Multicellular animals such as ourselves have a body plan consisting of distinct layers of cells. These germ cell layers are maintained as separate regions very early in embryonic development. New research by scientists from McGill University and from the University of Toronto has shed light on the process of differentiation and separation.
Our bodies are made up of three types of cells: ectoderm, mesoderm and endoderm. Broadly, ectoderm cells eventually make up the nervous system and skin; mesoderm cells become bone, muscle, and the blood system; and the endoderm develops into the gastrointestinal and respiratory tracts. Once the distinct layers are formed (at a time when the developing embryo is no more than a ball of cells), they remain separate for rest of the individual’s life.
Traditionally, it was thought that the three layers of cells remained separate due to differential attraction. It was proposed that endoderm cells, for example, had a higher affinity for other endoderm cells than for mesoderm or ectoderm cells. Thus, each type of cell would stick to its own kind in preference to other cell types. In this study, François Fagotto of McGill and Rudi Winklbauer of the University of Toronto took a closer look at developing frog embryos. They found that rather than simply adhering to their own types of cells, mesoderm and ectoderm cells were actively repelling each other.
Caption: These are frames from a high- resolution movie of live fluorescence confocal microscopy. The membranes (outlines) of ectoderm and mesoderm cells are labeled in green and orange, respectively. In the first frame, the two types of cells are in tight contact, and their membranes appear merged (green + orange = yellow). In frames 2 and 3, a repulsive reaction is triggered. While cells are being pulled away (arrows), adhesive contacts are ripped apart, leaving behind threads of membranes (arrowhead), just like when one tries to detach two objects that are glued together.
Credit: François FagottoNow it appears that both attraction and repulsion are important factors in embryo development.