Eric Davidson spent a career mapping the gene regulatory networks of the sea urchin embryo.
The Visitor stands in the embryo and watches one fertilized cell become a body.
A Network That Writes Identity
- One fertilized cell divides into 200+ distinct cell types with the same 3.2 billion base pairs. What differs is which genes are expressed.
- Davidson's gene regulatory networks (GRNs) are dense webs of transcription factors, with the output of one gene feeding the next.
- The page describes three tiers. Kernel circuits (about 20 transcription factors) specify cell type identity, and the Pax6 kernel specifies eye identity across bilaterians. Plug-in circuits (about 200 transcription factors) are reusable subcircuits such as the Wnt, Notch, and Hedgehog cascades. Differentiation genes (about 20,000) form the output layer of structural and functional proteins.
- The page highlights that the most essential kernels are the most conserved across animal groups, including Pax6 for eyes, Nkx2.5 for heart, and GATA-SCL for blood.
One cell branches into tiers until the body's cell types appear. The site page carries the kernel, plug-in, and output tiers.
Which tier of the network specifies a cell's identity?
The kernel circuits.
Identity is written into a network of regulators, and the same genome produces every cell type.
The Visitor lights node 10 and sees Signaling point toward it on the map.
See the map