Epiboly

Epiboly is one of the five major types of cell movements that happen during the gastrulation stage of embryonic development in some organisms. It describes how the outer layer of cells, called the ectoderm, spreads out and becomes thinner. At the same time, the layers that will become the inner parts of the body, the endoderm and mesoderm, move to the inside of the embryo.

During epiboly, cells change shape so they can spread out. In some cases, many layers of cells move together as they change position or fit in between each other. Although human embryos do not go through epiboly, scientists can study this process in sea urchins, tunicates, amphibians, and most commonly in zebrafish. This helps us understand how embryos develop and grow.

Zebrafish

Epiboly in zebrafish is the first big movement of cells, starting at a stage called the dome and lasting through gastrulation. At this time, the embryo has three main parts: a thin layer of cells called the enveloping layer (EVL), a layer of nuclei on top of the yolk called the yolk syncytial layer (YSL), and the deep cells (DEL) that will become the embryo’s three layers — the ectoderm, mesoderm, and endoderm. All three layers go through epiboly.

During this process, the deep cells move outward, thinning as they spread toward the bottom of the embryo. The EVL also spreads, growing larger as it moves. The YSL moves along the yolk’s surface. Once this movement is complete, all three layers have covered the yolk, forming a closing point called the blastopore.

Other vertebrates

Epibolic movements happen in many animals with backbones. Most studies of epiboly have been done using fish, but we also know about it from the African clawed frog, Xenopus laevis. When we compare epiboly in fish, frogs, and animals like birds and mammals (called amniotes), we see different ways it works. In fish and frogs, cells move by fitting in between each other, called radial intercalation. In amniotes, it seems to happen more through cell division. In all these animals, the cells also change shape, flattening out to cover more space.