Although genetic lineage tracing is a powerful tool for studying development, most systems rely on tracing the expression of a single gene. Now, Takashi Serizawa and colleagues have developed a ‘TRiple Coloured germ layer Knock-in’ mouse or ‘TRiCK’ system, which allows the simultaneous tracing of all three germ layers in vivo. The system employs a new transgenic technique that precisely labels the neuroectoderm, mesoderm and endoderm, using the Sox1, brachyury (T) and Sox17 promoters, respectively, to drive the expression of three different fluorescent proteins. In addition, to maximise the imaging potential of the TRiCK mice, the authors describe an improved clearing protocol that preserves fluorescent protein intensity. To evaluate the TRiCK system, the researchers study the composition of different organs and validate their observations through immunostaining. The brain, for example, is almost entirely composed of neuroectoderm-derived cells (as would be expected), but also contains populations of mesoderm-derived cells identified as microglia. Finally, the authors make some interesting observations that warrant further investigation, such as populations of unlabelled cells – of unidentifiable lineage – in the liver and heart. Together, these resources are powerful tools to help understand the relationships between diverse cell types and germ-layer patterning.
