Changes in cellular composition shape the inductive properties of Hensen's Node.

The establishment of the vertebrate body plan is orchestrated by the organizer, a specialized group of cells with inductive properties that guide axial specification during early development. In avian embryos, organizer cells reside within Hensen's node, a transient structure located at the tip of the primitive streak. Despite its pivotal role during gastrulation, the cellular architecture of the Hensen's node remains poorly understood. Here, we show that Hensen's node is composed of two transcriptionally and functionally distinct organizer populations. In addition to anterior GSC-expressing cells associated with head induction, we identify a posterior population co-expressing organizer and mesodermal genes. These posterior cells exhibit trunk-inducing activity when transplanted into naïve tissue. Our findings reveal that the organizer is a dynamic and spatially compartmentalized structure, and that temporal changes in the relative abundance of anterior and posterior populations underlie shifts in its inductive capacity, ensuring coordinated patterning along the body axis.