠🔬