Cellular scaling laws in the mammalian brain.

Deep subcortical structures like the hypothalamus and brain stem harbor the greatest neuronal cell type diversity in the mammalian brain, not the cortex or cerebellum.

• Why it matters: Understanding the organization of neuronal diversity is crucial for insights into brain function, evolution, and disease, yet the reasons for this distribution remain unclear.
• What they did: The authors analyzed whole-brain cell atlases and proposed a framework distinguishing a "learning subsystem" that scales via module replication and a "steering subsystem" that scales through diversification of cell types.
• The result: This division explains the evolutionary divergence of brain regions, offers a unified view of brain architecture and disease, and may guide artificial intelligence model design.