Polyps of the starlet sea anemone Nematostella vectensis. Their single body opening (mouth) is surrounded by extended tentacles. The cells producing digestive enzymes and insulin-like peptides are part of the folds running internally along animal.PATRICK STEINMETZHumans and most other animals that belong to the phylum Bilateria have embryos that form three layers of cells called germ layers—ectoderm outside, endoderm inside, and mesoderm in between—that give rise to the organism’s organization. Embryos from the phlyum Cnidaria, which includes jellyfish and sea anemones, only have two layers of cells, which scientists have correspondingly classified into two germ layers—ectoderm on the outside and on the inside, mesendoderm, which produces cell types that in bilaterians come from either the endoderm or the mesoderm.
A study published today (September 11) in Nature Ecology & Evolution suggests that instead of two distinct germ layers, cnidarians may indeed have three. The authors show that the mesendoderm of the sea anemone (Nematostella vectensis) resembles bilaterian mesoderm, based on cell type specification and gene expression. And Nematostella derives gut-like cells from a portion of the embryonic layer that has been previously characterized as ectoderm, but expresses genes orthologous to those in bilaterian endoderm.
Nematostella embryos’ two cell layers arise during gastrulation by the movement of a group of cells into the interior of the animal to form a pouch with one open end. The two layers created by this invagination were historically described as endoderm and ectoderm. More recent work suggested that what ...
