Research in bilaterians (e.g. Drosophila and vertebrates) has highlighted the important role of the mesoderm in orchestrating branching morphogenesis. The jellyfish, Cladonema pacificum, also forms branched structures, despite being a diploblastic species that lacks mesoderm. Now, Gaku Kumano and colleagues reveal the role of Cnidarian stem cells, known as interstitial cells (I-cells), in Cladonema tentacle branching morphogenesis. Through in situ hybridisation and antibody staining experiments, the authors identify that Nanos1-expressing I-cells accumulate at tentacle branch buds. Small molecule treatments that inhibit MEK signalling and proliferation reveal that I-cells accumulate at these regions through MEK-mediated proliferation. In addition, antagonising FGF activity using a small molecule inhibitor prevents bud elongation, potentially through altering I-cell behaviours, such as self-renewal or differentiation. Indeed, phosphorylated MAPK (a downstream target of FGF signalling) is lost in I-cells treated with the inhibitor. Finally, the researchers label and map of the fates of I-cells in the developing branch. They show that the I-cell population contributes to differentiated cell types of the new branch, such as nematocytes (stinging cells) and epithelial cells. Although there is still much to elucidate, these data indicate the importance of I-cell proliferation and FGF signalling for branch morphogenesis in Cladonema.