The mouse dorsal raphe nucleus as understood by temporal Fgf8 lineage analysis

Area of Science:

• Neuroscience
• Developmental Biology
• Genetics

Background:

• Fibroblast growth factor 8 (Fgf8) is crucial during embryogenesis, particularly at the midbrain-hindbrain border, which forms key neuronal populations like the dorsal raphe (DR) nucleus.
• Understanding the developmental origins of neuronal subtypes is essential for deciphering brain circuitry and function.

Purpose of the Study:

• To identify neuronal populations derived from Fgf8-expressing cells during embryogenesis.
• To correlate the temporal dynamics of Fgf8 lineage with the organizational patterns of serotonin neurons in the DR and median raphe (MnR) nuclei.

Main Methods:

• Utilized an inducible Fgf8-cre allele to trace Fgf8 lineage in mice at different embryonic timepoints (E7.5, E9.5, E11.5).
• Analyzed the distribution of Fgf8-lineage cells in the adult brain, specifically focusing on the DR and MnR nuclei.
• Characterized serotonin neuron populations within these nuclei based on their Fgf8 lineage status.

Main Results:

• Induction at embryonic day 7.5 (T-E7.5) resulted in Fgf8 lineage encompassing the entire DR and part of the MnR in adults.
• Later induction timepoints showed a progressive loss of Fgf8 lineage from caudal and ventral regions, particularly on the midline.
• Specific serotonin neuron populations in the MnR and DR were sequentially excluded from Fgf8 lineage, with the last cells showing lineage being rostral-dorsal and lateral wing serotonin neurons.

Conclusions:

• The temporal progression of Fgf8 lineage during embryogenesis directly correlates with the spatial organization of serotonin neuron subtypes within the DR and MnR.
• Fgf8 signaling plays a critical role in defining the developmental trajectories of distinct neuronal populations in the brainstem.
• This study provides a detailed map of Fgf8-derived neurons, offering insights into the developmental mechanisms underlying raphe nuclei formation.