Ventricular zone in a human brain organoid.

The dark arch along the bottom is the ventricular zone, a cavity that appears when stem cells arrange themselves into rings, exactly as they do in a real developing brain.

The cells hugging that wall are the earliest stem cells of the lot, and they're the ones that divide and turn into neurons. You can spot them by 🟪, a protein that switches on in brain stem cells before they commit to becoming anything in particular.

🟩 DNA 🟪 SOX2 — the badge of a brain stem cell