Figure 3.

Ephrin reverse signaling and Rap1b contribute to Integrin clustering/activation, which results in Fn polymerization at the nascent somite boundary. Fn (green) is available for polymerization in the presomitic mesoderm but is not organized. Signaling through Ephrin receptors results in Integrin clustering (and presumably activation). Next, inside-out Integrin signaling potentiates Fn polymerization, leading to formation of intersomitic clefts. The alignment and enlargement of these clefts, along with epithelialization of the border cells, results in the formation of stable somite boundaries.