Most animals — including birds, fish and mammals — have symmetrical left and right sides, and are known as bilaterians. During early life, the embryos of animals in this group develop three distinct layers of cells: the ectoderm (outer layer), the endoderm (inner layer), and the mesoderm (middle layer). These layers then go on to form the animal’s tissues and organs. The ectoderm produces external tissues, such as the skin and the nervous system; the endoderm forms internal tissues, like the gut; and the mesoderm creates all tissues in between, like muscles and blood.
Another, smaller group of animals, called cnidarians, do not have left and right sides. Instead, they have a ‘radial symmetry’, meaning they have multiple identical parts arranged in a circle. These animals — which include corals, jellyfish and sea anemones — only develop two distinct layers of cells, equivalent to the outer and inner layers of bilaterians. Cnidarians evolved before bilaterians, but their genetic material is equally complex. So why did these two groups evolve to have different layers of cells? And how exactly do animal embryos develop these distinct layers?
To address these questions, Salinas-Saavedra et al. studied embryos of the sea anemone Nematostella vectensis. Molecules called Par-proteins play an important role in controlling how cells behave and attach to one another (and therefore how they form layers). So, using a technique called immunohistochemistry to look inside cells, Salinas-Saavedra et al. examined these proteins in the two layers of cells in sea anemone embryos.
The experiments found that in the sea anemones, Par-proteins are arranged differently in cells that form the ‘skin’ compared to cells that form the ‘gut’. In other words, cells in the outer layer attach to one another in a different way than cells in the inner layer, where the Par-proteins are degraded by ‘mesodermal’ genes. The findings also show that these sea anemones have all they need to form a third middle layer of cells. Like bilaterians, they could potentially move cells in and out of sheets that line surfaces inside the body — but they do not naturally do this.
Understanding how animals form different layers of cells is important for scientists studying evolution and the development of embryos. However, it also has wider applications. For instance, some cells involved in developing the mesoderm are also involved in forming tumors. Future research in this area could help scientists learn more about how cancer-like cells form in animals.
To find out more
Read the eLife research paper on which this eLife digest is based:
