Morphogenesis of the Starfish Embryo: Exploring the Mechanisms by Which Cells Create Body Shape

Starfish are primitive animals that have existed on Earth for 600 million years. I am researching how animals develop their unique shapes during the developmental process from a fertilized egg to a larva, focusing on the function of mesenchymal cells (see next paragraph) in starfish larvae. One reason for using starfish is that echinoderms, the phylum to which they belong, are ancestors of vertebrates. Through the study of starfish, we can also learn a great deal about humans. The body of a starfish larva is only about 1 millimeter long, but its organs, such as the mouth, esophagus, stomach, and intestine, are arranged much like those in humans. When viewed in this light, its shape comes to resemble a human head, shoulders, and waist (see figure).

The cells that make up the larva can be broadly divided into epithelial cells and mesenchymal cells. The epithelial cells adhere to each other to form a sheet that covers the entire surface of the larva. In contrast, the mesenchymal cells are located inside the larva, scattered individually. During the process of embryonic development leading to the larva, the epithelial sheet transforms from a simple hollow ball shape into a unique form with "shoulders" and a "waist." It has become clear that during this process, mesenchymal cells act on filamentous substances secreted by epithelial cells into the embryonic interior, playing a role in maintaining the unique shape of the starfish larva. I believe the actual mechanism of this action is that in different locations within the embryo, mesenchymal cells weave the filamentous substances into thicker fibers or, conversely, unravel them.

Mesenchymal cells have other functions as well. When a foreign substance, such as sea urchin sperm, is injected into the larva's body, the mesenchymal cells gather and consume it (phagocytosis). Unlike with sea urchin sperm, this phagocytic action does not occur in response to starfish sperm. So, what happens when a mixture of sea urchin and starfish sperm is injected? The cells consume only the sea urchin sperm. Mesenchymal cells also have the ability to distinguish between species. Recently, we have also discovered that mesenchymal cells have the function of inducing the proliferation of epithelial cells.

The starfish embryo, in the stage before it becomes a larva, possesses a morphogenetic ability not found in other animal embryos. Even when dissociated into individual cells, the cells can reaggregate and reconstruct the larval form in a way that appears different from normal development. By comparing this process with the mechanisms of normal development, new discoveries are likely to be made. In the future, while shifting my focus to elucidating this reconstruction—a feat unique to starfish embryos—I also hope to nurture the next generation of researchers who will carry on the study of starfish morphogenesis.