A cellular basis for the hourglass pattern in vertebrate embryogenesis.

Vertebrate embryogenesis follows a conserved trajectory, exhibiting divergence in early and late stages and conservation during mid-embryogenesis across species. This pattern, known as the developmental hourglass, was first described at the morphological level and later supported by molecular studies, establishing it as a hallmark of vertebrate development. The "waist" of the hourglass, representing the period most resilient to evolutionary change, coincides with the emergence of the body plan, when embryos across species appear most alike. Yet development is not simply an organism-level process; it arises from the coordinated behaviors of individual cell lineages that collectively generate form and function. If the hourglass reflects a fundamental principle of vertebrate development, might it also be rooted in the dynamics of cells themselves? In this Perspective, we revisit the hourglass model through the lens of cellular lineages, asking whether the conservation of mid-embryogenesis is underpinned by universal constraints at the level of individual cells. Could the vertebrate developmental hourglass truly have a cellular basis?